Halogens (Group 17)
Fluorine, chlorine, bromine, iodine, astatine and tennessine: group 17, one electron short of a full shell.
What they have in common
Halogens end in ns² np⁵. Picking up one more electron completes the octet, so they form 1− ions (halides) and are powerful oxidising agents. Fluorine is the most electronegative element of all (3.98 on the Pauling scale).
Reactivity falls down the group, the reverse of group 1, because the incoming electron is held less tightly further from the nucleus. At room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.
- Seven valence electrons
- Form 1− ions
- Exist as diatomic molecules (F₂, Cl₂, Br₂, I₂)
- Reactivity decreases down the group
Halogens at a glance
| Element | Z | Mass | Configuration | Valence e⁻ | Charges | EN | Radius (pm) |
|---|---|---|---|---|---|---|---|
| Fluorine (F) | 9 | 18.998 | [He] 2s2 2p5 | 7 | − | 3.98 | 71 |
| Chlorine (Cl) | 17 | 35.453 | [Ne] 3s2 3p5 | 7 | − | 3.16 | 99 |
| Bromine (Br) | 35 | 79.904 | [Ar] 3d10 4s2 4p5 | 7 | − | 2.96 | 114 |
| Iodine (I) | 53 | 126.904 | [Kr] 4d10 5s2 5p5 | 7 | − | 2.66 | 133 |
| Astatine (At) | 85 | [210] | [Xe] 4f14 5d10 6s2 6p5 | 7 | − | 2.2 | — |
Questions students ask
Why are halogens so reactive?
They need just one electron to fill their outer shell, and they attract electrons strongly.
What does “halogen” mean?
Salt-former, from Greek: they combine with metals to make salts such as NaCl.