Atomic Mass

The mass number is a simple count of the number of protons and neutrons. This is always close to the mass of the atom because both protons and neutrons weigh approximately 1 amu and because the mass of electrons is very very small compared to the masses of the protons and neutrons.

However, protons and neutrons do not weigh EXACTLY 1 amu, nor do they even have the same exact mass as each other.

That means that the mass number is NOT the actual mass of the atom. The actual mass is called atomic mass, and it is determined experimentally using a device called a mass spectrometer. Even this device cannot give actual masses, it can only give relative masses (for example atom B is exactly 3.4651 times as heavy as atom A).

As a result, we has to choose a standard to compare everything else to. Initially we chose hydrogen (the lightest element) and gave it a value of 1 amu (atomic mass unit). Since oxygen is about 16 times heavier than hydrogen, that made the mass for oxygen 16 amu. This system allowed us to calculate atomic masses for many elements, however hydrogen gas is difficult to obtain in pure form and can be difficult to work with, so we decided to change standards. We now use carbon as the standard to which everything is compared.

Changing standards can be difficult because it means that everyone would have to learn new values for everything. To avoid that problem, we did NOT set carbon's mass to 1 amu.  Instead, we set it’s value to be 12 (since it is 12 times as heavy as hydrogen). That means that everything maintained the mass that people were used to, but it allowed us an easier standard to obtain and work with.

However, things still aren't quite that easy. The numbers recorded on the periodic table are actually average atomic masses.