how many daltons is the mass of a helium atom?
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 the atomic mass. Helium is the second lightest element, and is larger than hydrogen. It consists of two protons and two neutrons. Because protons and neutrons are approximately the same size, the atomic mass of a helium atom is four daltons. Let's look at a larger atom. Barium has an atomic mass of 137 daltons.