Atomic Theory timeline.

Alchemists were ancient philosophers. They were the first to theorize seven elements that started the way we think about chemistry today. This allowed the foundation of modern chemistry of elements and atoms by invoking the way we see the things that make up our world differently, creating many famous scientists and philosophers throughout centuries.

Throughout Democritus' life, he theorized many things. One of his most famous works is the theory about atoms. Democritus was a philosopher, so he didn't scientific support to back up his claim. Nevertheless, Democritus and his pupil Leucippus theorized that "all matter (matter is a substance that has a mass or takes up space) is made of tiny units called atoms, greek for invisible." The duo also suggested that atoms were solid particles without a structure that didn't have a color or a taste.

As an ancient Indian philosopher, Kanada proposes that everything is made up of paramanu (atoms). Although Democritus theorizes the same thing, Kanada lived on the other side of the world and considering the time of the theory, this made Kanada unable to copy Democritus. Kanada didn't play a huge role in the overall atomic theory, but he still contributed to atomic theory by starting/ theorizing the research of atomic theory on the eastern side of the world, giving its foundation.

As one of the smartest and most recognized chemist in the 1800s, there is no surprise that Dalton from the United Kingdom played a huge role in atomic theory. By conducting experiments and using complicated maths, John Dalton concluded that each chemical element had a unique atom to themselves that can't be altered or destroyed by any means. Dalton's conclusion was the first scientific theory about atoms and it also sparked a chain reaction of curiosity to chemists around the world about atoms.

As Nobel prize winner, J.J Thomson was able to contribute to atomic theory. In 1897, Thomson conduced an experiment using cathode ray tubes and was able to find a negative charge in atoms. Not only did J.J Thomson discover the negative charge of the atom, electrons, but Thomson also proposed a model of an atom. In 1903, Thomson proposed a model of an atom containing positive and negative charges. Today, we don't use Thomson's model due to its flaws but, it helped build the model we have today.

As the predecessor of Werner Heisenberg, Max Planck, a German physicist, contributed to atomic theory by theorizing quantum theory, a theory that helps describe the behaviour of matter at the atomic level. Chemist before Planck couldn't describe atoms' behaviours because of the small size of the atom. In 1900, Planck's quantum theory, helped many chemists understand the behaviours of matter at an atomic level. The discovery of the quantum theory dramatically changed the atomic theory research.

Marie Curie was the first woman to win a Nobel prize, making it no surprise that she contributed to the atomic theory. In 1903, Curie was the first to discover radioactivity. The discovery of radioactivity changed modern technology and warfare, but also discovered radioactive atoms. Marie Curie's discoveries did come at a cost since her body was exposed to high levels of radioactivity during her research, it caused her to lose her life.

As a Japanese Physicist, Hantaro wasn't famous during the time of prime years of atomic theory, but he still played an important role. In 1904, Hantaro was the first to predict that atoms have a massive atomic center in which electrons revolve around. Although he didn't prove his prediction, Hantaro Nagaoka's hypothesis might have lead Rutherford to conduct his experiment, helping us find the nucleus, one of the most important features of an atom.

Known as the theorizer of special relativity, Einstein also contributed to atomic theory. In 1905, Using Planck's works, Einstein was able to expand and prove the quantum theory. Also, Albert Einstein used the photoelectric effect and Brownian motion to prove the atom mathematically exist. Even after many centuries of theorizes of atoms, no one actually proved the existence of atoms. This is until Einstein proved it, making any doubts of atoms gone.

Ernest Rutherford was able to contribute to atomic theory by using high energy streams that shoot through a thin gold foil. If the energy streams deflected it will show a positive charge, but if the streams went through the gold foil, it would've been a empty space. After the lab, Rutherford was able to conclude that atoms have a concentrated small volume of positive charges in the center of the atom, the nucleus. Along with the nucleus, he also concluded that atoms mostly have empty space.

As a Danish and parters with Rutherford, Bohr was able to play a major role in atomic theory. In 1913, Bohr proposed that electrons only move around the nucleus in prescribed orbits. Along with this information, Bohr was able to create a model of the atom with Rutherford. With Bohr proposing that electrons can only move in prescribed orbits he was able to create a model containing orbitals around the nucleus. Today, we believe that the Bohr-Rutherford model is the best model expressing the atom.

As a French physicist, Louis de Broglie contributed to atomic theory by researching more on quantum theory. In 1924, with his research, de Broglie had thesis that suggested that all matter has wavelengths and discovered how to predict the wave nature of the electron. Also, his research on waves of electrons also describes why electrons have a restriction on their motion.

With the popularity of Quantum mechanics of the atom in the 1900s, Pauli, an Austrian, was able to discover the Pauli exclusion principle. The principle was discovered in 1925 and it states that in an atom, no more than one electron can take on the same quantum state simultaneously. With this work, it helped many chemists explain, and understand the activity of quantum mechanics in an atom. Pauli's work on Quantum Mechanics was one of the first, so this allowed chemists to explore deeper.

In 1926, Erwin Schrödinger was able to contribute to atomic theory by using math equations and de Broglie equation of wavelengths. By using math equations, Schrödinger was able to improve on the Bohr model of an atom. Schrödinger created the quantum mechanical model of an atom that allows us to predict the location of an electron. It improves on the Bohr model because, although the Bohr model gives us the location of an electron in an atom, it is not as precise as the quantum mechanical model.

As a German physicist, in 1925 Heisenburg was able to formulate quantum mechanics in a series of numbers, which made quantum mechanics of atoms much easier to deal with. Along with this discovery, in 1927 Heisenberg theorizes the uncertainty principle. The principle states that the velocity and position of an electron can not be exactly possible to know because an electron is a small amount of mass making light hard to bounce off the electron. Thus making the electron unobservable.

James Chadwick, was able to contribute to the atomic theory by conducting an experiment. In 1932, many thought atoms were only made up of only negative and positive charges, but James Chadwick was able to prove them wrong and discovered the neutral charge of the atom, neutron. James Chadwick discovers this charge by giving beryllium atoms alpha particles, this caused unknown radiation. This made Chadwick believe this radiation as being composed of particles with a neutral charge, neutrons.

As a German-born Canadian, Gerhard Herzberg contributed to atomic theory by dealing with molecules. Around 1936, Gerhard Herzberg determined the electronic structure of molecules and also its geometry. Gerhard Herzberg's most famous works involve free radicals, a type of group of atoms. Gerhard Herzberg was the first to identify certain radicals in interstellar gas. Herzberg's helped advance the research on radicals and molecules.

In 1991, Richard Bader, a Candian chemist, contributed to atomic theory by discovering that electron density has a huge importance in explaining the behaviours of electrons of the atom in molecules. Along with this discovery of behaviours of electrons, Bader suggests that there are no atomic orbitals in molecules. Bader's work helps many modern chemist behaviours of electrons in the atom when it is in a molecule.