2.2 The Atomic Structure

Historical development of atomic models

Model Structure Characteristic

Dalton’s atomic model

proposed by John

Dalton in 1805

The atom was

imagined as a small

indivisible ball similar

to a very tiny ball.

Thomson’s atomic

model proposed by J.J.

Thomson in 1897

J.J Thomson discovered

electron, a negatively-

charged particle.

The atom was

described as a sphere

of positive charge

embedded with

electrons.

Model Structure Characteristic

Rutherford’s atomic

model proposed by

Ernest Rutherford

in 1911

Ernest Rutherford

discovered proton,

a positively-charged

particle in an atom.

The central region

of atom has a very

small positively-

charged nucleus,

which contains

almost all the mass

of the atom.

Bohr’s atomic

model proposed by

Neils Bohr in 1913

The electrons in an

atom move in shells

around the nucleus

which contains

protons.

Model Structure Characteristic

Chadwick’s atomic

model proposed

by James Chadwick

in 1932

Chadwick proved

the existence of

neutrons, the

neutral particle in

the nucleus.

The nucleus of the

atom contains

protons &

neutrons, and the

nucleus is

surrounded by

electrons.

Subatomic particles in atom

Proton

Neutron

Electron

Subatomic particles

Relative electric charge: +1

Relative mass: 1

Relative electric charge: 0

Relative mass: 1

Properties of subatomic particles

Nucleus

Proton, p

Neutron, n

Relative electric charge: -1

Relative mass: ≈ 0.0005

Electron, e

Proton number and nucleon number

Proton number:

The number of proton in its atom

** also shows the number of electrons in the atom

Nucleon number:

The total number of protons and neutrons in its atom

** also known as mass number

What is the relationship??

Nucleon

number

Proton

number

Number of

neutrons= +

Nucleon

number

Number of

protons

Number of

neutrons= +

OR

Reminder!!!

Proton number

Nucleon number

Neutron number

√

√

XNumber of neutron

Symbol of elements

XA

Z

Nucleon

number

Proton

number

Symbol of

element