6.13 - Explaining halogen reactivity
Explain why halogen reactivity decreases down Group 7 using occupied electron shells, shielding and attraction for an incoming electron.
Electronic configurations and ion formation
A halogen atom has seven electrons in its outer shell. When forming a halide ion, it gains one electron to complete that shell. Halogens can also form covalent bonds; this lesson uses electron gain to explain their displacement reactivity.
For halide formation, greater reactivity means a greater tendency to gain an electron.
For example, a chlorine atom gains one electron to form a chloride ion:
This is the opposite pattern from Group 1 metals. Group 1 atoms form ions by losing their outer electron. Halogen atoms form ions by gaining an electron. That difference matters, because the most reactive halogen is the one that attracts and gains an electron most easily.
Displacement reactions from earlier Group 7 work show the order of reactivity, but the explanation in this lesson comes from electronic configurations. Redox language also links to this idea, because gaining electrons is reduction, but this lesson focuses on why the gain is easier for some halogens.
Elements in the same group have the same number of electrons in their outer shell. Group 7 atoms all have seven outer-shell electrons, so they are one electron short of a full outer shell.
| Halogen | Electronic configuration point needed for this lesson |
|---|---|
| Fluorine | 2,7: two occupied shells, seven outer electrons |
| Chlorine | 2,8,7: three occupied shells, seven outer electrons |
| Bromine | 2,8,18,7: four occupied shells, seven outer electrons |
| Iodine | 2,8,18,18,7: five occupied shells, seven outer electrons |
You do not need to derive the full electronic configurations of bromine and iodine from scratch here. The useful pattern is simpler:
- all halogens have seven electrons in the outer shell
- moving down Group 7 adds more occupied electron shells
- the outer shell is further from the nucleus down the group
Be precise with the wording. Iodine has more occupied electron shells than chlorine, but iodine still has only one outer shell. Saying "iodine has more outer shells" is wrong because every atom has one outermost shell.
Same group means the same number of outer electrons; moving down the group means more occupied shells between the nucleus and the outer shell.
Why attraction changes down Group 7
A halogen atom becomes a halide ion by gaining an electron. That incoming electron is negatively charged, and the nucleus is positively charged, so there is electrostatic attraction between them.
The strength of that attraction depends on the electronic configuration:
- if the atom has fewer occupied shells, the incoming electron is closer to the nucleus
- if the atom has more occupied shells, inner electrons shield the incoming electron from the nucleus more
- stronger attraction means the atom gains the electron more easily
Continue this lesson.
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