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Millikan discovered the charge of an electron.

He did an oil drop experiment to determined the charge of an electron. First, he sprayed oil droplets onto the top chamber. Then, he let them fall until they reached a constant velocity. He measured the constant velocity using a microscope and used a formula to figure out the mass of each drop. Next, he applied a charge to the falling drops by charging the bottom chamber with x-rays. The air particles lost electrons and a part of the oil droplets captured one or more of the extra electrons and became negatively charged. Finally, he attached batteries to the plates and created an electric field between the plates that would act on the charged oil drops. He adjusted the voltage until the force would balance the force of gravity on a drop and the drop would be sitting in mid-air.

Oil Drop Experiment -

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Erwin developed Schrödinger's wave equation. He established the Schrödinger's equation by applying it to the hydrogen atom and predicted many of its properties with great accuracy. Created a model which introduced the concept of sub-energy levels.

Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position.

Schrödinger viewed electrons as:
1. Continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.

This atomic model is known as the quantum mechanical model of the atom. The quantum mechanical model does not define the exact path of an electron. However, it predicts the odds of the location of the electron. The model can be portrayed as a nucleus surrounded by an electron cloud. The cloud is most dense and the probability of finding the electron is greatest, and conversely. The electron is less likely to be in a less dense area of the cloud.

The electron cloud model suggests that there is a cloud of electrons in the center of an atom.

Electron Cloud Model -

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Developed an explanation of  atomic structure that underlies regularities  of the periodic table of elements. His atomic model had atoms built up of sequential orbital shells of electrons.

Niels investigated atomic structure and radiation.  He calculated the possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit. He later developed the planetary model.

His experiments proved that:
1. An atom has electrons orbiting around it.
2. Electrons travel in stationary orbits defined by their angular momentum.

The Bohr model shows the nucleus with a positive charge and then orbiting shells with electrons on them. Also, there are energy shells, which contain a certain amount of electrons on each one.

Bohr Model -

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Thomson showed that cathode rays were particles with a negative charge and smaller than atoms. These particles were thought to be contained inside of an atom and were later named electrons. He also studied canal rays and found that they were associated with the proton H+ (Hydrogen Ion).

Thomson used a cathode tube to demonstrate that cathode rays were electrically charged and could be deflected by a magnetic field.

He believed that atoms:
1. Contained smaller particles, known a electrons.
2. Cathode rays could travel much further through air than expected for an atom-sized particle.

This model represents the atom with electrons inside of it.

"Plum Pudding" Model -

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In 1898, he studied radiations emitted from uranium and thorium and named them alpha and beta. In 1911, he discovered the nucleus model of an atom. He also established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

Using alpha models as atomic bullets, he shot then onto a gold foil. As a result, this experiment proved that the theory of J.J Thomson was incorrect because atoms have a central charge.

Rutherford established that atoms;
1. Have a central charge.
2. Have a tiny nucleus surrounded by tiny orbiting electrons.
3. Are not indivisible.

Rutherford's model suggests that atoms have a nucleus with protons and neutrons inside of it and orbiting electrons.

Nuclear Model -

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Along with his teacher, he was the first to propose an atomic universe.

In his experiment, he took a seashell and broke it in half. then, he took a half of that and kept breaking it smaller and smaller until it was a little piece of fine powder. He tried to break the smallest piece of the shell but he couldn't. This led him to believe that an atom is an indivisible building block to life.

Democritus believed that:

1.All matter is made up of invisible particles (atoms).

2. Atoms are indestructible.

3. Atoms are solid and invisible.

4. Atoms are homogenous.

5. Atoms differ in size, shape, mass, position, and arrangement.

6. Solids consist of small, pointy atoms.

7. Liquids consist of large, round atoms.

8. Oils are made of very fine, small atoms that are very slippery.

Democritus' model is a small, solid sphere.

Atomos -

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Proposed the first useful atomic theory of matter.

He performed many experiments on mixtures of gases to determine how each gas effected the mixture. While explaining the results of the experiments, he hypothesized that the size of the particles making up the gases must be different.

He believed that atoms:

1. Make up all matter
2. Cannot be created or destroyed
3. Elements can vary and can be distinguished by their weight
4. They can combine with and separate from other atoms

5. Atoms of different elements can form compounds in whole number ratios.

Dalton's model is also like Democritus' however, this atom can form compounds with ither atoms.

"Billiard Ball" Model -