History of Atomic Theory

Democritus

400 BCE

Democritus's model stated that matter consists of invisible particles called atoms. His atomic model was solid and stated all atoms differ in size, shape, mass, position, and arrangement

John Dalton

1803

Dalton's atomic theory was the first theory to describe all matter in terms of atoms and their properties.
Dalton based his theory on the law of conservation of mass and the law of constant composition.ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ
1: States that all matter is made of atoms, which are indivisible.
2: All atoms of a given element are identical in mass and properties.
3: Compounds are combinations of two or more different types of atoms.
4: states that a chemical reaction is a rearrangement of atoms.

J.J Thomson

1897

Thomson's experiments showed that all atoms contain tiny negatively charged subatomic particles or electrons. He proposed the plum pudding model of the atom, which had negatively-charged electrons fixed.

Ernest Rutherford

1911

Rutherford overturned Thomson's experiment with his experiment called "Gold Foil Experiment". This experiment led to the discovery of the nucleus of the atom as a small, dense, and positively charged atomic core.

Niels Bohr

1913

Niels Bohr proposed a theory for the hydrogen atom based on quantum theory (He is a father of quantum theory) that energy is transferred only in certain well-defined quantities. His model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons.

Erwin Schrödinger

1926

Schrödinger took the Bohr atom model to the next level. He used mathematical equations to describe the probability of finding an electron in a certain position. This model can be represented as a nucleus surrounded by an electron cloud.

James Chadwick

1932

Chadwick used a version of Rutherford's experiment. Thinking that the neutron mass was close to that of the proton, Chadwick attacked hydrogen atoms with his produced neutrons to learn the speed of the protons after the crash. In 1930 he discovered that Beryllium, when attacked by alpha particles, discharged a very energetic stream of radiation.