In Group 7, the reactivity of the elements decreases going down the group.

The idea

Group 7 halogens react by gaining one electron to form a 1⁻ ion. Going down the group, the outer shell becomes further from the nucleus and there are more inner shells shielding the outer electrons. Although nuclear charge increases, the increased distance and shielding mean the attraction for an incoming electron is weaker, so reactivity decreases. This is shown by displacement reactions: a more reactive halogen higher in the group displaces a less reactive halide lower down. For example, chlorine displaces bromine from potassium bromide solution, but bromine cannot displace chlorine from potassium chloride solution. The trend is assessed by explaining electron gain and by predicting displacement reactions.

What earns the marks

• Halogens react by gaining one electron to form a 1⁻ ion, so reactivity depends on how easily the outer shell attracts an electron.
• Down the group, the outer shell is further from the nucleus and there are more inner shells, increasing shielding.
• The increased distance and shielding outweigh the increased nuclear charge, so the attraction for an incoming electron is weaker.
• A more reactive halogen higher in the group displaces a less reactive halide lower in the group from its compound.
• Displacement observations, such as a colour change in solution, provide evidence for the reactivity trend.

Common mistakes and how to put them right

• Saying reactivity increases down the group because atoms get larger.

  Put it right by explaining that larger atoms have weaker attraction for an incoming electron, so reactivity decreases.

• Ignoring shielding and only referring to distance.

  Put it right by stating that more inner shells shield the outer shell and reduce the effective nuclear attraction.

• Predicting that a lower halogen can displace a higher halide.

  Put it right by applying the rule that the more reactive halogen higher in the group does the displacing.

Exam tips

• Link reactivity to electron gain and the formation of a 1⁻ ion, then explain the effect of distance and shielding.
• For displacement questions, name the halogen and the halide, state which is more reactive, and describe the expected observation.
• Use comparative language such as more reactive than and less reactive than to make the trend clear.