When elements are written as symbols, such as on the periodic table, they are expressed in the form

The symbols are where X is the letter(s) that represent the element, Z is the atomic number (the number of protons) and A is the atomic mass (number of protons plus neutrons).

For example, carbon-12 is represented as

C is the letter that represents carbon, and it has a mass of 12, and 6 protons.

In the periodic table Z will often be a decimal number, even though you can’t get protons or neutrons in a non-whole number. This is because it is an average of the different amounts of neutrons that can be found in occurrences of the element, which will be explained in more detail later.