Electrons in shells — answer key

1. AQ6 · Practice

Options as numbered on the pupil sheet: 1 — 2,1; 2 — 2,6; 3 — 2,8,1; 4 — 2,8,7; 5 — 2,8,8,1; 6 — 2,8,8,2.

Model answer. Lithium (3) is 2,1; oxygen (8) is 2,6; sodium (11) is 2,8,1; chlorine (17) is 2,8,7; calcium (20) is 2,8,8,2. In each atom the electrons equal the proton number, and the shells fill from the inside: 2, then up to 8, then up to 8. 2,8,8,1 is potassium (19), which isn't in the list.

A full answer includes

• Lithium 2,1; oxygen 2,6; sodium 2,8,1; chlorine 2,8,7; calcium 2,8,8,2.
• I checked that the numbers in each configuration add up to the proton number.

Also accept

• Li 2,1; O 2,6; Na 2,8,1; Cl 2,8,7; Ca 2,8,8,2.

Watch for

• “Calcium is 2,8,10, because the third shell isn't full until it has more.” — For the first 20 elements the third shell takes 8, then the fourth shell starts. Calcium is 2,8,8,2.

2. AQ7 · Practice

Model answer. 1 two (2); 2 Group; 3 Period; 4 full. The first shell holds up to 2 electrons. In Groups I to VII the number of outer-shell electrons is the Group number, and the number of occupied shells is the Period number. The noble gases in Group VIII have a full outer shell.

A full answer includes

• 1: 2: the first shell holds up to 2 electrons.
• 2: Group: in Groups I to VII, the outer-shell electrons equal the Group number.
• 3: Period: the number of occupied shells equals the Period number.
• 4: full: the noble gases in Group VIII have a full outer shell.

Also accept

• 1 2; 2 Group; 3 Period; 4 full (or complete).

Watch for

• “The number of shells is the Group number.” — Sodium has three shells and is in Period 3, Group I. Shells give the Period; outer-shell electrons give the Group.

3. AQ8 · Practice

Model answer. 2,8,5. The first shell takes 2 (13 left), the second takes 8 (5 left), and the last 5 go in the third shell. With 5 outer-shell electrons phosphorus is in Group V, and with three occupied shells it is in Period 3.

A full answer includes

• I chose 2,8,5: 2 in the first shell, 8 in the second, the remaining 5 in the third.
• I stated that phosphorus is in Group V (5 outer electrons) and Period 3 (three occupied shells).

Also accept

• 2,8,5; Group V, Period 3.

Watch for

• “2,13: all the rest go in the second shell.” — The second shell holds up to 8. After 2 + 8, the next 5 electrons start the third shell.
• “2,5,8: the outer shell fills first.” — Shells fill from the inside. The second shell must be full, with 8, before the third shell gets any.

4. AQ9 · Practice

Model answer. (a) A neutral atom has as many electrons as protons: 2 + 8 + 8 + 1 = 19, so the proton number is 19. That's potassium. (b) One electron in the outer shell, so Group I. (c) Four shells have electrons in them, so Period 4.

Checked automatically

• (a) Proton number: 19 (exact value)
  • Common wrong answer 4 : 4 is the number of shells. Count every electron: 2 + 8 + 8 + 1.
  • Common wrong answer 18 : Count the outer electron too: 2 + 8 + 8 + 1 = 19.
• (b) Group number, as a digit (1 for Group I): 1 (exact value)
  • Common wrong answer 4 : 4 is the number of shells, which gives the Period. The Group is the number of outer-shell electrons.
  • Common wrong answer 8 : The outer shell is the last one with electrons in it. It holds 1.
• (c) Period number: 4 (exact value)
  • Common wrong answer 1 : 1 is the number of outer-shell electrons, which gives the Group. The Period is the number of occupied shells.
  • Common wrong answer 3 : Count the rings with electrons on them: 2, 8, 8 and 1 make four.

A full answer includes

• (a) 2 + 8 + 8 + 1 = 19 electrons, so the proton number is 19 (potassium).
• (b) 1 electron in the outer shell: Group I.
• (c) 4 occupied shells: Period 4.

Also accept

• (a) 19; (b) Group I (1); (c) Period 4.

Watch for

• “It's in Group IV, because it has four shells.” — The number of shells gives the Period. The Group is the number of outer-shell electrons: 1.

5. AQ10 · Practice

Model answer. (a) A charge of +2 means 2 fewer electrons than protons: 12 − 2 = 10 electrons, so 2,8. (b) A charge of −1 means 1 more electron than protons: 9 + 1 = 10 electrons, so 2,8. (c) Both ions have 10 electrons, so both are 2,8. But the magnesium ion has 12 protons and the fluoride ion has 9. The proton number decides the element, so they are still magnesium and fluorine.

A full answer includes

• (a) Mg²⁺ has 12 − 2 = 10 electrons: 2,8.
• (b) F⁻ has 9 + 1 = 10 electrons: 2,8.
• (c) I explained that the element is decided by the number of protons (12 and 9), not the electrons, so they are different elements with the same number of electrons.

Also accept

• (a) 2,8; (b) 2,8; (c) different numbers of protons (12 and 9) decide the element.

Watch for

• “Mg²⁺ is 2,8,4, because the plus means two electrons are added.” — A positive charge means electrons have gone. Mg²⁺ has 12 − 2 = 10 electrons, so 2,8.
• “Mg²⁺ is 2,8,2, the same as the atom.” — That's the magnesium atom, with 12 electrons. The ion has lost two of them.

6. AT2 · Apply

Model answer. (a) Period 3: three occupied shells, so the first two are full (2,8). Group VI: 6 electrons in the outer shell. So X is 2,8,6, with 16 electrons and so 16 protons: sulfur. (b) Lithium 2,1; sodium 2,8,1; potassium 2,8,8,1. They have different numbers of shells, but each has just one electron in its outer shell, which puts them all in Group I. The outer electrons are the ones that meet other atoms in a reaction, so the same outer shell can explain the similar reactions. (c) Helium is in Group VIII. Its 2 electrons are in the first shell, which holds only 2, so its outer shell is full. That full outer shell makes it a noble gas like neon (2,8) and argon (2,8,8). The number of outer electrons gives the Group only in Groups I to VII, so helium is not in Group II.

A full answer includes

• (a) 2,8,6: three shells, 6 outer electrons; proton number 2 + 8 + 6 = 16; sulfur.
• (b) Lithium 2,1; sodium 2,8,1; potassium 2,8,8,1.
• (b) I suggested that each has one electron in its outer shell (all in Group I), so they react in a similar way.
• (c) Group VIII: helium’s only shell, the first, holds up to 2, so with 2 it is full. I explained that a full outer shell makes it a noble gas, like neon and argon; the outer-electron rule gives the Group only in Groups I to VII.

Also accept

• (a) 2,8,6, proton number 16, sulfur; (b) 2,1; 2,8,1; 2,8,8,1: each has one outer-shell electron; (c) Group VIII, because its only shell is full with 2.

Watch for

• “X is 2,6,8 because it has three shells and is in Group VI.” — Inner shells fill first. With three shells, the first two are full (2,8), and the Group gives the outer shell: 6.