There have been several different models of the atom in history and these models have led to the current structure of the atom.
Timeline of the atomic model
THE SCIENTIST
WHEN?
DESCRIPTION OF THE MODEL
John Dalton
1803
A solid model of the atom, without any details of inside the atom.
J.J. Thomson
1897
The discovery of the electron in the atom.
J.J. Thomson
1904
Plum pudding model. The atom was suggested to be filled with positive charge containing negative electrons dotted around.
Ernest Rutherford
1911
Positive alpha particles were fired at gold atoms. Most particles passed through and were not deflected. Discovered an area of positive charge, the nucleus, surrounded by orbiting negative electrons.
Niels Bohr
1913
Electrons orbit the nucleus in different energy levels or shells. Electrons are not arranged randomly.
Ernest Rutherford
1918
Discovered the proton.
James Chadwick
1932
Discovered the neutron. Negative charge stabilises the nucleus.
Structure of the atom
1. Protons, 2. Neutrons,3. Electrons,4. Electron shells, 5. Nucleus
The atom has a central nucleus which contains protons and neutrons.
Protons have a positive charge and neutrons have a neutral charge, so the overall charge of the nucleus is positive.
The nucleus is surrounded by orbiting electrons in electron shells.
Subatomic particle
RELATIVE CHARGE
RELATIVE MASS
Proton
+1
1
Neutron
0
1
Electron
−1
18361
Atoms have a neutral charge, with equal numbers of positive and negative charges. Atoms contain an equal number of protons and electrons.
Electrons have a small mass, with most of the mass in an atom being concentrated in the nucleus.
Example
In an atom, which two subatomic particles are equal in number?
The answer is protons and electrons. Protons have a positive charge whereas electrons have a negative charge. An atom has a neutral charge, so they must be equal in number.