What makes sodium sodium? — answer key

1. AQ1 · Practice

Options as numbered on the pupil sheet: 1 — Proton; 2 — Neutron; 3 — Electron.

Model answer. 1 proton; 2 neutron; 3 electron; 4 electron; 5 proton. A proton has a relative charge of +1 and a relative mass of 1, and the number of protons decides which element the atom is. A neutron also has a relative mass of 1 but no charge. Both are in the nucleus. An electron has a relative charge of −1 and a relative mass of only 1/1840, and it is found in shells around the nucleus.

A full answer includes

• I matched 1 and 5 to the proton: charge +1, and its number decides the element.
• I matched 2 to the neutron: no charge.
• I matched 3 and 4 to the electron: relative mass 1/1840, found in shells around the nucleus.

Also accept

• Proton: 1, 5. Neutron: 2. Electron: 3, 4.

Watch for

• “The electron has no mass at all.” — It has a tiny mass, 1/1840 of a proton's. It's so small that we leave it out of the mass number, but it isn't zero.
• “The neutron decides the element, because it's in the nucleus.” — Neutrons are in the nucleus, but adding one only made a heavier sodium atom. Only the number of protons decides the element.

2. AQ2 · Practice

Model answer. The proton number is 9, so the atom has 9 protons. The mass number is 19, so protons + neutrons = 19 and neutrons = 19 − 9 = 10. The atom is neutral, so it has 9 electrons, the same as its protons.

Checked automatically

• Protons: 9 (exact value)
  • Common wrong answer 19 : 19 is the mass number, protons and neutrons together. The proton number is the bottom number.
• Neutrons: 10 (exact value)
  • Common wrong answer 19 : 19 counts protons and neutrons together. Take away the 9 protons.
  • Common wrong answer 28 : You added the two numbers. The mass number already includes the protons, so subtract: 19 − 9.
• Electrons: 9 (exact value)
  • Common wrong answer 10 : That's the number of neutrons. In an atom the number of electrons equals the number of protons.
  • Common wrong answer 19 : Electrons aren't counted in the mass number. A neutral atom has as many electrons as protons.

A full answer includes

• Protons: 9, the proton number.
• Neutrons: 19 − 9 = 10, the mass number minus the proton number.
• Electrons: 9, because an atom has as many electrons as protons.

Also accept

• 9 protons, 10 neutrons, 9 electrons.

Watch for

• “The atom has 19 neutrons.” — 19 is the mass number: protons and neutrons together. Take away the 9 protons to find the neutrons.

3. AQ3 · Practice

Model answer. 3 and 5. The element is decided by the number of protons. Adding a proton gives 12 protons, magnesium; taking one away gives 10 protons, neon. Changing the electrons makes an ion of sodium, and changing the neutrons makes a lighter or heavier sodium atom: 11 protons is still sodium.

A full answer includes

• I chose 3 and 5: adding a proton or taking one away changes the proton number, so the element changes.
• I explained that taking away an electron leaves 11 protons, so it's still sodium: a sodium ion with a charge of +1.

Also accept

• 3 and 5: only changes to the number of protons.

Watch for

• “Adding two electrons changes the element, because it changes the charge.” — It changes the charge, from 0 to −2, but the nucleus still has 11 protons. It would be an ion of sodium, not a new element.
• “Adding a neutron changes the element, because the mass number changes.” — The mass number goes from 23 to 24, but the proton number stays 11. In the experiment the atom stayed sodium.

4. AQ4 · Practice

Model answer. (a) The proton number is the number of protons: 20, so the element is calcium. (b) The mass number is protons + neutrons = 20 + 20 = 40. (c) The charge is protons − electrons = 20 − 18 = +2, so it's a calcium ion, Ca²⁺.

Checked automatically

• (a) Proton number: 20 (exact value)
  • Common wrong answer 18 : 18 is the number of electrons. The proton number counts the protons.
  • Common wrong answer 40 : 40 is the mass number. The proton number counts the protons only.
• (b) Mass number: 40 (exact value)
  • Common wrong answer 58 : Electrons aren't counted in the mass number: only protons and neutrons.
  • Common wrong answer 20 : 20 is the number of protons, or of neutrons. The mass number counts both together.
• (c) Charge, sign first (e.g. +1): +2 (exact value)
  • Common wrong answer -2 : There are fewer electrons than protons, so the protons' positive charge wins: +2.

A full answer includes

• (a) Proton number 20: the number of protons.
• (b) Mass number 20 + 20 = 40: protons and neutrons.
• (c) Charge 20 − 18 = +2, written with its sign (+2, not just 2): two more protons than electrons.

Also accept

• (a) 20; (b) 40; (c) +2.

Watch for

• “The charge is −2, because two electrons are missing.” — Missing electrons leave the protons' positive charge uncancelled. Two more protons than electrons gives +2.

5. AQ5 · Practice

Model answer. (a) The nucleus at the centre contains 19 protons and 39 − 19 = 20 neutrons. Around it, 19 electrons are arranged in shells. (b) Each proton has a charge of +1 and each electron −1. There are 19 of each, so the charges cancel and neutrons add no charge. (c) An electron's relative mass is only 1/1840 of a proton's. Even 19 electrons together have about 19/1840 ≈ 0.01 of the mass of one proton, while the nucleus has a mass of 39.

A full answer includes

• (a) I stated 19 protons and 39 − 19 = 20 neutrons in the nucleus, and 19 electrons in shells around it.
• (b) I explained that 19 protons, each +1, are cancelled by 19 electrons, each −1.
• (c) I explained that an electron has a relative mass of 1/1840, compared with 1 for a proton or neutron, so 19 electrons add only about 1/100 of the mass of one proton.

Also accept

• (a) 19 p and 20 n in the nucleus, 19 e in shells; (b) 19 protons +1 cancel 19 electrons −1; (c) each electron has only 1/1840 of the mass of a proton.

Watch for

• “The atom is neutral because neutrons cancel the charges.” — Neutrons have no charge, so they can't cancel anything. The atom is neutral because it has as many electrons (−1) as protons (+1).
• “Potassium has 39 electrons.” — 39 is the mass number. The number of electrons in an atom equals the proton number, 19.

6. AT1 · Apply

Model answer. (a) Proton number 1: 1 proton. Mass number 1: 1 − 1 = 0 neutrons. Neutral atom: 1 electron. (b) Charge = protons − electrons, so +1 = 1 − electrons, which gives 0 electrons. (c) This hydrogen atom has no neutrons, and the ion has lost its only electron. What is left is a nucleus made of one proton and nothing else, so H⁺ really is just a proton.

A full answer includes

• (a) 1 proton, 1 − 1 = 0 neutrons, 1 electron.
• (b) +1 = 1 − electrons, so H⁺ has 0 electrons.
• (c) I explained that H⁺ has lost its only electron and has no neutrons, so all that is left is one proton.

Also accept

• (a) 1 p, 0 n, 1 e; (b) 0 electrons; (c) no neutrons and no electrons, so only one proton is left.

Watch for

• “Every atom has at least one neutron.” — Most hydrogen atoms have none: mass number 1 minus proton number 1 is 0. It's the only kind of atom without neutrons.
• “H⁺ has 2 electrons, because + means one is added.” — A + charge means fewer electrons than protons. One proton and no electrons gives +1.