Chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3 : 1 respectively. What is the average atomic mass of the Chlorine atom ?
- A.36.1 u
- B.35.5 u
- C.36.5 u
- D.35.1 u
Correct answer
Explanation
The correct answer is B, 35.5 u. The average atomic mass is the weighted mean of the isotope masses, taking each in proportion to how common it is.
In a 3 : 1 ratio, three out of every four chlorine atoms are chlorine-35 and one is chlorine-37. So the average mass = (35 × 3 + 37 × 1) ÷ 4 = (105 + 37) ÷ 4 = 142 ÷ 4 = 35.5 u. That is why chlorine's atomic mass in the periodic table is not a whole number. Isotopes have the same number of protons, 17 in chlorine, but different numbers of neutrons, 18 and 20 here, so they behave the same chemically.
A is wrong because 36.1 u does not come from any correct weighting of the two masses. C is wrong because 36.5 u is the molar mass of hydrogen chloride, HCl, not of a chlorine atom. D is wrong because 35.1 u gives far too little weight to chlorine-37.
Exam tip: average atomic mass = Σ(mass × fraction); chlorine 35.5 u, from Cl-35 and Cl-37 in a 3 : 1 ratio.
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