Answers · Isotopes

Isotopes — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. IK1 · Final check

Model answer. (a) Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Both atoms have proton number 7; nitrogen-14 has 14 − 7 = 7 neutrons and nitrogen-15 has 15 − 7 = 8. (b) The nucleus is unchanged, so the mass number is 15 and the proton number 7; the charge is 7 − 10 = −3: ¹⁵₇N³⁻. (c) Both isotopes have 7 electrons in the configuration 2,5. Reactions involve only the electrons, so both isotopes react in the same way.

A full answer includes

(a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons: both have 7 protons; nitrogen-14 has 7 neutrons and nitrogen-15 has 8.
(b) ¹⁵₇N³⁻, or N with mass number 15, proton number 7 and charge 3−: the charge is 7 − 10 = −3.
(c) I explained that both have 7 electrons arranged 2,5, the same electronic configuration, and chemical properties depend on the electrons.

Also accept

(a) Same protons (7), different neutrons (7 and 8). (b) ¹⁵₇N³⁻. (c) Same electrons, 2,5.

Watch for

“Isotopes are atoms with the same mass number.” — Isotopes have the same proton number; their mass numbers differ because their neutrons differ.
“The nitride ion of nitrogen-15 has charge +3, because it has 3 more electrons.” — Extra electrons give a negative charge: 7 − 10 = −3.

2. IK2 · Final check

Model answer. (a) 28 × 92.2 = 2581.6, 29 × 4.7 = 136.3 and 30 × 3.1 = 93. The total is 2810.9, and 2810.9 ÷ 100 = 28.109, so Ar = 28.1. (b) The relative atomic mass is an average weighted by abundance. Almost all the atoms (92.2 %) are silicon-28, so the average is very close to 28; the silicon-29 and silicon-30 atoms raise it by only about 0.1.

A full answer includes

(a) 28 × 92.2 + 29 × 4.7 + 30 × 3.1 = 2581.6 + 136.3 + 93 = 2810.9; 2810.9 ÷ 100 = 28.109, so 28.1.
(b) I explained that 92.2 % of the atoms are silicon-28, so the average is pulled very close to 28; the few heavier atoms raise it only slightly.

Also accept

(a) 28.1. (b) 92.2 % of the atoms are silicon-28.

Watch for

“Ar is 29, the mean of 28, 29 and 30.” — The isotopes are far from equally common. Weight each mass number by its abundance.
“Ar is 28, the most common isotope.” — The heavier isotopes still count: they raise the average to 28.1.