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Democritus was a Greek philosopher who theorized that all matter is made up of tiny particles. He came up with this theory based on the concept of what happens when you cut a piece of material into smaller pieces. For example, when you cut a piece of cheese in half, you would be able to halve those pieces, and then halve them again, and so on. Democritus theorized that eventually you would reach at point at which the pieces would be too small to divide any further. He called these tiny indivisible pieces "atoms," based on the Greek word atomos, which means "uncuttable."

Democritus also theorized about the properties of these atoms. He believes that they made up all matter, that they were too small to see, that they were solid and uncuttable, and that they were seperated by empty space or "void."

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John Dalton was a British scientist who came up with his own atomic theory to describe the composition of matter and the properties of atoms. His theory states the following:

1. All elements are made up of atoms. Atoms are the smallest existing particles and that they could not be divided, destroyed, or created. He thought that they were hard, solid, and spherical, properties that are demonstrated by Dalton's "Billiard Ball" model of the atom.

2. Atoms of a certain element will have identical properties, such as size, mass, and boiling point. Atoms of one element will be different from elements of other elements.

3. Compounds are combinations of two or more types of atoms, and these atoms will always combine in fixed ratios. Chemical compounds will always contain the same proportions of elements.

4. Atoms are not created or destroyed in a chemical reaction, just rearranged. This is referred to as the law of conservation.

Dalton was able to support the existence of atoms through his experiments. He investigated the pressure of gases and was able to infer that gases are made up of tiny particles that exist in constant motion. He also researched the properties of compounds and was able to determine that elements in given compounds are present in consistent ratios. He was also able to support the idea that different elements have different properties through his investigation of atomic mass.

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J.J. Thomson made a great contribution to atomic theory through his discovery of the electron. Thomson was able to support the existence of electrons by experimenting with cathode rays. When Thomson placed positively and negatively charged plated on either side of a cathode ray tube, it appeared to be repelled by the negative and attracted to the positive, showing that the ray must be negative in charge and be made up of particles with mass. These particles later came to be known as electrons. Thomson was also able to measure the mass of electrons by determining the change in the bend of the cathode rays at various voltages. He found that their mass was extremely small, only a fraction of the weight of any atom, which disproved the theory that atoms are the smallest unit of matter.

Thomson created a new atomic model to support his theory, known as the "plum pudding" model. He knew that atoms had a neutral charge, so he knew that there must be an element of the atom that is positively charged which cancels out the electrons' negative charge. In his model, electrons float within a soup of positively charged matter.

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Rutherford's contribution to atomic theory was his discovery of the atom's nucleus. In an experiment, Rutherford shot alpha particles at a very thin sheet of gold. Rutherford expected the alpha particles to simply pass through the foil, because based on Thomson's atomic model, the particles would be able to pass through the weak positive charge evenly distributed throughout the atom. However, the results of the experiment were suprising, as a very small fraction of the alpha particles were deflected drastically. From his results, Rutherford concluded that the positive charge of the atom must be contained in a very small area within it, that that area must also contain most of the atom's mass, and that therefore the rest of the atom must be primarily empty space. Rutherford's model is referred to as the "nuclear model," which depicts the atom as having a small nucleus with a positive charge surrounded by the negative electrons.

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Henry Moseley's work on atomic theory showed the relationship between the characteristics of elements and their atomic numbers. Moseley discovered that when x-rays were diffracted off of different metals, the frequency of the diffracted rays were related to the elements' atomic number. He later concluded that the atomic numbers reflected the number of protons in each atom. Moseley's experiments provided the basis for the modern periodic table. The accepted table prior to his discovery was organized based on atomic weight, which was not an effective way to represent the chemical properties of each atom. Moseley's discovery of the relationship between protons and atomic number in atoms allowed the periodic table to be arranged in a more logical manner.

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Niels Bohr proposed that the electrons in atoms exist in orbits located at fixed distances around the nucleus. Based on known rules of physics, if the electrons floated around freely as depicted in Rutherford's nuclear model, they would spiral into the nucleus and cause the atom to collapse. In the "Bohr model", each orbit is associated with a specific energy level, and atoms can move between orbits with the addition of energy. When that energy is removed, the electrons move back to their original orbits and emit light. Bohr's work helped explain the behaviour of electrons and the spectrometer of light emitted by atoms.

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Rutherford made another important contribution to atomic theory when he discovered managed to split the atom. He discovered that sending alpha rays at nitrogen gas causes ogygen atoms and hydrogen nuclei to be emitted. Rutherford later showed that the "hydrogen nucleus" existed as a building block in other atoms as well, and he later named this particle the proton.

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Shrodinger and Heisenburg developed a different way of representing the atom based on quantum mechanics. Although it was groundbreaking, Bohr's atomic model failed to accurately represent the atom and the properties of electrons. Between the time of Bohr's model and Schrodinger and Heisenburg's discoveries, it was found that electrons behaved in ways characteristic of both particles and waves, which meant that they could not exist on fixed orbits as shown in the Bohr model.

Shrodinger developed an equation known as the Schrodinger equation in order to define the three-dimensional space in which an electron is most likely to be found, known as an atomic orbitals. Shortly after, Heisenburg realized that there were limitations on how accurately the position of electrons could be described. He created the Heisenburg Uncertainty Principle, which states that there is an inverse relationship between knowing the momentum and the position of a particle. Shrodinger and Heisenburg's research showed that the positions of electrons can be predicted statistically, but we can never be sure of their precise locations. This is represented in the "quantum mechanical model" of the atom, which depicts the electrons in orbitals around the nucleus, sometimes called electron clouds.

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Chadwick's major contribution to atomic theory was the discovery of the neutron. Prior to his discovery, it was assumed that the atom was made up of only protons and electrons, and they cancelled each other out in order to create a neutral charge, and that these particles made up the entire mass of the atom. Chadwick discovered that when he bombarded beryllium with alpha particles, it released radiation. He found that the radiation was made up particles with mass similar to that of protons but with a neutral electrical charge: neutrons. The discovery of neutrons provided explanation for the mass and spin of atoms, and allowed for atomic models to more accurately represent the atom by depicting both neutrons and protons in the nucleus.