How to Calculate the Mass of Atoms in AMU's ?

To calculate the masses of atoms we need only know the numbers of the various subatomic particles within the nucleus. For our purposes, we will disregard the electrons in our computations of mass because, for all intensive purposes, they have no mass. We will assign a mass of zero to the electrons in this lesson.

The total mass of an atom is the sum of all the protons and neutrons. Since protons and neutrons both weigh 1 ATM (Atomic Mass Unit), we simply need to add up the total number of each to arrive at the mass of the atom.

As a simple example, a helium atom (symbol He) has two protons and two neutrons in the nucleus. It also has two electrons, but we can ignore their mass and assign their weight a value of zero. Since the two protons weigh 2 AMU's and the two neutrons also weigh 2 AMU's, the total mass of a helium atom is 4 AMU's. That's it, it's that easy (at this level!) Of course, to compute the precise mass of an atom, we need to take into account the exact mass of protons and neutrons, and the electrons, and also to know the what type of isotope we are talking about. But that is far more complex, and we will not go into that level of detail in this lesson.

For a visual guide to the structure and mass computations of simple atoms, please see the multimedia section.