Atomic Mass or Relative Atomic Mass

Atomic mass and relative atomic mass are closely related but slightly different concepts in chemistry.

1. Atomic Mass: This refers to the mass of an atom of a chemical element. It is typically expressed in atomic mass units (amu) or unified atomic mass units (u). The atomic mass of an atom is determined by the total number of protons and neutrons in its nucleus. Electrons, being much lighter than protons and neutrons, contribute negligible mass to the atom's overall mass. The atomic mass of an element is usually found on the periodic table and is an average of the masses of all the naturally occurring isotopes of that element, weighted by their relative abundance.

2. Relative Atomic Mass: This is also known as atomic weight. It is the average mass of an element's atoms compared to the mass of an atom of carbon-12, which is assigned a relative atomic mass of exactly 12. Relative atomic mass is a dimensionless quantity because it is a ratio of two masses (the mass of an atom of the element and the mass of an atom of carbon-12) with the same units. The relative atomic mass listed on the periodic table is a weighted average of the atomic masses of all the isotopes of that element, weighted by their natural abundances.

In summary, atomic mass refers to the mass of a single atom of an element, while relative atomic mass refers to the average mass of all the atoms of that element relative to the mass of a carbon-12 atom.