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Compounds formed with bonds between different Halogen atoms are known as interhalogen compounds. The binary compounds have formulae XY, XY3, XY5 and XY7 (where X is the heavier halogen atom). Ternary species are also formed, and the species can be neutral, cations or anions.
The reactions of the halogen oxoanions have a strong dependence on the conditions. The oxidizing power is greatly altered by pH: in acidic solution, the standard reduction potential for the perchlorate ion ClO4– is +1.20 V, whereas in basic solution it is +0.37 V.
Fluorine, the lightest of the halogens, has several unique features to its chemistry which merit individual discussion: The bond strength of the F2 molecule is unexpectedly low when compared to the other dihalogens. This is due to the repulsion between the lone pairs on the fluorine atoms (non-bonding pair repulsion) which becomes important when they are so close together.
The halogens form a range of compounds with oxygen, but many of these are unstable. Oxides are formed in the range E2O to E2O7.
The halogens have electronic configuration [NG]ns2np5 (where NG implies the relevant Noble Gas), and the accessible oxidation states range from -1 to +7. The stability of the highest oxidation state increases down the group: Fluorine only occurs in the -1 and 0 oxidation states, because it is the most electronegative element and so is never found in a positive oxidation state.