Cl and Br also show +4 and +6 oxidation states in a compound like ClO 2, BrO 2 and Cl 2O 6, BrO 3. Bromine does not show +7 oxidation state. The various positive oxidation states exhibited by Cl, Br and I are +3 (in ClF 3, BrF 3, IF 3), +5 (in HClO 3, HBrO 3, HIO 3) and +7 (in HClO 4, Cl 2O 7, HIO 4). The valence electrons from the ground state can be raised to the excited states to produce 3, 5 and 7 half-filled orbitals for chemical bonding. These halogens (Cl, Br, I) also show other positive oxidation states due to the presence of vacant d-orbitals in their respective valence shell. In hydrohalides (HF, HCl,HBr, HI) all the halogens are in -1 oxidation state. Halogens like Cl, Br and I show mostly positive oxidation state like +1 due to their comparatively low electronegativity in inter-halogens compounds like Br-Cl (Bromine Chloride), I-Br, Cl-F as well as in oxy-halogen compounds like HClO (Hypochlorous acid), HBrO, HIO etc. Thus, halogens show an oxidation state of -1 or +1 depending upon whether the element with which they combine is less electronegative or more electronegative than halogens. (ix) Oxidation States- The elements of group 17 are one electron short of the nearest noble gas configuration and have a strong tendency to attain the stable configuration of the nearest noble gas either by gaining an electron to form an ionic bond or by sharing an electron to form a covalent bond. However, on moving down the group from F to I, ionisation energy shows a decreasing trend due to increase in atomic size and screening effect which causes decrease in the forces of attraction between nucleus and valence shell electrons and, hence, less energy is required for the removal of electrons. (viii) Ionisation energy- Due to their small atomic size, the members of group 17 have quite high values of ionsiation energy. There is a repulsion between the electrons already present and the electron being added and consequently, Fluorine has less tendency to accept the electron and, hence, its electron affinity is low. The electron affinity of ‘F’ is, however, lower than that of ‘Cl’ due to the small size of F atom. However, on moving down the group from F to I, the atomic size increases due to which the effective nuclear attraction for the electron decreases and, hence, the electron-affinities of the members of group 17 shows a decreasing trend. (vii) Electron Affinity- The halogens have strong tendency to hold an additional electron in their valence shell to acquire the stable configuration of the nearest noble gas and as such have high values of electron affinity. However, on moving down the group 17, the electronegativity values gradually decrease due to an increase in the size of atoms. Fluorine, the first members of the family is the most electronegative element in the periodic table with an electronegativity value of 4.0. (vi) Electronegativity- Electronegativity of the members of the halogen family is very high. (v) Melting points and Boiling points- The melting points and boiling points of the members of halogen family shows an increasing trend on moving down the group due to increase in the magnitude of van der Waal’s forces. (iii) Density- The densities of elements of group 17 increases gradually on moving down the group from ‘F’ to ‘At’. Fluorine and Chlorine are gases, Bromine is a fuming liquid while I 2 is solid at room temperature.
(ii) Physical State- These elements exist as diatomic covalent molecules like F 2, Cl 2, Br 2 and I 2. (i) Electronic Configuration- The general outer electronic configuration of the members of the halogen family is ns 2 np 5 (n = 2 to 6) indicating that these elements have seven electrons in the valence shell (2 in s-orbital and 5 in p-orbitals). General Trends in Physical Properties of Halogen Family:
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