Proton number

The proton number of an atom is the number of protons it has in its nucleus.

We usually represent it with the letter Z, and sometimes call it the atomic number.

Proton number and elements

The proton number is important because it determines which element an atom is.

An element is defined by its proton number. For example:

• All atoms with Z=1 are hydrogen.
• All atoms with Z=6 are carbon.
• All atoms with Z=8 are oxygen.
• …and so on.

You should be able to apply this pattern to all the elements on the periodic table!