# What does $\mathrm{A}_r(\mathrm{H})$ represent when calculating the average natural weight of an elemental atom?

I have a question that explains the weights and abundances of various hydrogen isotopes, and the question concludes by asking to:

Calculate the atomic mass of hydrogen $\mathrm{A}_r(\mathrm{H})$ (with the $r$ in subscript).

The answer simply summed the mass of each isotope multiplied by its natural abundance. However, the text never mentioned the meaning of $\mathrm{A}_r(\mathrm{H})$ and I couldn't find anything online.

• To use subscripts etc, use $\LaTeX$ and enclose between \$, e.g. for$A_r$, use \$A_r\$. – Ramashalanka Jun 7 '13 at 3:31 ## 1 Answer The relative atomic mass$A_r$is 'the ratio of the average mass of atoms of an element ... to 1/12 of the mass of an atom of carbon-12' (from http://en.wikipedia.org/wiki/Relative_atomic_mass). The atomic mass$m_a$is the mass of a particular isotope of an atom, and if it is in 'u' (unified atomic mass units) it is relative to 1/12 of the mass of an atom of carbon-12. So, to get$A_r$you take a weighted average of$m_a\$ in 'u' for each isotope.

Note also that the standard atomic weight is the relative atomic mass of an element using natural abundance on Earth (http://en.wikipedia.org/wiki/Relative_atomic_mass).