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404 BCE
Democritus
Democritus was born in 460 BC. He was an ancient Greek philosopher who is known today as the “ father of modern science”. His theory that the universe is made up of tiny “atoms” brought him fame. Democritus elucidated the atomic theory which had many principles but most famously that, everything is composed of “atoms” which are physically, but not geometrically indivisible. -
John Dalton
John Dalton was a chemist, physicist, and meteorologist from Eaglesfield, United Kingdom. He graduated from Harris Manchester College in Oxford. Dalton, alongside John Gough and Peter Crosthwaite, created a book full of essays about meteorology. They found that the atmosphere was made up of 80% Nitrogen and 20% Oxygen. Dalton’s theory was based on the idea that only like atoms in a mixture of gases repel one another -
First evidence based on atomic theory
John Dalton formulated the first modern description and evidence of the atomic theory. He described it as the fundamental building block of chemical structures. -
Discovery of the Electron
The electron was discovered by J.J. Thomson in 1887 when he was studying the properties of the cathode ray. He constructed a glass tube which was partially evacuated. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of a particle was coming out from the negatively charged electrode to the positively charged electrode. -
Cathode Ray Tube Experiment
The cathode-ray tube experiment was first performed by J.J Thomson. He constructed a glass tube with wires inserted in both ends and pumped out as much air as he could. By doing this an electric charge passe through the tube from the wires and it made a fluorescent glow. He also found that applying a magnetic field across the tube, there was no activity recorded by an electrometer so the charge had been bent away by the magnet. -
Gold Foil Experiment
Ernest rutherford studied the absorption of radioactivity by thin sheets of metal foil and found two things, alpha, and beta radiation. Alpha is best absorbed by a few thousandths of a centimeter of metal foil while beta, can pass through 100 times as much foil before being absorbed. He shot minute particles at a thin sheet of gold. It was found a small number of particles were deflected, while the rest passed through. This caused him to conclude that the mass of an atom was at its center. -
Robert Millikan
Born on March 22, 1868, in Morrison Ill ( USA). He graduated with a Ph.D. Millikan measured the charge on an electron with his oil-drop apparatus. He also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. This means that we know that all the electrons in every atom have the same charge. -
Determination of the size of the charge of an electron
Robert Millikan was the first person to measure the electron's charge. He determined it through his oil-drop experiment in 1909. -
Oil Drop Experiment
This experiment was performed by Robert Millikan in 1909 and it determined the size of the charge on an electron. He put charge on a tiny drop of oil and measure how strong the electric field had to be in order to stop the oil from falling. Since then he was able to work out the mass of the oil drop, he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must-have. -
Ernest Rutherford
“ father of nuclear physics”. Born on August 30, 1871, in Nelson, New Zealand. He became the Langworthy Professor of Physics at the University of Manchester. He performed the gold foil experiment which demonstrated that the atom’s nucleus is tiny and heavy. In 1911, he discovered that atoms have a small charged nucleus that is surrounded by a large empty space and are circled my tiny electrons. This became known as the Rutherford model or planetary model of the atom. -
Discovery of the Nucleus
Ernest Rutherford used his gold foil experiment. Once the results of the gold foil experiment were analyzed, they showed the alpha particles being deflected. Rutherford noted that having an alpha particle deflected backwards was akin to firing a cannon at a sheet of tissue paper and having projectile bounce back. These findings led him to conclude that the atomic nucleus is an extraordinary tiny bundle of particles some 100,000 times smaller than the atom itself. -
J.J. Thomson
born on December 18, 1856, in Cheetham Hill, England. He attended Trinity College at Cambridge to study mathematics. Thomson later discovered that all matter is made up of tiny particles much smaller than atoms. In 1906 he began studying positive charge ions. This lead to one of his famous discoveries in 1912, when he discovered that neon is composed of two different types of atoms by channeling a stream of ionized neon through a magnetic and electric field. -
Niels Bohr
born October 7, 1885. He worked under J.J. Thompson at Cambridge. He later made discoveries of the electron and radioactivity at the end of the 19th century led to different models for the structure of the atom. In 1913, Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well-defined quantities. He said that electrons should move around the nucleus but only in prescribed orbits. -
Erwin Schrodinger
From Vienna, Austria, Erwin Schrodinger used equations to describe the likelihood of finding an electron in a certain position which resulted in the quantum mechanical model of the atom in 1926. Unlike the Bohr model, it does not define the exact path of an electron, but the likelihood of it being in a particular place. It is shown as a nucleus surrounded by an electron cloud. The denser the cloud is in one area, the more likely there are electrons and the less dense the fewer the electrons. -
James Chadwick
Born October 20, 1891. James Chadwich played a vital role in the atomic theory because of his discovery of neutrons. He gained his M.Sc. degree in 1913. He proved in 1932 that penetrating radiation emitted from alpha particles at beryllium consisted of a neutral particle with about the same mass as a proton. This particle is now known as a neutron. -
Discovery of the Neutron
James Chadwick discovered the neutron in 1932 and determined that it was an elementary particle, different from the proton.
