The atomic mass of an element is based on the sum of the protons and neutrons in the nucleus. The electrons in the nucleus have very little mass and influence atomic mass minimally. Most elements on the periodic table have different atomic masses. Moving from the top left of the periodic table to the bottom right, the atomic mass of the different elements increases.
Protons and neutrons each have a mass of 1 atomic mass unit (amu).
The most common form of carbon has 6 protons and 6 neutrons in its nucleus. Carbon with 6 protons and 6 neutrons has a mass of 12amu and can be written as carbon-12.
Hydrogen has an atomic mass of 1 because it has 1 proton and no neutrons.
