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https://www.youtube.com/watch?v=bIVI-rMLm54

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The Nuclear Atomic Model and Ernest Rutherford | Mr. Causey's Chemistry

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Mr. Causey discusses the Nuclear atomic model, the Gold-foil experiment and the atomic nucleus. http://www.yourCHEMcoach.com SUBSCRIBE for more chemistry videos: http://bit.ly/1jeutVl ABOUT MR. CAUSEY'S VIDEO ACADEMY Mr. Causey's Video Academy is an educational video series of short video lessons for chemistry, algebra and physics.

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https://www.youtube.com/watch?v=JUJPyQtoB5E

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Thomson's Plum Pudding Model of the Atom

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https://chemistrytalk.org/discovering-the-nucleus-rutherfords-gold-foil-experiment/

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History of Chemistry: Rutherford Gold Foil Experiment

In this article, you will learn the history behind the Rutherford Gold Foil Experiment and the events that led to the discovery of the atomic nucleus. If you enjoy this article, check out our other history of chemistry articles linked below!

Who was Ernest Rutherford?

Ernest Rutherford is known as the father of nuclear physics. Born in Brightwater, New Zealand on August 30th, 1871, Rutherford was the fourth of twelve children. His father was a farmer and his mother a school teacher. From a very early age, Rutherford understood the importance of hard work and the power of education. In school, he excelled greatly and at the age of fifteen won an academic scholarship to study at Nelson Collegiate School. Then, at the age of 19, he won another academic scholarship to study at Canterbury College in Christchurch. A few years later he won another scholarship, the exhibition science scholarship, and he left New Zealand to study at Trinity College, Cambridge in England. While there, he conducted research at the Cavendish Laboratory under his advisor J.J. Thomson.

Cavendish Lab: JJ Thomson front middle and Rutherford behind him

During his time at Cavendish Lab, Rutherford faced adversity from his peers. Because he was from New Zealand, he was often ostracized by fellow students. In the end, he used this as motivation to succeed. Which he did as he made a multitude of great discoveries through his research in gases and radioactivity. These included the discovery of different types of radiation, radiometric dating, and the nucleus of an atom.

The Rutherford Gold Foil Experiment

The Experiment

While working as a chair at the University of Manchester, Rutherford conducted the gold-foil experiment alongside Hans Geiger and Ernest Marsden. In this experiment, they shot alpha particles-which Rutherford had discovered years prior- directly at a piece of thin gold foil. As the alpha particles passed through, they would hit the phosphorescent screen encasing the foil. When the particles came into contact with the screen, there would be a flash.

Observations

Going into the experiment, Rutherford had formed preconceptions for the experiment based on J.J. Thomson's plum pudding model. He predicted the alpha particles would shoot through the foil with ease. Some of the particles did manage to pass directly through the foil, but some veered from the path either bouncing back or deflecting. Rutherford found this to be an exciting observation and compared it to shooting a bullet at a piece of tissue and having it bounce back.

From this observation, two deductions were made. Firstly, he concluded most of the atom is composed of empty space. Secondly, he concluded there must be something small, dense, and positive inside the atom to repel the positively charged alpha particles. This became the nucleus, which in Latin means the seed inside of a fruit.

Top: How Rutherford expected the experiment to go. Bottom: How the experiment actually went.

The Nuclear Model

The gold-foil experiment disproved J.J. Thomsons plum pudding model, which hypothesized the atom was positively charged spaced with electrons embedded inside. Therefore, giving way to the nuclear model. In this model, Rutherford theorized the atomic structure was similar to that of the solar system. Where the nucleus was in this middle and surrounded by empty space with orbiting electrons.

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Discovering the Nucleus: Rutherford's Gold Foil Experiment | ChemTalk

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In this article, you will learn the history behind the Rutherford Gold Foil Experiment and the events that led to the discovery of the atomic nucleus. If you enjoy this article, check out our other history of chemistry articles linked below! Ernest Rutherford is known as the father of nuclear physics.

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https://orise.orau.gov/resources/k12/documents/harnessed-atom/build-an-atom/index.html

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Interactive module that introduces atomic structure

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https://plato.stanford.edu/entries/physics-experiment/app7.html

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Appendix 7: Evidence for a New Entity: J.J. Thomson and the Electron

In discussing the existence of electrons Ian Hacking has written, "So far as I'm concerned, if you can spray them then they are real" (Hacking 1983, p. 23). He went on to elaborate this view. "We are completely convinced of the reality of electrons when we set out to build-and often enough succeed in building-new kinds of device that use various well-understood causal properties of electrons to interfere in other more hypothetical parts of nature" (p. 265).

Hacking worried that the simple manipulation of the first quotation, the changing of the charge on an oil drop or on a superconducting niobium sphere, which involves only the charge of the electron, was insufficient grounds for belief in electrons. His second illustration, which he believed more convincing because it involved several properties of the electron, was that of Peggy II, a source of polarized electrons built at the Stanford Linear Accelerator Center in the late 1970s. Peggy II provided polarized electrons for an experiment that scattered electrons off deuterium to investigate the weak neutral current. Although I agree with Hacking that manipulability can often provide us with grounds for belief in a theoretical entity, his illustration comes far too late. Physicists were manipulating the electron in Hacking's sense in the early twentieth century. They believed in the existence of electrons well before Peggy II, and I will argue that they had good reasons for that belief.

The position I adopt is one that might reasonably be called "conjectural" realism. It is conjectural because, despite having good reasons for belief in the existence of an entity or in the truth of a scientific law, we might be wrong. At one time scientists had good reason to believe in phlogiston and caloric, substances we now have good reason to believe don't exist. My position includes both Sellars' view that "to have good reason for holding a theory is ipso facto to have good reason for holding that the entities postulated by the theory exist" (Sellars 1962, p. 97), and the "entity realism" proposed by Cartwright (1983) and by Hacking (1983). Both Hacking, as noted above, and Cartwright emphasize the manipulability of an entity as a criterion for belief in its existence. Cartwright also stresses causal reasoning as part of her belief in entities. In her discussion of the operation of a cloud chamber she states, "...if there are no electrons in the cloud chamber, I do not know why the tracks are there" (Cartwright, 1983, p.99). In other words, if such entities don't exist then we have no plausible causal story to tell. Both Hacking and Cartwright grant existence to entities such as electrons, but do not grant "real" status to either laws or theories, which may postulate or apply to such entities.

In contrast to both Cartwright and Hacking, I suggest that we can also have good reasons for belief in the laws and theories governing the behavior of the entities, and that several of their illustrations implicitly involve such laws. I have argued elsewhere for belief in the reality of scientific laws (Franklin 1996). In this section I shall concentrate on the reality and existence of entities, in particular, the electron. I agree with both Hacking and Cartwright that we can go beyond Sellars and have good reasons for belief in entities even without laws. Hacking and Cartwright emphasize experimenting with entities. I will argue that experimenting on entities and measuring their properties can also provide grounds for belief in their existence.

In this section I will discuss the grounds for belief in the existence of the electron by examining J.J. Thomson's experiments on cathode rays. His 1897 experiment on cathode rays is generally regarded as the "discovery" of the electron.

The purpose of J.J. Thomson's experiments was clearly stated in the introduction to his 1897 paper.

The experiments discussed in this paper were undertaken in the hope of gaining some information as to the nature of Cathode Rays. The most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the aether to which-inasmuch as in a uniform magnetic field their course is circular and not rectilinear-no phenomenon hitherto observed is analogous: another view of these rays is that, so far from being wholly aetherial, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity (Thomson 1897, p. 293).

Thomson's first order of business was to show that the cathode rays carried negative charge. This had presumably been shown previously by Perrin. Perrin placed two coaxial metal cylinders, insulated from one another, in front of a plane cathode. The cylinders each had a small hole through which the cathode rays could pass onto the inner cylinder. The outer cylinder was grounded. When cathode rays passed into the inner cylinder an electroscope attached to it showed the presence of a negative electrical charge. When the cathode rays were magnetically deflected so that they did not pass through the holes, no charge was detected. "Now the supporters of the aetherial theory do not deny that electrified particles are shot off from the cathode; they deny, however, that these charged particles have any more to do with the cathode rays than a rifle-ball has with the flash when a rifle is fired" (Thomson 1897, p. 294).

Thomson repeated the experiment, but in a form that was not open to that objection. The apparatus is shown in Figure 14. The two coaxial cylinders with holes are shown. The outer cylinder was grounded and the inner one attached to an electrometer to detect any charge. The cathode rays from A pass into the bulb, but would not enter the holes in the cylinders unless deflected by a magnetic field.

Figure 14. Thomson's apparatus for demonstrating that cathode rays have negative charge. The slits in the cylinders are shown. From Thomson (1897).

When the cathode rays (whose path was traced by the phosphorescence on the glass) did not fall on the slit, the electrical charge sent to the electrometer when the induction coil producing the rays was set in action was small and irregular; when, however, the rays were bent by a magnet so as to fall on the slit there was a large charge of negative electricity sent to the electrometer.... If the rays were so much bent by the magnet that they overshot the slits in the cylinder, the charge passing into the cylinder fell again to a very small fraction of its value when the aim was true. Thus this experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays (Thomson 1897, p. 294-295, emphasis added).

This experiment also demonstrated that cathode rays were deflected by a magnetic field in exactly the way one would expect if they were negatively charged material particles.

Figure 15. Thomson's apparatus for demonstrating that cathode rays are deflected by an electric field. It was also used to measure \(\bfrac{m}{e}\). From Thomson (1897).

There was, however, a problem for the view that cathode rays were negatively charged particles. Several experiments, in particular those of Hertz, had failed to observe the deflection of cathode rays by an electrostatic field. Thomson proceeded to answer this objection. His apparatus is shown in Figure 15. Cathode rays from C pass through a slit in the anode A, and through another slit at B. They then passed between plates D and E and produced a narrow well-defined phosphorescent patch at the end of the tube, which also had a scale attached to measure any deflection. When Hertz had performed the experiment he had found no deflection when a potential difference was applied across D and E. He concluded that the electrostatic properties of the cathode ray are either nil or very feeble. Thomson admitted that when he first performed the experiment he also saw no effect. "on repeating this experiment [that of Hertz] I at first got the same result [no deflection], but subsequent experiments showed that the absence of deflexion is due to the conductivity conferred on the rarefied gas by the cathode rays". On measuring this conductivity it was found that it diminished very rapidly as the exhaustion increased; it seemed that on trying Hertz's experiment at very high exhaustion there might be a chance of detecting the deflexion of the cathode rays by an electrostatic force (Thomson 1897, p. 296). Thomson did perform the experiment at lower pressure [higher exhaustion] and observed the deflection.

Thomson concluded:

As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter. (Thomson 1897, p. 302)

Having established that cathode rays were negatively charged material particles, Thomson went on to discuss what the particles were. "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision" (p. 302). To investigate this question Thomson made measurements on the charge to mass ratio of cathode rays. Thomson's method used both the electrostatic and magnetic deflection of the cathode rays. The apparatus is shown in Figure 15. It also included a magnetic field that could be created perpendicular to both the electric field and the trajectory of the cathode rays.

Let us consider a beam of particles of mass \(m\) charge \(e\), and velocity \(v\). Suppose the beam passes through an electric field F in the region between plates D and E, which has a length \(L\). The time for a particle to pass through this region \(t = \bfrac{L}{v}\). The electric force on the particle is \(Fe\) and its acceleration \(a = \bfrac{Fe}{m}\). The deflection d at the end of the region is given by

\[ d = \frac{1}{2} at^2 = \frac{1}{2}\left(\frac{eF}{m}\right)\frac{L^2}{v^{2}}. \]

Now consider a situation in which the beam of cathode rays simultaneously pass through both \(F\) and a magnetic field \(B\) in the same region. Thomson adjusted \(B\) so that the beam was undeflected. thus the magnetic force was equal to the electrostatic force.

\[ evB = eF \text{ or } v = \frac{F}{B}. \]

This determined the velocity of the beam. Thus,

\[ \frac{e}{m} = \frac{2dF}{B^2L^2}. \]

Each of the quantities in the above expression was measured so the \(\bfrac{e}{m}\) or \(\bfrac{m}{e}\) could be determined.

Using this method Thomson found a value of \(\bfrac{m}{e}\) of \((1.29\pm 0.17) \times 10^{-7}\). This value was independent of both the gas in the tube and of the metal used in the cathode, suggesting that the particles were constituents of the atoms of all substances. It was also far smaller, by a factor of 1000, than the smallest value previously obtained, \(10^{-4}\), that of the hydrogen ion in electrolysis.

Thomson remarked that this might be due to the smallness of \(m\) or to the largeness of \(e\). He argued that \(m\) was small citing Lenard's work on the range of cathode rays in air. The range, which is related to the mean free path for collisions, and which depends on the size of the object, was 0.5 cm. The mean free path for molecules in air was approximately \(10^{-5}\) cm. If the cathode ray traveled so much farther than a molecule before colliding with an air molecule, Thomson argued that it must be much smaller than a molecule.

Thomson had shown that cathode rays behave as one would expect negatively charged material particles to behave. They deposited negative charge on an electrometer, and were deflected by both electric and magnetic fields in the appropriate direction for a negative charge. In addition the value for the mass to charge ratio was far smaller than the smallest value previously obtained, that of the hydrogen ion. If the charge were the same as that on the hydrogen ion, the mass would be far less. In addition, the cathode rays traveled farther in air than did molecules, also implying that they were smaller than an atom or molecule. Thomson concluded that these negatively charged particles were constituents of atoms. In other words, Thomson's experiments had given us good reasons to believe in the existence of electrons.

Return to Experiment in Physics

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Experiment in Physics > Appendix 7: Evidence for a New Entity: J.J. Thomson and the Electron (Stanford Encyclopedia of Philosophy)

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In discussing the existence of electrons Ian Hacking has written, "So far as I'm concerned, if you can spray them then they are real" (Hacking 1983, p. 23). He went on to elaborate this view.

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https://www.footprints-science.co.uk/index.php?quiz=Plum_pudding_model_vs_nuclear_model_(2)

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Plum pudding model vs nuclear model (2) quiz questions - Footprints-Science | GCSE science animations and quizzes

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Plum pudding model vs nuclear model (2) quiz including multiple choice, drag and drop, flashcards and anagrams | Footprints-Science | GCSE science animations and quizzes

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https://www.youtube.com/watch?v=4HOWi-vhy7Q

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Additional Physics (P2) Plum Pudding vs Nuclear Model of the Atom

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This led Rutherford to propose the nuclear model. The nuclear model consists of a small, positively charged nucleus surrounded by the negatively charged electrons. Based on the number of alpha particles deflected in his experiment, Rutherford was able to determine that the nucleus took up a tiny fraction of the total volume of the atom.

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Based on Thomson's plum pudding model, Rutherford hypothesized that most of the alpha particles would pass straight through the gold foil. This is because the positive charge in the plum pudding model was believed to be spread out throughout the entire volume of the atom. Therefore, the electric field from the positively charged "soup" would be too weak to drastically affect the path of the relatively large and fast-moving alpha particles.

The results of the experiment, however, were not as expected. While the vast majority of the alpha particles passed straight through the gold foil, a select few alpha particles were deflected more than
90 degrees from their path.

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https://sciencenotes.org/law-of-definite-proportions-law-of-constant-composition/

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The law of definite proportions says that all samples of a single compound contain the same proportion of elements by mass.

The law of definite proportions states that any samples of a single compound contain the same proportion of elements by mass. Other names for the law are Proust's law or the law of constant composition. Whichever name you use, this law together with the law of multiple proportions form the basis for stoichiometry in chemistry.

Examples of the Law of Definite Proportions

For example, the mass ratio between hydrogen and oxygen is the same in any sample of pure water (H ₂O). The molar mass of water is the mass of the hydrogen atoms added to the mass of the oxygen atoms. These numbers are on the periodic table.

mass of water = (2 x mass of hydrogen) + mass of oxygen = (2 x 1.01) + 16.00 = 18.02

Hydrogen accounts for 11.19% o the mass of the compound:

% hydrogen = 2.02/18.02 = 0.112 = 11.2%
% oxygen = 16.00/18.02 = 0.888 = 88.8%

Another way of looking at it is that water is always 1/9 hydrogen and 8/9 oxygen by mass.

The law of definition proportions applies to ionic compounds, too. For example, the formula for table salt or sodium chloride is NaCl. The atomic mass of sodium is ~23 and the mass of chlorine is ~35. Based on the mass ratio between the elements, you expect dissociating 58 grams of NaCl yields 23 grams of sodium and 35 grams of chlorine.

If two compounds consisting of the same elements have different mass ratios from one another, you know they are two different compounds. So, if you have samples with two formulas using the same elements, such as CO and CO ₂, you know they are completely different compounds.

Exceptions to the Law of Definite Proportions

The law of definite proportions has exceptions. Non-stoichiometric compounds vary in mass composition from one sample to the next. For example, the formula for the iron oxide mineral called wustite is Fe 0.95 O and not FeO because the crystal structure allows for slight deviations from the stoichiometric ratio.

Isotopes also influence the mass composition of a compound. The ratio of isotopes of a given element varies according to its source. This leads to samples from different locations having different element mass ratios. Usually, the mass difference is small, unless a compound contains hydrogen.

Polymers also vary in element composition by mass, depending on how many monomers they contain. However, their chemical formulas usually are stoichiometric ratios that adhere to the law of definite proportions.

History

Credit for discovering the law goes to French chemist Joseph Proust, who described it in 1797. Joseph Priestley and Antoine Lavoisier also observed the way elements combine in constant proportions.

The law of definition proportions is important in everyday chemistry calculations, but it also has historical significance. It was among the laws supporting John Dalton 's 1803 atomic theory.

References

Gamow, George (1987). One Two Three... Infinity: Facts and Speculations of Science (Bantam Science and Mathematics ed.). Bantam. ISBN 978-0486256641.

Geng, Hua Y.; et al. (2012). "Anomalies in nonstoichiometric uranium dioxide induced by a pseudo phase transition of point defects". Phys. Rev. B. 85 (14): 144111. doi:10.1103/PhysRevB.85.144111

Greenwood, N. N.; Earnshaw, A. (2012). Chemistry of the Elements (2nd ed.). Elsevier. ISBN 0080501095.

Proust, J.-L. (1806). "Sur les mines de cobalt, nickel et autres". Journal de Physique. 63:566-8.

Zumdahl, S. S. (1986) Chemistry. Lexington, MA. ISBN 0-669-04529-2.

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Law of Definite Proportions - Law of Constant Composition

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The law of definite proportions states that any samples of a single compound contain the same proportion of elements by mass. Other names for the law are Proust's law or the law of constant composition. Whichever name you use, this law together with the law of multiple proportions form the basis for stoichiometry in chemistry.

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https://education.nationalgeographic.org/resource/conservation-matter-during-physical-and-chemical-changes/

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Image

Photograph by OJO Images Ltd.

Matter makes up everything visible in the known universe, from porta-potties to supernovas. And because matter is never created or destroyed, it cycles through our world. Atoms that were in a dinosaur millions of years ago-and in a star billions of years before that-may be inside you today.

Matter is anything that has mass and takes up space. It includes molecules, atoms, fundamental particles, and any substance that these particles make up. Matter can change form through physical and chemical changes, but through any of these changes matter is conserved. The same amount of matter exists before and after the change-none is created or destroyed. This concept is called the Law of Conservation of Mass.

In a physical change, a substance's physical properties may change, but its chemical makeup does not. Water, for example, is made up of two hydrogen atoms and one oxygen atom. Water is the only known substance on Earth that exists naturally in three states: solid, liquid, and gas. To change between these states, water must undergo physical changes. When water freezes, it becomes hard and less dense, but it is still chemically the same. There are the same number of water molecules present before and after the change, and water's chemical properties remain constant.

To form water, however, hydrogen and oxygen atoms must undergo chemical changes. For a chemical change to occur, atoms must either break bonds and/or form bonds. The addition or subtraction of atomic bonds changes the chemical properties of the substances involved. Both hydrogen and oxygen are diatomic-they exist naturally as bonded pairs (H ₂ and O ₂, respectively). In the right conditions, and with enough energy, these diatomic bonds will break and the atoms will join to form H ₂ O (water). Chemists write out this chemical reaction as:

2H ₂ + O ₂ → 2H ₂ O

This equation says that it takes two molecules of hydrogen and one molecule of oxygen to form two molecules of water. Notice that there are the same number of hydrogen atoms and oxygen atoms on either side of the equation. In chemical changes, just as in physical changes, matter is conserved. The difference in this case is that the substances before and after the change have different physical and chemical properties. Hydrogen and oxygen are gases at standard temperature and pressure, whereas water is a colorless, odorless liquid.

Ecosystems have many chemical and physical changes happening all at once, and matter is conserved in each and every one-no exceptions. Consider a stream flowing through a canyon-how many chemical and physical changes are happening at any given moment?

First, let's consider the water. For many canyon streams, the water comes from higher elevations and originates as snow. Of course that's not where the water began-it's been cycled all over the world since Earth first had water. But in the context of the canyon stream, it began in the mountains as snow. The snow must undergo a physical change-melting-to join the stream. As the liquid water flows through the canyon, it may evaporate (another physical change) into water vapor. Water gives a very clear example of how matter cycles through our world, frequently changing form but never disappearing.

Next, consider the plants and algae living in and along the stream. In a process called photosynthesis, these organisms convert light energy from the sun into chemical energy stored in sugars. However, the light energy doesn't produce the atoms that make up those sugars-that would break the Law of Conservation of Mass-it simply provides energy for a chemical change to occur. The atoms come from carbon dioxide in the air and water in the soil. Light energy allows these bonds to break and reform to produce sugar and oxygen, as shown in the chemical equation for photosynthesis:

6CO ₂ + 6H ₂O + light → C ₆H ₁₂O ₆ (sugar)+ 6O ₂

This equation says that six carbon dioxide molecules combine with six water molecules to form one sugar molecule and six molecules of oxygen. If you added up all the carbon, hydrogen, and oxygen atoms on either side of the equation, the sums would be equal; matter is conserved in this chemical change.

When animals in and around the stream eat these plants, their bodies use the stored chemical energy to power their cells and move around. They use the nutrients in their food to grow and repair their bodies-the atoms for new cells must come from somewhere. Any food that enters an animal's body must either leave its body or become part of it; no atoms are destroyed or created.

Matter is also conserved during physical and chemical changes in the rock cycle. As a stream carves deeper into a canyon, the rocks of the canyon floor don't disappear. They're eroded by the stream and carried off in small bits called sediments. These sediments may settle at the bottom of a lake or pond at the end of the stream, building up in layers over time. The weight of each additional layer compacts the layers beneath it, eventually adding so much pressure that new sedimentary rock forms. This is a physical change for the rock, but with the right conditions the rock may chemically change too. In either case, the matter in the rock is conserved.

The bottom line is: Matter cycles through the universe in many different forms. In any physical or chemical change, matter doesn't appear or disappear. Atoms created in the stars (a very, very long time ago) make up every living and nonliving thing on Earth-even you. It's impossible to know how far and through what forms your atoms traveled to make you. And it's impossible to know where they will end up next.

This isn't the whole story of matter, however, it's the story of visible matter. Scientists have learned that about 25 percent of the universe's mass consists of dark matter-matter that cannot be seen but can be detected through its gravitational effects. The exact nature of dark matter has yet to be determined. Another 70 percent of the universe is an even more mysterious component called dark energy, which acts counter to gravity. So "normal" matter makes up, at most, five percent of the universe.

National Geographic Society

October 19, 2023

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The Conservation of Matter During Physical and Chemical Changes

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Matter makes up all visible objects in the universe, and it can be neither created nor destroyed.

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https://www.purposegames.com/game/parts-of-an-atom-game

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https://www.nrc.gov/reading-rm/basic-ref/students/science-101/what-is-an-atom.html

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Printable Version
The Nuclear Regulatory Commission's Science 101: What is an Atom?

The atom is considered the basic building block of matter. Anything that has a mass-in other words, anything that occupies space-is composed of atoms. While its name originally referred to a particle that couldn't be divided any more-the smallest thing possible-we now know that each atom is generally made up of smaller particles. Given that these particles make up atoms, they are often referred to as subatomic particles. There are three subatomic particles: protons, neutrons and electrons.

Two of the subatomic particles have electrical charges: protons have a positive charge while electrons have a negative charge. Neutrons, on the other hand, don't have a charge. A fundamental rule is that particles with the same charge are repulsed from each other, while particles with opposite charges are attracted to each other. So, much like opposite ends of a magnet, protons and electrons are attracted to each other. Likewise, just as when you experience resistance trying to push the same ends of two magnets together, protons are repelled from other protons and electrons are repelled from other electrons.

The nucleus (or center) of an atom is made up of protons and neutrons. The number of protons in the nucleus, known as the "atomic number," primarily determines where that atom fits on the Periodic Table. The number of protons in the nucleus also defines in large part the characteristics of an atom-is it a gas or a metal, for example.

Two atoms with an identical number of protons in their nuclei belong to the same element. An element, like hydrogen, oxygen or iron, is a substance that cannot be broken down-outside of a nuclear reaction-into anything else. In other words, one element cannot be transformed into another (again, with the exception of nuclear reactions).

Now, while the protons are the same in an element, the number of neutrons may vary from atom to atom. The number of neutrons determines what isotope an atom is. This is important to the NRC because the number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay. While radioactive decay can occur in a variety of ways, it is, simply put, the process by which unstable atoms break down, releasing particles (and energy).

Generally speaking, atoms with roughly matching numbers of protons and neutrons are more stable against decay.
The nucleus of an atom is surrounded by a cloud of electrons. Remember, electrons are negatively-charged and are attracted to the positively-charged protons in the nucleus. An atom is considered to be electrically neutral if it has an equal number of protons and electrons. If an atom has a different number of electrons and protons, it is called an ion.

An important principle to know is electrons may be transferred from one atom to another or even shared between atoms (allowing atoms to bind together). These bonds allow for the formation of molecules, combinations of atoms (including those of different elements). Just as several atoms make up a molecule, many molecules make up a chemical.
The U.S. Nuclear Regulatory Commission is an independent federal government agency responsible for regulating the commercial use of nuclear materials. This document is free of copyright and can be reproduced for educational purposes.
Page Last Reviewed/Updated Thursday, March 19, 2020

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Printable Version The atom is considered the basic building block of matter. Anything that has a mass-in other words, anything that occupies space-is composed of atoms. While its name originally referred to a particle that couldn't be divided any more-the smallest thing possible-we now know that each atom is generally made up of smaller particles.

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Rutherford came to the following conclusions about the structure of the atom:

1.) The positive charge must be localized over a very tiny volume of the atom, which also contains most of the atom's mass. This would explain how a select few of the alpha particles were deflected drastically. This is presumably due to the rare collision with a gold nucleus.

2.) Since most of the alpha particles passed straight through the gold foil, the atom must mainly consist of empty space.

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Ernest Rutherford's gold foil experiment

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Ernest Rutherford's gold foil experiment

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In his famous gold foil experiment, Rutherford fired a thin beam of alpha particles at a very thin sheet of pure gold. Alpha particles are helium nuclei that are released in various radioactive decay processes.

In this case, Rutherford placed a sample of radium (a radioactive metal) inside a lead box with a small pinhole in it. Most of the radiation was absorbed by the lead, but a thin beam of alpha particles escaped out of the pinhole in the direction of the gold foil. The gold foil was surrounded by a detector screen that would flash when hit with an alpha

particle.

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Thomson knew that atoms had an overall neutral charge. This led him to the hypothesis that there has to be a positive charge within the atom to counterbalance the negative charge on the electrons. This led Thomson to propose that atoms could be described as negative particles floating within a soup of diffuse positive charge.

Fun Fact:
This model is called the plum pudding model because it's appearance is very similar to plum pudding!

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2023-03-22 10_27_43-Plum Pudding Model of Atom (JJ Thomson's Model) - Postulates, Limitati

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Matter
Atoms
Protons
Neutrons
Electrons
Nucleus
Molecule
Element
Compound
J.J. Thomson
law of conservation of mass
law of constant composition
plum pudding model
Ernest Rutherford
gold foil experiment
nuclear model

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Matter is a substance made up of various types of particles that occupies physical space.

According to the principles of modern physics, the various types of particles each have a specific mass and size.

Examples of material particles are the electron, the proton and the neutron.

The combination of these particles form atoms.

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Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.

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The law of conservation of mass states that matter cannot be created nor destroyed in a closed system. This means if a chemical reaction occurs, the amount of each element must be the same in both the starting materials and the products.

Fun Fact:
We use the law of conservation of mass every time we balance equations!

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The law of constant composition says that a pure compound will always have the same proportion of the same elements.

An example of this is table salt (NaCl). NaCl contains the same proportions of the elements sodium (Na) and chlorine (Cl) no matter how much salt you have or where the salt came from.

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Dalton's theory states:

All matter is composed of atoms

Atoms cannot be subdivided, created, or destroyed (Law of Conservation of Mass).

Atoms of any given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties (Law of constant composition).

Atoms of different elements can combine to form what is known as compounds.

In a chemical reaction, atoms are rearranged, combined, or separated.

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J.J. Thomson's discovery of the electron

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J.J. Thomson's discovery of the electron

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In the late 19th century, physicist J.J. Thomson began experimenting with cathode ray tubes. Cathode ray tubes are sealed glass tubes That has essentially been vacuum sealed. A high voltage of electricity is applied to two electrodes at one end of the tube, which causes a beam of particles to flow from the cathode (the negatively-charged electrode) to the anode (the positively-charged electrode).

To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged plate and towards the positively-charged plate. This showed that the cathode ray was composed of negatively-charged particles.

Thomson also placed two magnets on either side of the tube. He observed that this magnetic field also deflected the cathode ray. Thomson repeated this experiment using many different metals as the electrode materials. He concluded that the properties of the cathode ray were constant no matter what cathode material was used.

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The Atom: Protons, Neutrons, and Electrons

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The Atom: Protons, Neutrons, and Electrons

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All matter is made of atoms. Atoms are made up of protons, neutrons and electrons. Both protons and neutrons are located within the nucleus located at the center of an atom. Protons are positively charged particles, while neutrons are neutrally charged. Electrons are negatively charged, and they exist in orbitals surrounding the nucleus.

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When more than one atom combines, a molecule is formed. Large groups of atoms and molecules are what form the matter we see, such as your book, computer, pencil, even you! There are more than 100 different kinds of atoms listed in the periodic table, with each one representing a unique chemical element. Atoms and/or molecules from two or more elements can join together to form what is called a compound.

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Atoms are the smallest particles of every element!