Answers · Inside the atom

Inside the atom — final check key

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

1. AK1 · Final check

Model answer. (a) Proton number 18: 18 protons; 40 − 18 = 22 neutrons; a neutral atom, so 18 electrons. (b) Mass number = 19 + 20 = 39. Charge = 19 − 18 = +1. Proton number 19 is potassium, so this is a potassium ion, K⁺. (c) The element depends only on the proton number. Argon atoms have 18 protons and the potassium ion has 19, so they are different elements even though both have 18 electrons.

A full answer includes

(a) Argon: 18 protons, 40 − 18 = 22 neutrons, 18 electrons.
(b) Mass number 19 + 20 = 39; charge 19 − 18 = +1; potassium (proton number 19), so it's a potassium ion, K⁺.
(c) I explained that the element is decided by the number of protons: argon has 18, potassium 19. The number of electrons doesn't decide the element.

Also accept

(a) 18 p, 22 n, 18 e; (b) mass number 39, charge +1, potassium; (c) different numbers of protons (18 and 19).

Watch for

“The particle with 18 electrons is argon, because argon has 18 electrons.” — The number of electrons can change without changing the element. 19 protons makes it potassium: it's a potassium ion that has lost one electron.
“Argon has 40 neutrons.” — 40 is the mass number, protons and neutrons together. Neutrons = 40 − 18 = 22.

2. AK2 · Final check

Model answer. (a) Nitrogen has 7 electrons: 2,5. Argon has 18: 2,8,8. (b) Nitrogen has 5 electrons in its outer shell, so Group V, and 2 occupied shells, so Period 2. Argon's outer shell is full with 8, so it's a noble gas in Group VIII, and it has 3 occupied shells, so Period 3. (c) A charge of +3 means 3 fewer electrons than protons: 13 − 3 = 10, so 2,8.

A full answer includes

(a) Nitrogen 2,5; argon 2,8,8.
(b) Nitrogen: 5 outer-shell electrons, Group V; 2 shells, Period 2. Argon: a full outer shell, Group VIII (a noble gas); 3 shells, Period 3.
(c) Al³⁺ has 13 − 3 = 10 electrons: 2,8.

Also accept

(a) 2,5 and 2,8,8; (b) N Group V Period 2, Ar Group VIII (full outer shell) Period 3; (c) 2,8.

Watch for

“Argon is in Group VIII because it has 8 shells.” — Argon has 3 shells: that's its Period. It's in Group VIII because its outer shell is full.
“Al³⁺ is 2,8,3, the same as the atom.” — The 3+ charge means three electrons have gone: 10 are left, so 2,8.