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.

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    $\begingroup$ To use subscripts etc, use $\LaTeX$ and enclose between \$, e.g. for $A_r$, use \$A_r\$. $\endgroup$ – Ramashalanka Jun 7 '13 at 3:31

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).

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