... Atomic Structure In the last lesson we learned that atoms were particles of elements, substances that could not be broken down further. In examining atomic structure though, ... ways: using the atomic number (Z) or using the atomic mass (A, also known as the mass number). The atomic number describes the number of protons in an atom. For hydrogen the atomic number, Z, ... tells you the number of electrons in the atom. The atomic mass tells you the number of protons plus neutrons in an atom. Therefore, the atomic mass, A, of hydrogen is 1. For helium A = 4....