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Democritus is best known for his Atomic Hypothesis.

He theorized that everything was made up of indivisibly small “atoms".

Atoms that varied in size and shape depending on the substance they compose.

More info:

https://www.britannica.com/biography/Democritus

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Antoine Lavoisier and an example explaining the Law of Conservation of Mass.

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Antoine Lavoisier's work in defining the Law of Conservation of Mass helped shape the atomic theory.

It confirmed what matter was made of.

It proved that atoms are not created or destroyed when a reaction happens.

More info:

https://www.britannica.com/biography/Antoine-Lavoisier

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Dalton claimed that:

Atoms cannot be subdivided, created, or destroyed.

Atoms of a given element are identical in size, mass, and other properties.

Atoms of different elements differ in size, mass, and other properties.

More info:

https://www.britannica.com/biography/John-Dalton

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JJ Thompson and his model of the atom, also known as the "Plum Pudding Model".

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In 1887, he discovered the electron. He claimed that atoms are packed spheres of positive matter filled with negatively charged electrons.

He used a cathode ray tube to conduct experiments that led to this discovery.

He proved that cathode rays have a negative charge.

He also investigated positively charged neon gas particles.

More info:

https://www.britannica.com/biography/J-J-Thomson

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Marie Curie and her theory of radioactive decay.

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Her theory of radioactive decay proved the existence of subatomic particles.

She concluded that radioactivity does not depend on how atoms are arranged into molecules, but rather that it originates within the atoms themselves.

She proved that unstable atoms could undergo changes and could give rise to completely different atoms as a result.

More info:

https://www.atomicheritage.org/profile/marie-curie

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In 1900, according to Planck's quantum theory, different atoms and molecules can emit or absorb energy in discrete quantities only.

He studied blackbody radiation

He discovered that it always emitted or absorbed energy proportional to the frequency of the electromagnetic waves it absorbed.

More info:

http://byjus.com/chemistry/planks-quantum-theory/

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One of Albert Einstein's mathematical equations.

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In 1903, he did mathematical equations to mathematically prove and finally confirm the existence of the atom.

With Einstein's calculations, one could determine the size of these invisible atoms and molecules.

The idea that heat is the result of the motion of atoms and molecules was also confirmed with his calculations.

More info:

https://www.ans.org/news/article-969/albert-einstein-and-the-most-elemental-atomic-theory/

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Robert Milikan discovered that electrons have a charge and the way to calculate their mass.

He experimented on small electrically charged oil drops hung between two charged metal plates, exposed to both an electrical field's upward attraction and gravity's downward force.

The drops lined up in a specific way between the plates.

Millikan demonstrated that the charge of each oil drop was always a multiple of an exact known charge of the electron by observing how the different oil drops moved.

More info:

www.britannica.com/biography/Robert-Millikan

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The gold foil experiment is Ernest Rutherford's most well-known experiment.

A piece of gold foil was the target of an alpha particle beam.

While some alpha particles were scattered backward, the majority of them went through the foil.

This demonstrated that atoms are mostly empty spaces with
dense, positively charged particles, called the nucleus, at the center.

More info:

www.nobelprize.org/prizes/chemistry/1908/rutherford/biographical/

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In 1913, Niels Bohr discovered that electrons orbit the nucleus at fixed distances and that they can jump from one level to another but not exist in the space between.

He introduced the "planetary model".

He stated that electrons behaved like waves.

More info:

https://www.britannica.com/biography/Niels-Bohr

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Louis de Broglie and his model of the atom.

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In 1924, Louis de Broglie introduced the idea that particles, such as electrons, have wave-like properties.

The way streams of electrons were reflected against crystals and spread through thin metal foils proved this.

More info:

www.britannica.com/biography/Louis-de-Broglie

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In 1927, Werner Heisenberg explored using a gamma ray microscope to track the position of an electron.

The electron would receive a shock from the high-energy photon, altering its momentum in an unpredictable way.

The momentum would become more uncertain when trying to determine the location.

He stated that this uncertainty is not a limitation of any specific experimental instrument; rather, it is a fundamental aspect of quantum mechanics.

More info:

https://www.britannica.com/biography/Werner-Heisenberg

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In 1932, Chadwick proved the existence of neutrons.

To get this conclusion, he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium.

This showed high penetration through a lead shield, which could not be explained via the particles known at that time.

More info:

www.nobelprize.org/prizes/physics/1935/chadwick/facts/

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Erwin Schrodinger and his thought experiment.

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In 1935,  Erwin Schrodinger had a thought experiment.

Schrödinger's cat
- A cat is imprisoned in a steel box with a radioactive substance.
-  A device crushes a vial of deadly gas, killing the cat if the atom decays.
- The atom's wave function is in a superposition of decay and non-decay up until the box opens and the atom's wave function collapses.

As a result, the cat is simultaneously living and dead.

More info:

www.britannica.com/biography/Erwin-Schrodinger