Explaining the trend in the reactivity of the halogens

5 marks · Hard

This question is about explaining halogen reactivity in terms of electronic configuration.

(a)

State, in terms of electron shells, the difference in the position of the outer electrons of a chlorine atom and a bromine atom.

[1 mark]

(b)

Explain, in terms of electronic configuration, why a chlorine atom is more reactive than a bromine atom.

[2 marks]

(c)

A chlorine atom reacts to form a chloride ion. Write the electron configuration for the chloride ion.

[1 mark]

(d)

Using your answers and knowledge of shells, describe what happens, in terms of electrons, when bromine forms a bromide ion.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) The outer electron of chlorine is in the third shell; the outer electron of bromine is in the fourth shell. · must show 3rd shell (Cl), 4th shell (Br)

(b) Chlorine is more reactive than bromine because the extra electron is added to a shell closer to the nucleus in chlorine, so it feels a stronger attraction, making it easier for chlorine to gain an electron and form a 1– ion. · must show Chlorine’s extra electron added closer to nucleus, stronger attraction, easier to gain electron

(d) The bromine atom gains one electron to form a bromide ion. · must show Bromine atom gains one electron to form Br-

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States chlorine’s outer electron in third shell, bromine’s in fourth shell, or equivalent wording.

M1 (b) · 1 mark · Mentions that in chlorine, the extra electron is added to a shell closer to the nucleus/stronger attraction.

A1 (b) · 1 mark · Correctly links stronger attraction to greater reactivity; explains that chlorine more easily gains an electron.

B2 (c) · 1 mark · Gives the correct electronic configuration for the chloride ion: 2,8,8.

B3 (d) · 1 mark · States that bromine atom gains one electron to form bromide ion (Br–).

Worked solution

a The outer electron of chlorine is in the third shell; the outer electron of bromine is in the fourth shell.

b Chlorine’s extra electron is added to a shell closer to the nucleus, so the attraction is stronger. This makes it easier for chlorine to gain an electron compared to bromine, so chlorine is more reactive.

c The electron configuration for the chloride ion is 2,8,8.

d The bromine atom gains one electron to form a bromide ion.