2.2	The Development of the Atomic Model

Subatomic particles

• An atom is made up of smaller subatomic particles which are proton, electron and neutron. 
• Proton and neutrons are located at the nucleus of the atom. 
• Electrons move around the nucleus at a high speed. 
• The mass of the atom is concentrated at the centre of the atom. 

 

Subatomic particle

 

Comparison between the subatomic particles 

 

Proton	Electron	Neutron
Relative charge
+1	-1	0
Position
Nucleus	Moves around the nucleus	Nucleus
Relative mass
1	\(\dfrac{1}{1840}\)	1

 

Development of atomic structure models 

 

John Dalton

All matter is composed of extremely small particles called atoms. All atoms are the same for one kind of element. Atom can not be created or divided into smaller particles. In a chemical reaction, atoms are separated, combined, or rearranged.

 

J.J Thomson

An atom consists of small negatively charged particles called electrons.  The atom is a positively charged spherical cloud with electrons positioned uniformly throughout the atom.

 

Ernest Rutherford

Proton, which is a positively charged particle, is discovered.  Proton and most of the mass of the atom is located at the nucleus, which is the centre of the atom.  The atom is mostly empty space.

 

Neils Bohr

Electrons move around the nucleus in their fixed shells. The shells formed are circles with a fixed radius from the nucleus.  Electrons at each shell have their own fixed energy level.

 

James Chadwick

Discovered the presence of the neutrons in the nucleus through an experiment where the beryllium-9 atom is bombarded with alpha particles.  Proposed that half of the mass of an atom is contributed by the neutron.