Isotopic Abundance & Relative Atomic Mass (Cambridge International AS Chemistry)

Calculating Relative Atomic Mass

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.

  • These are atoms of the same elements but with different mass numbers

• Because of this, the mass of an element is given as relative atomic mass (A_r) **by using the average mass of the isotopes

• The relative atomic mass of an element can be calculated by using the** relative abundance values

  • The relative abundance of an isotope is either given or can be read off the mass spectrum

• $A_{r = \frac{( \text{relative} abundance_{isotope} 1} mass_{isotope 1} ) + ( \text{relative} abundance_{isotope 2} mass_{isotope 2} ) etc.}{100}$

Worked Example Calculate the relative atomic mass, A_r, of oxygen to 2 d.p.

Isotope	Percentage abundance
16O	99.76
17O	0.04
18O	0.20

Answer:

• A_r = $\frac{( 99.76 \times 16 ) + ( 0.04 \times 17 ) + ( 0.20 \times 18 )}{100}$
• A_r = 16.0044
• A_r = 16.00 (to 2 d.p)

Worked Example Calculate the relative atomic mass of boron using its mass spectrum, to 2 d.p.

Answer:

• $A_{r} = \frac{( 19.9 \times 10 ) + ( 80.1 \times 11 )}{100}$
• A_r = 10.801
• A_r = 10.80 (to 2 d.p)