The Historical Development

of Atomic Models

Vocabulary

• Proton

• Neutron

• Electron

• Atom

• Nucleus

The first idea of atoms came from

two Greek philosophers, Democritus

and Leucippus in the 5th century BC.

The word atom comes from the

Greek word “atomos” which means

not “cut-able” or divisible.

Democritus Leucippus

J. Dalton

Dalton viewed the atom as a

small solid ball that cannot be

broken.

However, in the late 19th century,

scientists began to obtain

experimental evidences which

showed that atoms are not like

solid balls, and they could be

broken.

J. J. Thomson

• English physicist

• Discovered the electron, a

negatively-charged particle.

• In 1903 Thomson proposed

a new model of atom. The

atom was described as

positively-charged sphere

embedded with electrons

E. Rutherford

• In 1911, he discovered

the proton, a positively-

charged particle.

• He proved that atom has

nucleus.

• He described the new

model as tiny, dense,

positively-charged

electrons.

N. Bohr

In 1913, Bohr solved

this problem by

proposing that the

electrons could move in

shells around the

nucleus with particular

energies.

Bohr’s Atomic Model

J. Chadwick

• In 1932, James Chadwick

proved the existence of

neutrons, the neutral

particles in the nucleus.

• He discovered that nucleus

is surrounded by electrons.

• Neutrons contribute

approximately of the half of

the mass of an atom.

This model is known as the nuclear atom

and includes the three subatomic particles.

• The atom has a nucleus at its center.

The nucleus has protons and neutrons.

The nucleus has positive charges

because of the protons.

• A “cloud” of electrons surrounds the

nucleus. The electrons move at nearly

the speed of light.

• Most of the atom is empty.