What is atomic mass?

Hello, The atomic mass for an atom of hydrogen is one dalton, which is calculated from the lone proton. Remember, electrons are too small to be considered in theatomic mass. Helium is the second lightest element, and is larger than hydrogen. It consists of two protons and two neutrons.

The mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundances of different isotopes.