The atomic mass of chlorine is 35.5 and that of copper is 63.5. Does it mean that atoms of these two elements have half a proton or half a neuron? Explain.

As discussed earlier, isotopes have the same number of protons but different number of neutrons. Boron for example, has two isotopes, namely, 10B and 11B. The mass of an atom is the total mass of neutrons and protons. Hence, 10B has a mass of 10 amu and 11B has a mass of 11 amu. However, when we determine the atomic mass of boron, the masses of the two isotopes should be considered.

Relative atomic mass is the average mass of the isotopes of a given element. For example, the two isotopes of chlorine exist in nature in different percentages: 75% of the atoms are ³⁵Cl and 25% are ³⁷Cl.

Thus the relative atomic mass of chlorine is calculated as the average mass of these two isotopes.

[(75 × 35)] + [(25 × 37)] / 100
=(0.75 × 35) + (0.25 × 37) = 35.5 of chlorine