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Democritus was a Greek philosopher, and around the year 450 BC, he became the grandfather of the atom when he theorized what would happen if you were to take an object, say, an apple, and cut it down until it becomes uncuttable. He called it an "atomos", which is where the term atom comes from.

Unfortunately for Democritus, possibly the most influential name in philosophy at the time, Aristotle, thought his idea was absolutely ridiculous. Aristotle's doubt was accepted for the next 2000 years.

Despite this he laid the groundwork for the discoveries of the coming centuries, indeed making him the true grandfather of atoms in every sense

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Prior to Joseph Priestly's time, it had been defined that the elements were air, earth, fire and water. While it seems silly now, back then it made perfect sense due to the lack of knowledge of any idea that air itself is a composition

It was Joseph Priestly, an English Chemist, who made the discovery of  oxygen by studying the gasses of our world in great depth. As a result he disproved the elemental ideas that had ruled the world, and set in a new age of enlightened views.

This didn't come without backlash, as in England, his support of the French Revolution along with his controversial theology almost cost him his life via an angry mob. He only barely escaped to Pennsylvania so he could continue his work.

By showing air was just a composition of oxygen and other things, the whole scientific community would take a step back and reevaluate their understanding.

As a result, what Priestly did kickstarted the in depth look into molecules and eventually atoms.

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John Dalton is important as he revived the idea of the atom after Democritus. The meterologist and chemist proposed that all matter was made up of atoms. Dalton was able to discover this  because of the way he studied pressure.  He concluded that gases must consist of tiny particles in constant motion.

From this, he proposed his Atomic Theory that all substances are made of atoms. Atoms also cannot be further divided into smaller particles for they are the smallest particles of matter. Finally, atoms of the same element are alike and have the same mass.

Besides his point about atoms being the smallest particle, the rest of his ideas are still widely accepted today.

Not bad for an idea that's over 200 years old.

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Sir Joseph Jon Thomson (J. J. for short)'s main contributions to the atom are in the discovery that all atoms have a negatively charged subatomic particle, or electron. With the discovery of the electron, J. J. created the earliest atomic model, the Thomson atomic model. Thomson held that atoms are uniform spheres of positively charged matter in which electrons are embedded.

This model ended up being incorrect, as it was replaced later Ernest Rutherford's model, which you will find out about very shortly.

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Although there was currently an atomic model,  the existence hasn't exactly proven, and it was mostly still just theoretics. It was merely an idea to show how matter worked

But it was Einstein who effectively proved that atoms were actually something that existed in our real world, not just theoretics. He did this in 1905, where he mathematically proved the existence of the atom.

As a result, not only did he prove the atom was real, a true milestone in the field of science, but he created a bridge between math and science, bringing the ideas of statistics and probability into play, changing the field permanently.
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After numerous experiments, Ernest Rutherford ended up finding out that an atom could not be a uniform solid, but rather filled up with mostly empty space, with its mass mostly coming from a tiny core in the middle of the atom.

He called this the nucleus, and it became the backbone of the Rutherford atomic model. His model also tells us that the lighter, negative constituents, known as electrons, would revolve around the nucleus at a distance.

While this discovery was completely revolutionary, it received very little attention and fanfare until 1913, when it's importance was shown by Danish physicist Niels Bohr.

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Erwin Schrödinger crown jewel achievement was the wave formula he created where one could now accurately find the energy level of the electrons in atoms.

Schrodinger's equation made many other physicists very unhappy as it replaced the definite sequence of events that came from Newton's equations with lots of probable occurrence based but not certain answers with quantum mechanics. A system based on the wave equation now became known as the Schrodinger's Equation. This became the basis for the famous Schrodinger's cat thought experiment, and also created the idea of superposition.

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