Helium

heliums atomic number is 2 and its atomic mass is 4

heliums atomic number

Heliums atomic number is 2 which means it has 2 protons and 2 electrons

heliums atomic mass and number

Helium's atomic mass is four so if you subtract the atomic mass from the number of protons (2) you will get your number of neutrons so for helium the number of neutrons is 2.