Atomic number and mass number

Atomic number

The elements in the Periodic Table have a number called the atomic number which increases from left to right across the table.

This number tells us how many protons an atom has.

As all atoms are neutral (have no overall electrical charge) the number of protons must equal the number of electrons.

atomic number = number of protons

For example:

Hydrogen has the symbol H and it has an atomic number of 1 because its atoms contain one proton.

Gold has the symbol Au and it has an atomic number of 79 because each of its atoms contain 79 protons.

The atomic number of all the elements can be found on page 4 of the National 5 Chemistry data booklet

Mass number

The mass of an atom is due mostly to the total number of protons and neutrons it has in its nucleus.

This mass is given by the element's mass number.

mass number = number of protons + number of neutrons

If you know the atomic number of an element then the mass number is equal to the atomic number plus the number of neutrons.

We can also use the mass number and atomic number to work out how many neutrons are in an atom of an element:

number of neutrons = mass number - atomic number

Nuclide Notation

The information about the number of particles in an atom (and its atomic and mass numbers) is often written as:

This is known as nuclide notation.

In the atom of sodium above, the atomic number is 11 the mass number is 23.

This means that the atom of sodium has 11 protons and 12 neutrons.

As the atom is neutral the number of electrons present is also 11.

Next section: Electron Arrangements