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https://www.ck12.org/chemistry/daltons-atomic-theory/lesson/Daltons-Atomic-Theory-MS-PS/

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Reintroduction of the idea of the atom and the billiard ball model.

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https://kids.kiddle.co/Atomic_theory

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Cathode Ray Tubes in Your Home

You might have used a cathode ray tube even if you've never even heard of it until reading this lesson. Before LCD and Plasma TVs became commonplace, most people used bulkier cathode ray tube (CRT) televisions. The CRT in a television is used to display images on your screen. However, cathode ray tubes have been used for more than entertainment. It was cathode ray tubes that allowed the English physicist J.J. Thomson to discover the existence of electrons in 1897.

How Cathode Ray Tubes Work

Before we see how J.J. Thomson used the cathode ray tube to discover the electron, we need to know how a cathode ray tube works. We'll look at a basic CRT like what J.J. Thomson would've used, as seen in the diagram below.

The CRT consists of several elements starting with a tube that's vacuum sealed to keep air out of it. On one side of the inside of the tube there's a cathode and an anode. The cathode is a negatively-charged conductor, and the anode is a positively-charged conductor. Electrons, which have a negative charge, flow off the cathode and are attracted towards the anode. A small hole in the anode allows some electrons to pass through it, creating a beam of electrons. On the opposite side of the tube is a coating that glows when struck by the electrons. This allowed J. J. Thomson to see where the electron beam was hitting.

Of course, before his experiment, we didn't know electrons existed. So, no one was calling it an electron beam. Instead, what flowed off the cathode toward the anode were called 'cathode rays.' Hence the name cathode ray tube.

The Three Experiments

J.J. Thomson's discovery of electrons didn't happen all at once. Instead, it was a result that was slowly built towards over the course of three different experiments. Before Thomson's experiments, it had already been discovered that the cathode rays deposit an electric charge. This was done by putting an electrometer at the opposite end from the cathode and anode. When the cathode rays hit the electrometer, it measured an electric charge.

The first experiment was done in order to see if the charge was separate from the cathode rays. Thomson used a magnet to bend the cathode rays away from the electrometers. When he did this, he discovered that the electrometers stopped measuring electric charge. From this he deduced that the electric charge and the cathode rays must be combined together and not separate entities.

Next, J.J. Thomson set out to determine if the charge carried by the cathode rays was negative or positive. To do this he would use an electric field. He put a negatively charged metal plate on one side of the cathode rays let out past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the CRT he now had a fluorescent coating that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent toward the positive plate and away from the negative plate. This showed that the charge carried by the cathode rays was negative.

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j.j. thomson's cathode ray tube (crt): definition

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experiment & diagram - video & lesson transcript | study.com

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J.J. Thomson's Cathode Ray Tube (CRT): Definition, Experiment & Diagram - Video & Lesson Transcript | Study.com

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You might have used a cathode ray tube even if you've never even heard of it until reading this lesson. Before LCD and Plasma TVs became commonplace, most people used bulkier cathode ray tube (CRT) televisions. The CRT in a television is used to display images on your screen.

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https://kids.kiddle.co/John_Dalton

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The Rutherford Experiment

This classic diffraction experiment was conducted in 1911 by Hans Geiger and Ernest Marsden at the suggestion of Ernest Rutherford. Details about the experiment and how to operate the tutorial are provided beneath the applet window.

The tutorial simulates diffraction of alpha particles (helium nuclei containing two positive charges) by a thin foil made of gold metal. Opposite the gold foil is a zinc sulfide screen that emits a flash of light when struck by an alpha particle. To operate the tutorial, use the slider to increase the slit width from a range 0.1 to 9.0 nanometers. As the slit is opened to widths greater than 2 nanometers, some of the alpha particles will collide with the gold nuclei causing them to deflect and deviate from the straight path dictated by the alpha particle source.

Geiger and Marsden expected to find that most of the alpha particles travel straight through the foil with little deviation, with the remainder being deviated by a percent or two. This thinking was based on the theory that positive and negative charges were spread evenly within the atom and that only weak electric forces would be exerted on the alpha particles that were passing through the thin foil at high energy.

What they found, to great surprise, was that while most of the alpha particles passed straight through the foil, a small percentage of them were deflected at very large angles and some were even backscattered. Because alpha particles have about 8000 times the mass of an electron and impacted the foil at very high velocities, it was clear that very strong forces were necessary to deflect and backscatter these particles. Rutherford explained this phenomenon with a revitalized model of the atom in which most of the mass was concentrated into a compact nucleus (holding all of the positive charge), with electrons occupying the bulk of the atom's space and orbiting the nucleus at a distance. With the atom being composed largely of empty space, it was then very easy to construct a scenario where most of the alpha particles passed through the foil, and only the ones that encountered a direct collision with a gold nucleus were deflected or scattered backwards.

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Molecular Expressions: Electricity and Magnetism: Interactive Tutorial - The Rutherford Experiment

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The Rutherford Experiment This classic diffraction experiment was conducted in 1911 by Hans Geiger and Ernest Marsden at the suggestion of Ernest Rutherford. Details about the experiment and how to operate the tutorial are provided beneath the applet window. The tutorial simulates diffraction of alpha particles (helium nuclei containing two positive charges) by a thin foil made of gold metal.

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The Bohr Model has an atom consisting of a small, positively-charged nucleus orbited by negatively-charged electrons. Here's a closer look at the Bohr Model, which is sometimes called the Rutherford-Bohr Model.

Overview of the Bohr Model

Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call Bohr's Model the Rutherford-Bohr Model.

The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of atomic theory without all of the high-level math of the modern version. Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of atomic hydrogen.

The Bohr Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

Main Points of the Bohr Model

Electrons orbit the nucleus in orbits that have a set size and energy.

The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit.

Radiation is absorbed or emitted when an electron moves from one orbit to another.

Bohr Model of Hydrogen

The simplest example of the Bohr Model is for the hydrogen atom (Z = 1) or for a hydrogen-like ion (Z > 1), in which a negatively-charged electron orbits a small positively-charged nucleus. Electromagnetic energy will be absorbed or emitted if an electron moves from one orbit to another.

Only certain electron orbits are permitted. The radius of the possible orbits increases as n ², where n is the principal quantum number. The 3 → 2 transition produces the first line of the Balmer series. For hydrogen (Z = 1) this produces a photon having wavelength 656 nm (red light).

Problems with the Bohr Model

It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.

The Bohr Model provides an incorrect value for the ground state orbital angular momentum.

It makes poor predictions regarding the spectra of larger atoms.

It does not predict the relative intensities of spectral lines.

The Bohr Model does not explain fine structure and hyperfine structure in spectral lines.

It does not explain the Zeeman Effect.

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The Bohr Model has an atom consisting of a small, positively-charged nucleus orbited by negatively-charged electrons. Here's a closer look at the Bohr Model, which is sometimes called the Rutherford-Bohr Model. Niels Bohr proposed the Bohr Model of the Atom in 1915.

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Gold Foil simulation- hit the blue "alpha particles" button 

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Gold Foil simulation- hit the blue "alpha particles" button 

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https://phet.colorado.edu/sims/html/rutherford-scattering/latest/rutherford-scattering_en.html

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https://www.ck12.org/chemistry/gold-foil-experiment/lesson/Rutherfords-Atomic-Model-MS-PS/

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Gold Foil Experiment