5. (a) Define (i) mass number of an atom, (ii) relative atomic mass of an element - Leaving Cert Chemistry - Question 5 - 2018

The relative atomic mass of an element is the average mass of an atom of the element, taking into account the different isotopes and their abundances. It is calculated compared to a standard, usually the 1/12th mass of a carbon-12 isotope.

1. Ionization: The sample is vaporized and then ionized to produce positively charged ions.
2. Acceleration: The ions are accelerated by an electric field towards a magnetic field.
3. Separation / deflection in a magnetic field: The accelerated ions are passed through a magnetic field which causes them to deflect based on their mass-to-charge ratio.

To calculate the relative atomic mass, we use the abundances and mass numbers from the spectrum:

Given:

• 79% of isotope 24
• 10% of isotope 25
• 11% of isotope 26

The calculation is as follows:

$\text{Relative Atomic Mass} = \frac{(79 \times 24) + (10 \times 25) + (11 \times 26)}{100}$

Calculating:

• $79 \times 24 = 1896$
• $10 \times 25 = 250$
• $11 \times 26 = 286$

Adding these together: $1896 + 250 + 286 = 2432$

Dividing by 100 gives: $\frac{2432}{100} = 24.32$

Thus, the relative atomic mass of magnesium is 24.32.