Why is the atomic mass of the chlorine taken as 35.5 u and not a whole number like 35 u or 36 u? Explain.

The atomic mass of chlorine is represented as 35.5 atomic mass units (u) because it is a weighted average of the masses of its naturally occurring isotopes. Chlorine primarily exists in two isotopes: chlorine-35 and chlorine-37.

Understanding Isotopes

Isotopes are variations of an element that have the same number of protons but different numbers of neutrons. For chlorine:

• Chlorine-35: Has 17 protons and 18 neutrons.
• Chlorine-37: Has 17 protons and 20 neutrons.

Natural Abundance

The atomic mass reflects the relative abundance of these isotopes in nature. Approximately 75% of chlorine is chlorine-35, while about 25% is chlorine-37. This distribution affects the average atomic mass calculation.

Calculating the Average

The atomic mass is calculated using the formula:

Atomic Mass = (Fraction of Cl-35 × Mass of Cl-35) + (Fraction of Cl-37 × Mass of Cl-37)

Plugging in the values:

• For Cl-35: 0.75 × 35 u = 26.25 u
• For Cl-37: 0.25 × 37 u = 9.25 u

Adding these gives:

26.25 u + 9.25 u = 35.5 u

Conclusion

This averaging process explains why the atomic mass of chlorine is not a whole number. It reflects the combination of its isotopes and their relative abundances in nature.