Question

Define atomic mass.

Answer 1

Atomic mass is defined as mass equal to $1/12$ of mass of a carbon-$12$ atom in its ground state. Atomic mass is the average relative atomic mass of all the isotopes of that element.

For example, the atomic mass of carbon ($\mathrm{C}$) is $12$$\mathrm{u}$ and aluminium ($\mathrm{Al}$) is $13\mathrm{u}$.