Isotopes — return questions key

1. IR1 · Return

Model answer. (a) Both have 8 protons and 8 electrons; oxygen-16 has 16 − 8 = 8 neutrons and oxygen-18 has 18 − 8 = 10. (b) The ion keeps the nucleus, 8 protons and 10 neutrons, and has 2 extra electrons: ¹⁸₈O²⁻. (c) Oxygen-18 atoms have the same electrons as oxygen-16, 2,6, so the water reacts in the same way and turns the powder blue. Each oxygen-18 atom has two more neutrons, so each molecule has more mass, and the same volume has a greater mass.

A full answer includes

• (a) Oxygen-16: 8 p, 8 n, 8 e; oxygen-18: 8 p, 10 n, 8 e.
• (b) ¹⁸₈O²⁻, or O with mass number 18, proton number 8 and charge 2− (8 protons, 10 neutrons, 10 electrons).
• (c) I explained that the electrons are the same (8, 2,6), so the reactions are the same; the two extra neutrons give each molecule more mass.

Also accept

• (a) 8, 8, 8 and 8, 10, 8. (b) ¹⁸₈O²⁻. (c) Same electrons, same reaction; extra neutrons, more mass.

Watch for

• “The oxide ion of oxygen-18 has mass number 20, because it has 2 extra electrons.” — Electrons are not counted in the mass number. It stays 18; the extra electrons give the 2− charge.
• “Heavy-oxygen water would not turn the powder blue.” — Its oxygen atoms have the same electrons as ordinary oxygen, so it reacts as water does.

2. IR2 · Return

Model answer. 79 × 50.7 = 4005.3 and 81 × 49.3 = 3993.3. The total is 7998.6, and 7998.6 ÷ 100 = 79.986, so Ar = 80.0 to one decimal place. Check: the abundances are almost equal, so Ar is almost exactly halfway between 79 and 81.

Checked automatically

• Relative atomic mass, Ar: 80.0 (within ±0.05)
  • Common wrong answer 79 : Bromine-81 is almost as common as bromine-79, so it pulls the average well above 79. Multiply each mass number by its abundance, add, and divide by 100.
  • Common wrong answer 7998.6 : That is the total for 100 atoms. Divide by 100.

A full answer includes

• 79 × 50.7 + 81 × 49.3 = 4005.3 + 3993.3 = 7998.6.
• 7998.6 ÷ 100 = 79.986, so 80.0.

Also accept

• 80.0

Watch for

• “Ar is 79, because bromine-79 is slightly more common.” — Bromine-79 is only a little more common, so the average is just below the halfway value of 80.