Like the first member of other groups in the periodic table, fluorine differs considerably from other halogens. This is due to following characteristics of fluorine.
(a) Small size.
(b) High electronegativity (fluorine is the most electronegative of all the elements).
(c) Non-availability of d-orbitals in valency shell.
(d) Low dissociation energy of the F-F bond in F2. This is due to strong repulsion between the non-bonded electrons and absence of the possibility of multiple bonding due to absence of d-orbitals.
The above pecularities of fluorine lead to the following differences.
1. It exhibits only -1 oxidation state. This is due to its most electronegative nature.
2. The maximum covalency of fluorine is one because it cannot expand its valency shell beyond the octet as there is no vacant d-orbital in the valency shell. Other halogens can exercise covalencies upto 7 because of vacant d-orbitals.
3. Although fluorine is more electronegative than chlorine, its electron affinity is low than that of chlorine atom. This is due to very small size of fluorine atom with high electron density.
4. Due to its high electronegativity and small size, fluorine forms hydrogen bonding in its hydrides. Thus under ordinary conditions, HF is liquid (b.p. 19°C) while HCl, HBr and HI are gases.
5. Due to low F-F bond energy (38.5 kcal/mole), fluorine is the most reactive among halogens as represented below.
(a) It displaces other halogens from the corresponding halides.
(b) It reacts violently with halogens at a very low temperatures and in dark.
(c) It decomposes water violently and liberates oxygen.
(d) It directly combines with carbon, while other halogens do not react even under drastie conditions.
(e) it forms strong bonds with most of the other elements and thus forms stable compounds.
6. Hydrofluoric acid is a weak acid while other halogen acids (HCl, HBr and HI) are strong acids. This is due to strong H-F bond.
7. Solubility of fluorides is almost opposite to that of other halides; AgF is soluble in water while other silver halides are insoluble; CaF2 is insoluble while the other calcium halides are soluble.
8. Fluorides have the maximum ionic character than the other halides. Thus only AlF3 and SnF4 are ionic while other aluminium and tin (IV) halides are covalent.
9. Fluorides are more stable than the corresponding chlorine compounds, e.g. UF6 is more stable than UCl6; NF3 is non explosive while NC13 is violently explosive.
10. Fluorine does not form any polyhalide while other halogens form polyhalides, e.g. 13, Br3 and Cl3 ions. This is due to absence of d-orbitals in the valency shell of fluorine.
11. The fluoride ion (F) can form the anion HF2 due to hydrogen bonding (F--H-F)¯. Hence we can explain the existence of KHF2 written as K+ (F--H-F)-, while KHCl2 or KHBr2 are unknown.
