Atoms and Molecules - Atomic Mass

Atomic mass is the mass of a single atom of a chemical element, typically expressed in atomic mass units ($u$).

According to Dalton's atomic theory, each element possesses a characteristic atomic mass.

Since atoms are extremely small, their absolute mass is difficult to measure. Instead, scientists use Relative Atomic Mass.

The standard reference for measuring atomic mass is the Carbon-12 isotope ($^{12}C$), which is assigned a mass of exactly $12$ atomic mass units.

One atomic mass unit ($1 \text{ u}$) is defined as a mass unit equal to exactly $\frac{1}{12}^{th}$ the mass of one atom of carbon-12.

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to $\frac{1}{12}^{th}$ the mass of one carbon-12 atom.

Most elements exist as a mixture of isotopes, so the atomic mass we use is the Average Atomic Mass, which accounts for the natural abundance of each isotope.