The general electronic configuration of halogens is ns2np5 The electronic configurations of halogens are given in Table 18.9.
All the halogens are very reactive but amongst them fluorine is the most reactive. As we move down the group, reactivity decreases. This is due to the decrease in electronegativity and increase in bond dissociation energy. In general, a halogen of low atomic number oxidises halide ions of higher atomic number i.e., F2 displaces CI2, Br2 and I2 from their salts, Cl2 displaces Br2 and I2 whereas Br2 displaces iodine from its salts, e.g.,
F2 + 2X à 2r + X2 (X = Cl , Br , I)
F2 + 2KX à 2KF + X2 (X = Cl , Br , I)
Cl2 + 2x- à 2CI- + X2 (X = Br , I)
Br2 + 2I- à2Br + I2
Halogens exist in the elemental form as diatomic molecule having characteristic colours. Fluorine exhibits only negative oxidation state of -1. Other halogens in addition to -1 oxidation state exhibit formal positive oxidation states.
All the elements of the halogen family have a tendency to acquire noble gas electronic configuration by either accepting an electron resulting in the formation of monovalent anion or by sharing one electron with those of other elements. Thus, they show an oxidation state of – 1 or + 1 depending on whether the element combining with halogens is less electronegative or more electronegative than halogen. Since fluorine is most electronegative element it always has -1 oxidation state and never has a positive oxidation state. Also higher oxidation states are not possible in fluorine as it does not have vacant d-orbitals.
(Oxidation State =- 1 + 1 )
First Excited State
(Oxidation State= +3)
Second Excited State
(Oxidation State= +5)
Third Excited State
(Oxidation State= +7)
Other elements of this group show oxidation states of + 1, + 3, + 5 and + 7 in addition to- 1 state which is most common. Higher oxidation states of these elements are due to the presence of_ vacant d-orbitals. The outer s or p electrons can easily be excited to these vacant d-orbitals as shown below in case of iodine.
All the halogens have high electron affinity and high electronegativity. Therefore, these show similarity in the chemical nature.
Table 18.9. Electronic Configuration of Halogens
In additions to neutral interhalogen compounds poly halide ions containing more than two halogen atoms have also been prepared. These are formed by the combination between the halide ion with another halogen atom or with some compound of halogen. For example, 13- (triodide / ion is formed by combination of~ molecule and iodide in (I-).
Some other examples of polyhalide anions are IC12 -, fBr2-, IC14-, BrF4- , IF6- , etc. Besides anions, I3+ is a common example of poly halide cation . This ion is found to exist in compounds like iodine triacetate, I(CH3COO)3 iodine phosphate (IP04) and iodine perchlorate, I(CI04)3.
Halogens can readily form halide ion by gaining an electron. The halide ions can be identified easily by adding silver nitrate solution to the solution of halide ion on the presence of dilute nitric acid. The halides ions can be easily identified on the basis of colour of precipitates or by reacting with ammonia solution. The tests are summarized in Table 18.10.