What are isotopes? Define relative atomic mass, A_r - Leaving Cert Chemistry - Question (a) - 2006

The relative atomic mass, A_r, of an element is defined as the average mass of an atom of the element compared to 1/12 of the mass of a carbon-12 atom, taking into account the different abundances of its isotopes.

The mass spectrometer operates on the principle of separating positive ions based on their mass-to-charge ratio. When ions are subjected to a magnetic field, they follow different paths based on their velocities and masses, allowing for their identification and quantification.

To calculate the relative atomic mass of lithium (Li), we can use the formula:

A_r = \frac{(\% \; of \; ^{6}Li \times mass \; of \; ^{6}Li) + (\% \; of \; ^{7}Li \times mass \; of \; ^{7}Li)}{100}

Substituting the values:

$A_r = \frac{(7.4 \times 6) + (92.6 \times 7)}{100}$

Calculating:

1. For (^{6}Li): (7.4 \times 6 = 44.4)
2. For (^{7}Li): (92.6 \times 7 = 647.2)
3. Sum: (44.4 + 647.2 = 691.6)

Finally, the relative atomic mass is:

$A_r = \frac{691.6}{100} = 6.916$

Thus, the calculated relative atomic mass of lithium is approximately 6.926.