An atom is the smallest particle of an element which still carries all the properties of the element. Everything except energy is made up of atoms.
An atom consists of three components, which are the protons, electrons and neutrons. Protons and neutrons make up the nucleus (center) of the atom, with electrons on the outside.
Protons and Electrons
Protons carry a positive charge, and electrons carry a negative charge, holding the atom together by the same principle seen in magnets. In a standard or neutral atom, the number of protons and electrons are the same.
Neutrons have the same mass as protons but are neutral (having no charge). Different isotopes have different numbers of neutrons but the same number of protons and electrons.
The atomic mass number is the number of neutrons plus the number of protons. Different isotopes of the same element have a different atomic mass due to the differing number of neutrons.
If an atom has too many or too few neutrons, it becomes unstable. Unstable atoms or isotopes are radioactive.
Although it may survive intact for a short time, the nucleus of an unstable isotope will decay or change, expelling radiation in particle form or electromagnetic waves.