The of an element is equal to the number of protons present in the nucleus

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)