Atomic mass, also known as atomic weight, is a physical property of an atomic species that represents the average mass of the atoms in a given sample of an element. It is expressed in atomic mass units (amu) or unified atomic mass units (u).
The atomic mass of an atom is determined by the combined masses of its protons, neutrons, and electrons. However, since the mass of an electron is much smaller compared to that of protons and neutrons, it is often neglected when calculating atomic mass. Therefore, atomic mass is primarily influenced by the number of protons and neutrons in the atom's nucleus.
The atomic mass of an element is typically represented as a decimal number on the periodic table. This value takes into account the natural abundance of different isotopes of the element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since different isotopes have different masses, their relative abundances affect the overall atomic mass.
For example, carbon has an atomic mass of approximately 12.01 amu. This value accounts for the presence of two main isotopes: carbon-12 (which is most abundant) and carbon-13. The atomic mass considers the relative abundance of each isotope and their respective masses to arrive at an average value.
It is important to note that atomic mass is different from atomic number, which represents the number of protons in an atom's nucleus. Atomic mass provides information about the mass of the atom, while atomic number determines the identity of the element itself.