Atomic mass refers to the weight or size of an atom, which includes adding up the number of protons and neutrons in an atom. This mass is denoted in atomic mass units (amu). Only protons and neutrons are considered when calculating atomic mass, since an electron is extremely light. Proton and neutron’s atomic mass is 1.007 and 1.008 respectively. The electron, however, is only 0.00055 in mass.

Generally, an atom’s atomic mass is calculated by averaging the mass figures of the atom’s isotopes, or variants of the particular atom. On the periodic table, the atomic mass denoted below the particular element’s name is the average atomic mass.

Atomic mass figures could be as low as 1.008 amu (hydrogen) or could be more than 250 amu. A molecule’s mass can be determined by adding the atomic masses of all atoms constituting the molecule. For example, the sodium chloride (NaCl) molecule has one sodium atom and one chlorine atom. The molecule’s mass would therefore be the sum of sodium atomic mass and chlorine atomic mass.


An atom is generally one of the several hundred thousand units of a gram. It’s so miniscule, it is practically impossible to feel its heft or presence. However, everything on Earth is made of atoms, and knowing the mass of an atom is therefore important. For example, to create a new substance in a laboratory or discover one, the chemical formula of the substance must be ascertained. In other words, the scientist should know the exact proportions and quantities of the element’s molecules. Atomic mass figures help find out the chemical composition when implementing the complex calculations.

Why Mass and Not Weight?

The mass of an atom is not the same as its weight. There’s a difference. The weight of an object can be correlated to the gravitational pull acting upon it. The higher the pull, the heavier the object. For example, an individual’s weight on earth and moon won’t be the same, thanks to earth’s gravity and the lack of such a force on moon. However, an object’s mass is more uniform and stays consistent irrespective of the external forces acting upon it. This makes mass more accurate than weight.