Atomic mass, also known as atomic weight, is a measure of the mass of an atom. It is a weighted average of the masses of all the naturally occurring isotopes of an element, taking into account their relative abundances.
The atomic mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. Protons and neutrons contribute significantly to the mass of an atom, while electrons, which have much smaller masses, are typically not included when calculating atomic mass.
The atomic mass is expressed in atomic mass units (amu) or unified atomic mass units (u). One atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Therefore, the atomic mass of carbon-12 is exactly 12 amu or 12 u.
It's important to note that atomic mass is different from atomic number. Atomic number (Z) represents the number of protons in the nucleus of an atom, whereas atomic mass represents the total mass of the atom, taking into account both protons and neutrons.