-
450 BCE
Democritus
In 450 BC, Democritus discovered the atom. He did this by cutting an apple into smaller and smaller pieces until he reached a piece that could not be cut anymore. -
John Dalton
In 1800, John Dalton developed a theory of the atom. The theory of the atom stated that all substances are made of atoms and that atoms are the smallest particle of matter. It also says that atoms cannot be divided, created or destroyed. To find this out he studied the pressure of gases and the properties of compounds. He concluded that must consist of tiny particles in constant motion. He also showed that a compound consists of the same elements in the same ratio. -
John Dalton
he showed that a compound always consists of the same elements in the same ratio. On the other hand, different compounds always consist of different ratios. This can happen only if elements are made of tiny particles that can combine in an endless variety of ways. From this research, Dalton developed a theory of the atom. -
J.J. Thompson
In 1897, J.J. Thompson discovered the first subatomic particle, the electron. An experiment he conducted was passing an electric current through a vacuum tube. These experiments showed that an electric current consists of flowing, negatively charged particles. Thompson was the first yo say they were negatively charged. -
J.J. Thompson
Many scientists of Thompson's time thought that electric current consists of rays, like rays of light, and that is positive rather than negative. his experiments also showed that the negative particles are all alike and smaller than atoms. J.J. said that an atom was like plum pudding, which has plums scattered through it. That's why his model of the atom is called the plum pudding model. -
Ernest Rutherford
In 1899, Ernest Rutherford discovered the nucleus and that some elements give off positively charged particles. He named them alpha particles. In 1911, Rutherford used alpha particles to study atoms. He aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it. -
Ernest Rutherford
Most of the alpha particles passed straight through the foil as though they were moving through empty space. A few of the alpha particles bounced back as though they had struck a wall. this happened in very small areas at the centers of the gold atoms. Based on his results, Rutherford concluded that all the positive charge of an atom is concentrated in a small central area. He called this area the nucleus. He also predicted the existence of neutrons but failed to find them. -
Niels Bohr
In 1913, Niels Bohr discovered the Bohr model of the atom in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level to another.
