Atomic mass is an important property of atoms that is used to understand and predict their behavior in chemical reactions. It is determined by the number of protons and neutrons in the nucleus of an atom, as well as the mass of those particles. The number of protons in the nucleus of an atom determines the atomic number of the element, which uniquely identifies the element and its place in the periodic table.
The atomic mass of an element is typically written as a decimal number on the periodic table. This number is an average of the atomic masses of all the naturally occurring isotopes of the element, which are atoms with the same number of protons but different numbers of neutrons. Isotopes of an element have the same atomic number, but different atomic masses due to their different neutron counts.
For example, the atomic mass of carbon is written as 12.01 on the periodic table. This number is an average of the atomic masses of the three naturally occurring isotopes of carbon: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope of carbon, and it makes up about 98.9percentage of all carbon atoms. Carbon-13 is less abundant, making up about 1.1percentage of all carbon atoms, and carbon-14 is even rarer, making up about 0.0000000001percentage of all carbon atoms. The atomic masses of these isotopes are 12.0, 13.0, and 14.0 amu, respectively, so their average is 12.01 amu.