The history of the atom

In a nutshell

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

Chemistry; Key concepts; KS4 Year 10; The history 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	$-1$	$\frac{1}{1836}$

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.