Democritus was a Greek philosopher born around 460 BCE. He's not exactly a spring chicken. He was known as the "laughing philosopher," as we can clearly see from the picture below. (That face is definitely doing something.) We have this guy to thank for coming up with the atomic theory.
Democritus was actually a philosopher, although it was hard to separate science and philosophy back in the day. He thought that if we kept breaking down an object over and over again we would eventually get to something tiny that is physically indivisible. He called little things "atoms."
He also guessed that these atoms were indestructible, the space in between them was empty, they were constantly moving around, and that there was an infinite number of atoms with different shapes and sizes. For being made over 2000 years ago, these guesses aren't all that bad. Nice job, Democritus.
Dmitri Mendeleev was a Russian chemist born in 1834. He's famous for having an amazing beard. Also, he's attributed as being the creator of the first periodic table. He liked to play card games (Go Fish?), and one day he tried writing out a card for each of the known elements at the time. On each card, he wrote the name of the element, and different properties about the element that he knew, like its atomic mass. As he arranged the cards, he noticed certain patterns that led him to create the periodic table. The table he created led him to predict the properties of elements that had not even been discovered yet. Take that, Nostradamus. Mendeleev's beard out-predicts your beard anytime.
John Dalton was an English scientist born in 1766 and is responsible for the development of modern atomic theory. Modern atomic theory consists of the following five main points:
1. Elements are made of extremely small particles called atoms.
2. For any specific element, atoms have the same size, mass. In general, other properties are similar, too. Atoms from different elements differ in size, mass, as well as other element-specific properties.
3. Atoms can't be subdivided, created, or destroyed. They are here to stay, and aren't changing for anyone.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, it's possible for atoms to be combined, separated, or rearranged.
An side note about Dalton—he was color blind. He was one of the first researchers to describe its effect on his work. This is a major challenge for chemists, but Dalton proved that it could be done. Color blindness has been referred to as Daltonism.
J. J. Thomson was a British physicist born in 1856 and is best known for discovering electrons. He won the Noble Prize in 1906 his mad electron-finding skills. Using a cathode ray tube, he produced electricity in the form of a ray. He used magnetic and electric fields to deflect the ray reliably in vacuum tubes. After comparing the deflection of the beam due to the electric and magnetic fields, he was able to get measurements of the mass-to-charge ratio of the electrons.
Ernest Rutherford was a British Chemist and Physicist born in 1871 in New Zealand. He's credited with the discovery of the nucleus. He was a science double-threat, and he rocked the socks off of both fields. He's famous for his gold foil experiment where he discovered the nucleus, a place where atoms have their charge concentrated in a very small space. He is also known for being the first to split an atom in a nuclear reaction between nitrogen and alpha particles. During these experiments, he discovered and named the proton. We would have liked to call it the double-negative-ton, but proton works, too.
Niels Bohr was a Danish physicist born in 1885. In the 1920's, he described an atomic model where electrons orbit around the nucleus. Get this—he was working in Rutherford's lab. That's a lot of smarty-pants in a small space, if you ask us. He also worked with quantum mechanics and theorized that electrons move from one energy level to another in discrete steps. He won the Nobel Prize in Physics for his contributions.
Erwin Schrödinger was an Austrian physicist born in 1887. He had a sweet sense of style, and wasn't afraid to show it. He's known for his work in the field of quantum theory and wave mechanics. He theorized that electrons are actually waves instead of particles. He's also known for his Schrödinger's Cat Paradox.
James Chadwick was an English scientist born in 1891. He was awarded the Noble Prize for his discovery of the neutron. He worked on the Manhattan Project, which was a research initiative for the development of the first atomic bomb during World War II.
Werner Heisenberg was a German theoretical physicist born in 1901. He came up with the idea of quantum mechanics. He also confirmed the idea that electrons were particles that move in waves, as Schrödinger had suggested. Additionally, their location and movement around the nucleus is based on a wave probability.
There is also controversy surrounding his work on atomic research for the Nazis during World War II. It's possible that he purposely sabotaged the Nazis be miscalculating the amount of Uranium needed to build an atomic bomb.
Robert Millikan was an American physicist born in 1868. He was awarded the Nobel Prize for first measuring the charge of an electron using his famous oil drop experiment. For more information please watch this video.
For more information on the "Fathers of the Atom" please see any of the following videos: