Nitrate (NO₃⁻) is not an atom, but an ion. The atomic mass is a property associated with individual atoms, not ions. The atomic mass of an atom is the weighted average mass of all the naturally occurring isotopes of that element.
Nitrate is composed of one nitrogen atom (N) and three oxygen atoms (O). The atomic mass of nitrogen (N) is approximately 14.007 atomic mass units (u), while the atomic mass of oxygen (O) is approximately 15.999 u. Therefore, to calculate the molecular mass of nitrate (NO₃⁻), you would add the atomic masses of the constituent atoms:
1(N) + 3(O) = 1(14.007 u) + 3(15.999 u) = 14.007 u + 47.997 u = 62.004 u
Therefore, the atomic mass of the nitrate ion (NO₃⁻) is approximately 62.004 atomic mass units.