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Contains the answers

Answers · Inside the atom

Inside the atom — return questions key

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

1. AR1 · Return

Model answer. (a) The proton number, or atomic number, is the number of protons in the nucleus of an atom. The mass number, or nucleon number, is the total number of protons and neutrons in the nucleus. (b) In the nucleus: 11 protons, each with relative charge +1 and relative mass 1, and 23 − 11 = 12 neutrons, each with charge 0 and mass 1. In shells around the nucleus: 11 electrons, each with charge −1 and mass 1/1840. (c) Only changing the number of protons changes the element: adding one proton makes magnesium.

A full answer includes

  • (a) Proton number (atomic number): the number of protons in the nucleus of an atom. Mass number (nucleon number): the total number of protons and neutrons in the nucleus.
  • (b) Nucleus: 11 protons (relative charge +1, relative mass 1) and 12 neutrons (charge 0, mass 1); shells around the nucleus: 11 electrons (relative charge −1, relative mass 1/1840).
  • (c) Only adding or removing a proton changes the element, because the proton number decides the element.

Also accept

  • (a) protons in the nucleus; protons + neutrons in the nucleus. (b) 11 p (+1, 1) and 12 n (0, 1) in the nucleus; 11 e (−1, 1/1840) in shells. (c) changing the protons.

Watch for

  • “Electrons are in the nucleus with the protons.” — Only protons and neutrons are in the nucleus. Electrons are in shells around it.

2. AR2 · Return

Model answer. The atom has 19 electrons: 2,8,8,1. The ion K+ has one electron fewer, 18: 2,8,8. With one outer-shell electron potassium is in Group I, and with four occupied shells it is in Period 4.

A full answer includes

  • Potassium atom: 2,8,8,1.
  • Potassium ion K+: 18 electrons, 2,8,8.
  • Group I, because it has 1 outer-shell electron; Period 4, because it has 4 occupied shells.

Also accept

  • K 2,8,8,1; K+ 2,8,8; Group I (1 outer electron), Period 4 (4 shells).

Watch for

  • “Potassium is 2,8,9.” — For the first 20 elements the third shell takes 8, then the fourth starts: 2,8,8,1.