Atoms are very small, they have a radius of around 1 × 10−10 metres.
Small molecules are at least ten times larger.
The radius of the nucleus is very much smaller than that of the atom.
Each particle has its own charge and its own mass.
|Relative charge||Relative mass|
|Electron||-1||close to 0 (1/2,000)|
Protons and neutrons are the heaviest particles in an atom and as a result they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.
The number of protons is what defines the element, ie an atom with six protons in its nucleus will always be carbon, and uranium will always have 92 protons.
In a neutral atom, the number of electrons is always the same as the number of protons. However, the number of electrons will change if the atom becomes ionised.