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Democritus and his Ancient Theory - 460-370 BC

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Democritus and his Ancient Theory - 460-370 BC

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Democritus was a Greek philosopher, living in 460-370 BC. He is the father of the first atomic theory, that there is one very small particle that cannot be subdivided, called "Atomos." In Greek, atomos literally translated to "indivisible." This theory set the stage for all of what would eventually become chemistry.

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John Dalton was a chemist who lived from 1776 - 1844, and found the Atomic Theory in 1803. His theory had four parts:
1) All matter is composed of miniscule, indivisible particles called atoms
2) All atoms of a given element are identical; atoms of different elements have different properties
3) When elements react to form compounds, they react in defined, whole-number ratios
4) Chemical reactions involve the combination of atoms, not their destruction.
This Theory started the study of atoms and changed chemistry forever.

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Dmitri Mendeleev is credited as the Father of the Periodic Table. He noticed elements had patterns in them, along with patterns in their properties. He made a card for each element and its properties, rearranging them according to their atomic weight. He used scientific theory to find out some of his atomic weights were incorrect, and rearranged them as such. This turned into the Periodic Law (aka Mendeleev's law), whereas elements were arranged according to their atomic weight. He was thinking so far ahead as to leave space for new elements that had yet to be discovered. He was able to arrange the table so the elements with similar properties were side by side, and even predicted the properties of some unknown elements correctly. He published his work in 1869, and his model has been the standard model used to this day.

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Henri Becquerel and Marie and Pierre Curie - 1896

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Henri Becquerel and Marie and Pierre Curie - 1896

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Henri Becquerel was the first to discover radiation in 1896, and he shared this discovery with his students Marie and Pierre Curie. It started when he noticed certain uranium salts became exposed after being left on a table, creating penetrating rays. This spontaneous emission happened even in the absence of light, which they eventually called "radioactivity." He and his students conducted experiments on other elements that also proved to be radioactive. Henri did not live long however, as the radioactivity was giving him serious burns and died suddenly at 55. He didn't know the side effects of radioactivity, and didn't know just how far his discovery would develop, and how much it is used in this age.

An isotope has the same number of protons as an element, but a different number of protons. Because they have the same number of protons, they keep their identity, but different isotopes can behave in different ways. There are both stable and unstable isotopes, where the unstable ones can emit radiation.

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J.J. Thomson was a British physicist in the 1800s, and the first to discover the electron in 1897. He used several cathode-ray experiments to discover it, where he would empty a tube of all its gasses and pass a positive charge though it. He would place a positive electrode (a cathode) at one end, and a negative electrode at the other, and light would go from one end of the tube to the other. He couldn't figure out the specifics of the light particle, so he set up more experiments to figure it out. He settled on using a magnet near the tube and saw the light bend away from the magnet on the negative side, and bend towards it on the positive. This brought him to the conclusion that atoms had small particles with a negative charge inside (electrons). To justify why the electron particles didn't fly apart from all the negative charges inside, he came up with the plum pudding model; negative particles were arranged in the atom like fruit in a plum pudding. His discovery of the electron and the Plum Pudding Model helped people understand the atom more and do more research on it.

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Ernest Rutherford was a New Zealand physicist who studied radiation and discovered important radioactive emissions. He first discovered alpha and beta rays, and led to the understanding heavy atoms decay into lighter ones. When an isotope becomes unstable, it can emit three different types of radiation rays.

Alpha rays (decay): Occurs when a particle is ejected from an unstable nucleus with high energy, and is made up of two protons and two neutrons. The two protons make it a helium nucleus, and although ejected with high energy, it will lose energy as it travels and can be stopped with paper.
Beta rays (decay): Beta radiation particles are identical to electrons with a charge of negative one. It has a much higher penetrating capability, but can be stopped by aluminum foil or wood. The particle is only present after it is created in the decay process.
gamma radiation (ray): Gamma radiation is a high energy photon that have shorter wavelengths than x-rays and more energy. These rays are very harmful and can pass through the human body, and can only be stopped by a block of lead or a meter of concrete.

Rutherford setup a famous Gold Foil Experiment to further test alpha rays and their behavior. He directed alpha rays towards gold atoms and watched them pass through. He noticed the rays being deflected in certain areas, and created a diagram with a cluster of particles in the middle of the atom and other smaller particles spread throughout. The experiment taught him most of the area in an atom is empty space. It also taught him there was a cluster of particles in the middle to create a positively charged center, called the nucleus. Rutherford would also go on to discover the Proton in 1918.

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Henry Moseley was a British Physicist who demonstrated the properties of an element were determined by its atomic number, not weight. He did a series of experiments on the relationship between frequencies of corresponding lines in the x-ray spectrum. He found that the frequencies are proportional to the squares of the atomic number, plus a constant. This became known as Moseley's Law, a breakthrough in atom study, and published his work in papers.