Why are atomic numbers always whole?

The of an element is equal to the number of protons present in the nucleus. Because you can only have a whole number of protons (you can't have half a proton), the atomic number is always a whole number.

For example when you count people you count them in whole numbers. 1 2 or 3 people not 3.1. The same way in which you count the number of protons.

If your also wondering: "then why isn't the a whole number if neutrons also have to be in whole numbers?" Well this is because of . Since isotopes contain DIFFERENT numbers of neutrons but SAME number of protons, they use an average number based on the abundance of different isotopes. For example ALL lithium atoms have 3 protons. Therefore atomic number will be 3. However some lithium atoms have 4 neutrons and some 3 neutrons. Hence why the atomic mass is 6.9... (protons + neutrons)