Which one of the following is the average atomic mass of chlorine atom?
- (a)35.9 u
- (b)35.5 u
- (c)35.0 u
- (d)37.0 u
Correct — B, 35.5 u. Chlorine occurs in nature as two stable isotopes, chlorine-35 and chlorine-37, in a ratio close to 3 to 1. The average atomic mass is the weighted mean of the two, so it is (3 × 35 + 1 × 37) divided by 4, which is 142 ÷ 4 = 35.5 u. That is why the periodic table prints a fractional value for chlorine even though no single chlorine atom weighs 35.5 units.
- (a)35.9 u — 35.9 u would need the heavier isotope to make up about 45 per cent of natural chlorine. The observed abundance of chlorine-37 is about a quarter.
- (c)35.0 u — 35.0 u is the mass of the lighter isotope alone. Taking it as the average ignores the chlorine-37 present in every natural sample.
- (d)37.0 u — 37.0 u is the mass of the heavier isotope alone, and it is the less abundant of the two, so the average must lie much closer to 35.
Isotopes are atoms of one element with the same proton number and different neutron numbers. A natural sample is a mixture of them in fixed proportions, and the atomic mass quoted for the element is the weighted average of the isotopic masses using those proportions as weights. Fractional atomic masses in the periodic table are the fingerprint of that mixture.
The arithmetic is quick when the ratio is simple: with a 3 to 1 split, the average lies three-quarters of the way from 37 back towards 35, that is 35 + (1/4 × 2) = 35.5. Working backwards is just as useful in the hall, since an average of 35.5 sitting between 35 and 37 immediately implies a 3 to 1 abundance. The same reasoning explains the awkward 63.5 of copper and the 24.3 of magnesium, and it is different from the mass number of a single atom, which is always a whole number.
- Chlorine has two stable isotopes of mass 35 u and 37 u, in a natural abundance ratio close to 3 to 1.
- The average atomic mass is (3 × 35 + 1 × 37) ÷ 4 = 35.5 u.
- Both isotopes have 17 protons; chlorine-35 has 18 neutrons and chlorine-37 has 20.
- A fractional atomic mass in the periodic table signals a mixture of isotopes, never a fractional particle count.
- The mass number of an individual atom is always a whole number, which is what distinguishes it from the average atomic mass.
Reading the calculation in reverse is the faster exam habit: the position of the average between the two isotopic masses gives the abundance ratio directly.
- Taking the plain arithmetic mean of 35 and 37, which gives 36 and ignores the abundances.
- Reporting the mass of the commoner isotope as the average.
- Reading a fractional atomic mass as implying fractional protons or neutrons.
Either as this straight recall or as a short calculation supplying the abundances. Both reduce to one weighted mean.
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
Answer(b) 35.5 u
The identical calculation set out as a numerical item, with the 3 to 1 abundance supplied in the stem. Its key reaches the same 35.5 u, which is a direct confirmation of the value asked for here.
- practice — not a real PYQ
An element has two isotopes of masses 10 u and 11 u present in the ratio 1 : 4. Its average atomic mass is
- (a)10.2 u
- (b)10.5 u
- (c)10.8 u
- (d)11.0 u
Answer(c) 10.8 u — (1 × 10 + 4 × 11) ÷ 5 = 54 ÷ 5 = 10.8.
- practice — not a real PYQ
The atoms chlorine-35 and chlorine-37 differ in the number of
- (a)protons
- (b)electrons
- (c)neutrons
- (d)protons and electrons both
Answer(c) neutrons — both have 17 protons, while the neutron counts are 18 and 20.