NCERT Solutions for Class 9th Science Chapter 8 .9.1 A. Average atomic mass — Pause and Ponder

Book page 1568 Updated on2026-09-08

Q17.

Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?

Answer

Same atomic number, different mass numbers — they are the same element (sodium), and they are isotopes of each other.

Atomic number Z = number of protons
First atom: Z = 11 Second atom: Z = 11 → equal

Mass number A = p⁺ + n⁰
First atom: A = 11 + 12 = 23 → ²³₁₁Na
Second atom: A = 11 + 13 = 24 → ²⁴₁₁Na
→ different, by 1

Z = 11 in both cases, and Z is what fixes the identity of an element, so both are sodium. Same Z, different A is the definition of isotopes.

Why it happens: both atoms are neutral, so both have 11 electrons, arranged 2, 8, 1. Identical valence shell ⇒ identical chemistry. Both are soft metals that react vigorously with water. The extra neutron only makes the second atom about 4% heavier, which shows up in physical properties, not chemical ones.

Check it yourself: would the answer change if one atom had 11 protons and the other 12? Yes, completely — 11 protons is sodium and 12 is magnesium. Different elements, and they would not be isotopes at all.

Q18.

If a bromine atom is available in the form of, say two isotopes, ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of the bromine atom.

Answer

Average atomic mass of bromine ≈ 80.01 u, i.e. about 80 u.

Formula (weighted average):

Average atomic mass = (mass₁ × % abundance₁/100) + (mass₂ × % abundance₂/100)

Substituting:

= (79 u × 49.7/100) + (81 u × 50.3/100)

= (79 × 49.7 + 81 × 50.3) ÷ 100 u

79 × 49.7 = 3926.3
81 × 50.3 = 4074.3
Sum = 3926.3 + 4074.3 = 8000.6

Average atomic mass = 8000.6 ÷ 100
= 80.006 u ≈ 80.01 u

Rounded sensibly for the two-significant-figure data, ≈ 80 u.

Why it happens: the two isotopes are almost equally abundant (49.7% and 50.3%), so the weighted average sits almost exactly halfway between 79 u and 81 u — and the halfway point is 80 u. The tiny excess above 80 comes from the heavier isotope being slightly the more common one. Contrast chlorine, where the abundances are 75% and 25%, so the average 35.5 u sits much closer to 35 u than to 37 u.

Tip: no single bromine atom weighs 80.01 u. Every bromine atom is either 79 u or 81 u. The average describes a large collection — in 1000 bromine atoms you would expect about 497 of ⁷⁹Br and 503 of ⁸¹Br.

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