The halogens exist as diatomic molecules X2 at room temperature and pressure. Melting and boiling points increase down the group.
| Halogen | Number of electrons | Boiling point °C | Appearance and state at RTP |
| F2 | 18 | -188 | pale yellow gas |
| Cl2 | 34 | -34 | pale green gas |
| Br2 | 70 | 59 | red-brown liquid |
| I2 | 106 | 184 | shiny grey-black solid |
| At2 | 170 | 230 | never been seen |