Relative atomic mass, often denoted as \( A_r \), is a dimensionless quantity that describes the mass of an atom of a chemical element relative to one-twelfth of the mass of a carbon-12 (\( ^{12}C \)) atom. It is essentially a weighted average of the masses of all the naturally occurring isotopes of an element, taking into account their relative abundances.
Because atomic masses are very small, they are typically expressed in atomic mass units (amu), where \( 1 , \text{amu} \) is defined as one-twelfth of the mass of a \( ^{12}C \) atom. The relative atomic mass gives a useful way to compare the masses of different elements and understand their behavior in chemical reactions.
For example, the relative atomic mass of hydrogen is approximately 1.008, indicating that a hydrogen atom is, on average, about 1/12 the mass of a carbon-12 atom (which has a relative atomic mass of exactly 12). The values can be found on the periodic table, where each element's relative atomic mass is listed.