Calculating relative atomic mass - Higher - Atomic structure - Edexcel - GCSE Combined Science Revision - Edexcel

Calculating relative atomic mass - Higher

The , A_r, of an is calculated from:

the of its
the of these isotopes

Chlorine

Chlorine naturally exists as two isotopes, \(_{17}^{35}\textrm{Cl}\) (chlorine-35) and \(_{17}^{37}\textrm{Cl}\) (chlorine-37). The abundance of chlorine-35 is 75% and the abundance of chlorine-37 is 25%. In other words, in every 100 chlorine atoms, 75 atoms have a mass number of 35, and 25 atoms have a mass number of 37.

To calculate the relative atomic mass, A_r, of chlorine:

\(\textit{A}_\textup{r}=\frac{total~mass~of~atoms}{total~number~of~atoms}=\frac{(35\times75)+(37\times25)}{(75+25)}\)

\(\textit{A}_\textup{r}=\frac{2625+925}{100}=\frac{3550}{100}\)

\(\textit{A}_\textup{r} = 35.5\) (to 1 decimal place)

Notice that the answer is closer to 35 than it is to 37. This is because the chlorine-35 isotope is much more abundant than the chlorine-37 isotope.

Question

The table shows the mass numbers and abundances of naturally-occurring copper isotopes.

Mass number	Abundance
63	69%
65	31%

Mass number	63
Abundance	69%

Mass number	65
Abundance	31%

Calculate the relative atomic mass of copper. Give your answer to 1 decimal place.

Next up

Watch a video

�鶹Լ��

Atomic structure - EdexcelCalculating relative atomic mass - Higher