The Halogens

The Halogens are the elements in Group 7 of the periodic table.

In order of atomic number they are Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At).

All of the elements in Group 7 have similar physical properties.

They all have coloured vapours and do not conduct heat or electricity.

They are all made of diatomic molecules (they go around as pairs of atoms covalently bonded together) - for example chlorine molecules have the formula Cl₂. 

They all have low melting and boiling points. Fluorine is a pale yellow gas at room temperature, with a melting point of -220^oC and boiling point of -188^oC. Chlorine is a green gas at room temperature, with a melting point of -101^oC and boiling point of -35^oC. Bromine is a red-brown liquid at room temperature, with a melting point of -7^oC and boiling point of 59^oC. Iodine is a purple-black solid at room temperature, with a melting point of 114^oC and boiling point of 184^oC, but it tends to sublime directly from solid to gas.

All halogen atoms have seven electrons in their outer energy levels (shells), so they have very similar chemical properties.

For example, all halogens react with metals to form ionic compounds in which the halide ion has a 1- charge. This is because the halogen atom receives one electron to produce a full outer shell. The most common example of a halide compound is sodium chloride, formed from the reaction between sodium and chlorine.

Halogens also react with other non-metals to form molecular compounds containing covalent bonds. For example, chlorine reacts with hydrogen to form hydrogen chloride molecules.

Going down Group 7, the halogens become less reactive. Fluorine is one of the most reactive non-metal elements, but iodine is much less reactive.

This is because the outer shells are further away from the nucleus. Halogens react by gaining an electron, but there is less attraction for electrons further away from the nucleus, so a reaction is less likely to happen.