18.7 GROUP-17 ELEMENTS-THE HALOGEN FAMILY
The non-metallic elements Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At) are grouped together to form group 17 of the Periodic Table. Their salts are present in sea-water and hence they are collectively known as halogens (Greek, Halos means sea salt producers). The group collectively is called halogen family. All halogens precede the noble gases in the periodic table because their atoms contain one electron less than the electronic configuration of the inert gas. They have very strong tendency to acquire stable inert gas configuration by accepting one electron. For this reason halogens exhibit non-metallic behaviour. Astatine the last member of halogen family is a radioactive element with a very short life. All other halogens are abundant in nature in the combined state.
The sources of halogens along with their relative abundance are given in Table 18.6.
PHYSICAL PROPERTIES OF HALOGENS (GROUP 17 ELEMENTS)
There is a regular gradation in the physical as well as chemical properties of halogens. These are:
1. Physical State. The halogens are all diatomic and exist as F2, Cl2, Br2 and I2 The intermolecular forces are very weak in halogens. The nature of forces is van der Waals’ and their magnitude increases down the group. Thus, F2 and C~ are gases, bromine is a volatile liquid and iodine is a volatile solid.
2. Colour. Halogens are coloured.
The colour of different halogens are as follows:
Table 18.6. Sources and Relative Abundance of Halogens
Halogen Fluorine Chlorine Bromine Iodine
Colour Pale Greenish Reddish Dark
Yellow yellow brown violet
Reason. The colour of the halogen is due to absorption of visible light by their molecules resulting in the excitation of outer electron to higher energy levels. Fluorine being smaller in size absorbs violet light (of high energy) for excitation and appears pale yellow while iodine absorbs yellow light (of less energy) for excitation and therefore appears violet black In between, the colour of chlorine is yellowish-green and of bromine is reddish-brown or orange.
Thus, the colour deepens down the group.
3. Melting and Boiling Points. Melting and boiling points increase with increase in atomic number. The heat of fusion as well as heat of vapourisation also increase with the increase in atomic number. This indicates that the strength of intermolecular forces of attraction between the molecules increases with the increase in atomic number.
4. Interatomic Bond Energy. The interatomic bond energy of group 17 elements shows a periodic variation. In general, the bond dissociation energy decreases down the group as atomic radius increases. With the increase in size of atoms, the shared electron cloud in the covalent bond experiences less force of attraction hence a weaker bond forms. The energy needed to break the weaker covalent bond therefore decreases. Fluorine shows an. Unexpected (anomalous) bond dissociation energy (atomization energy, Eat ) due to its very small size. The electrons in the shells of the two atoms forming the bond are packed near each atom because of their small size, and repel each other strongly. This reduces the covalent bond attraction between the positive nuclei and the shared electrons. The bond is weakened and the energy needed to break it is greatly reduced compared with that of the other halogens.
Some of the properties of group 17 elements are given in Table 18.7.
Table 18.7. Atomic and Physical Properties of Halogens
a Radioactive ; b Pauling scale; c For the liquid at temperatures (K) given in the parentheses; d solid; e The half-cell reaction is X2(g) + 2e- à 2x- (aq).
Table 18.8 Variation of Atomization Energy
Halogen Covalent radius / nm Eat / kj mol-1
F2 0.071 79
Cl2 0.099 121.7
Br2 0.114 111.9
I2 0.133 106.8
5. Atomic and Ionic Radii. Atomic and ionic radii are small and increase regularly down the group from fluorine to iodine because new electronic shells are added.
6. Ionization Energies. Ionization energies of all the halogens are very high. Therefore, they have a less tendency to lose electron. However, this tendency increases down the group because the distance of valence shell from nucleus increases. Iodine is capable of forming stable compounds in which it exists as r+ ion.
7. Non-metallic Character. All the halogens have very high values of ionization energies and exhibit non-metallic character. The non-metallic character, however, decreases down the group. Iodine shows some distinct metallic properties e.g. , it possesses metallic lustre and forms positive ions like I+, J3+ etc.
8. Electron Gain Enthalpy. Since halogens have strong tendency to accept electrons, they have high values of electron gain enthalpy. On moving down the group electron gain enthalpy values decrease because size of the atom increases. However, it has been observed that fluorine has less electron affinity than chlorine. A low value of electron affinity for fluorine is due to the small size of fluorine atom. In a compact 2p subshell of fluorine the added electron does not feel much attraction resulting in low value of electron gain enthalpy.
9. Nature of Bonds with Other Elements. The halogens combine with metals to form predominantly ionic halides. The ionic character of these halides decreases while covalent character increases on going down the halogen group. This is due to the reason that with increase in size of the halide ion, its polarizability increases and this imparts covalent character to the compound. For example, AIF3 is predominantly ionic whereas AIC13 and AIBr3 are predominantly covalent.
With metals in higher oxidation states or with non-metals, halogens form covalent compounds which are polar in character. Polarity of these compounds decreases as we move down the group.
Due to similarity in chemical nature these readily combine with each other to form polyhalide ions.