Atomic Number and Atomic Mass

When you learned about the Periodic Table during KS3, you were told that it lists all the elements in order of their atomic number. You should also have learned that every element has two numbers; the smaller value is the element’s atomic number and the larger value is the element’s atomic mass.

The value for the atomic number is equal to the number of protons (positively charged particles) in the nucleus of the atom. This number is also equal to the number of electrons (negatively charged particles) that surround the atom. The number of positive charges is therefore equal to the number of negative charges and this means that atoms have no overall charge.

The value for the atomic mass is equal to the sum of the number of protons and neutrons (uncharged particles) in the nucleus of the atom. It is common for atoms of the same element to exist in slightly different forms with different numbers of neutrons (but always the same number of protons and electrons). These variations are called isotopes, which are discussed in more detail in the next post.