Atoms are made up of protons, neutrons and electrons. The protons and neutrons are in the nucleus and the electrons are found around the nucleus.

The relative masses and charges of the particles are as follows:

Mass number and atomic number
The mass numbers and atomic numbers can be included with the symbol of the element as follows:

• The mass number is written at the top left-hand corner of the symbol.
• The atomic number is written at the bottom left-hand corner of the symbol.

The mass number is the total number of protons and neutrons in the atom.

The atomic number is the total number of protons in the atom. (This is also called the proton number).

In a neutral atom the atomic number is also equal to the number of electrons (because the number of positive and negative charges must be equal).


Isotopes

• All atoms of a particular element have the same number of protons.
• The number of protons defines the element.
• But you can have atoms of the same element with different numbers of neutrons.
• These are what we call isotopes.
• If we have different isotopes they will always have the same atomic number but different mass numbers.
• The number of electrons is identical in isotopes.