The halogens form a range of compounds with oxygen, but many of these are unstable.

Oxides are formed in the range E₂O to E₂O₇.

Oxoacids are formed in the range HOE to HOEO₃ (there is only HOF with fluorine).

Oxoanions are formed in the range EO^- to EO₄^-.

Halogen oxides

Fluorine forms two compounds with oxygen. Oxygen difluoride, OF₂, is formed from the reaction of F₂ with hydroxide ions, and has the C_2v structure of water as predicted by VSEPR. Dioxygen difluoride, FOOF, is produced by photolysis of a mixture of F₂ and O₂, and is a very good fluorinating agent, eg. in the fluorination of Plutonium.

Preparation of OF2:
Oxidation of Plutonium:

Chlorine oxides occur with many chlorine oxidation numbers. Cl₂O₇ is the most stable of the oxychlorides. Cl₂O has the bent structure of F₂O, but here the Cl-O-Cl bond angle is smaller due to the presence of Opπ-Cldπ interactions, which favour the use of p rather than sp³-hybrid orbitals on the O for bonding, and hence have a bond angle closer to the 90^o of the orthogonal p-orbitals.

ClO₂ is used as a bleaching agent and disinfectant when used in dilute solutions. The oxidizing properties of Cl^IV make it useful for these purposes.

Halogen Oxoacids and Oxoanions

The structure of the oxoacids of the halogens can be used to predict their acidities, using Pauling's rule, ie. pK_a = 8-5p for the acid (O)_pE(OH)_q. The usefulness of this relationship can be seen in the oxoacids of chlorine.

HOCl	hypochlorite		p = 0: 8-5p = 8	pKa = 7.53
HOClO	chlorite		p = 1: 8-5p = 3	pKa = 2.00
HOClO2	chlorate		p = 2: 8-5p = -2	pKa = -1.2
HOClO3	perchlorate		p = 3: 8-5p = -7	pKa = -10

The pK_a of HIO₆ is only 3.29, and this seems high, but if one looks at the structure, we see that it is (HO)₅IO, and so p=1 and the pK_a is predicted to be 3. The rule is therefore fairly accurate, and can also be used as a tool for predicting the structure.

It can be seen that HClO₄ is a very strong acid, and so the conjugate base, ClO₄^-, is a very weak base. It is also a very poor Lewis base, and does not form complexes. However, it is also a strong oxidizing agent, and solid perchlorate compounds can form metastable complexes which are explosive.

The oxidizing power of the halogen oxoanions leads to the instability of HOF. Above -40^oC, it reacts with water to give HF and hydrogen peroxide. The XO^- ion is also prone to disproportionation.