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Group-17 Elements (Halogen Family)

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 seawater, and hence they are also known as halogens (Halos, in Greek, means sea salt producers). The group collectively is called halogen family.
 
The general electronic configuration of halogens is ns2np5. They are just one electron short of stable electronic configuration of inert gas elements. They have a very strong tendency to acquire stable configuration by accepting one electron. For this reason they exhibit non-metallic behavior. Astatine, the last member of this group, is a radioactive element with very short half-life.

General trends in physical properties

Property Element
Fluorine
Chlorine
Bromine
Iodine
Astatine
Covalent radius (pm)
72
99
114
113
Ionic radius (X) (pm)
133
184
196
220
Ionization energy (kJ/mol)
1680
1256
1142
1008
Electronegativity
4.0
3.2
3.0
2.7
2.2
Melting point (K)
54.4
172
265.8
386.6
Boiling point (K)
84.9
239
332.5
458.2
Heat of dissociation X2 2X (kJ/mol)
158.8
242.6
192.8
151.1
  1. Melting and boiling points: Melting and boiling points increase with the increase in atomic number. The enthalpy of fusion as well as enthalpy of vaporization also increases as we go down the group. This indicates that the strength of intermolecular forces of attraction between the molecules increases with increase in atomic number.
  2. Atomic and ionic radii: Atomic radii of the elements of this group are the smallest in their respective periods. Both the atomic radii and ionic radii for the anion X increase regularly down the group because the electrons are added to higher and higher shells.
  3. Ionization energies: Ionization energies of the all the halogens are very high. Therefore, they have very little tendency to loose electrons. However, this tendency increases down the group because the nuclear force of attraction on valence electrons decreases. Iodine is capable of forming stable compounds in which it exists as I+ ion.
  4. Physical state: All the halogens are diatomic and exist as F2, Cl2, Br2, and I2. The intermolecular forces are very weak and their magnitude increases down the group. Thus, F2 and Cl2 are gases, bromine is a volatile liquid, and iodine is a volatile solid.
  5. Color: Halogens are colored. The color of the halogen is due to absorption of certain wavelengths of visible light by their molecules, resulting in the excitation of outer electron to higher energy orbitals.
  6. Non-metallic character: All the halogens are non-metals because of their very high ionization energies. The non-metallic character, however, decreases with the increase in atomic number.
  7. Electron affinities: The halogens have strong tendency to accept electrons. Their electron affinities are highest in their respective periods. On moving down the group, the electron affinity values generally decrease with the increase in size of the atom.
  8. Oxidation states: The most common oxidation state of all the halogens is –1, as they attain stable configuration by accepting one electron. In fact this is the only oxidation state shown by fluorine because it is the most electronegative element known. Other elements of this group also show oxidation states of +1, +3, +5, and +7. Higher oxidation states of these elements are due to the presence of vacant d-orbitals.
     
    HClO4 is an extremely strong acid, while HOCl is very weak acid. The dissociation of an oxoacids involves two energy terms:
    1. Breaking an O – H bond to produce a hydrogen ion and an anion.
    2. Hydrating both ions.
Plainly the Description: 51531.png ion is larger than the OCl ion, so the hydration energy of Description: 51538.png is less than that of OCl. This would suggest that HOCl should ionize more readily than HClO4. Since we know the reverse to be true, the reason must be the energy required to break the O – H bond.
 
Oxygen is more electronegative than chlorine. In the series of oxoacids—HOCl, HClO2, HClO3, HClO4—an increasing number of oxygen atoms are bonded to the chlorine atom. The more oxygen atoms that are bonded, the more the electrons will be pulled away from the O – H bond and the more this bond will be weakened. Thus, HClO4 requires the least energy to break the O – H bond and form H+. Hence, HClO4 is the strongest acid. In general, for any series of oxoacids, the acid with the most oxygen (that is the one with the highest oxidation number) is the most dissociated. Thus, the acid strengths decreases in the order HClO4 > HClO3 > HClO2 > HOCl. In exactly the same way, H2SO4 is a stronger acid than H2SO3, and HNO3 is a stronger acid than HNO2.

Pseudohalogens and pseudohalides

A few ions are known, consisting of two or more atoms of which at least one is N, that have properties similar to those of the halide ions. They are therefore called pseudohalide ions. Pseudohalide ions are univalent, and these form salts resembling the halide salts. For example, the sodium salts are soluble in water, but the silver salts are insoluble. The hydrogen compounds are acids like the halogen acids HX. Some of the pseudohalide ions combine to form dimers comparable with the halogen molecules X2. These include cyanogens (CN)2, thiocyanogen (SCN)2, and selenocyanogen (SeCN)2.
 
Anion
Acid
Dimer
CN
Cyanide ion
HCN
Hydrogen cyanide (CN)2
Cyanogen
SCN
Thiocyanate ion
HSCN
Thiocyanic acid (SCN)2
Thioxyanogen
SeCN
Selenocyanate ion
(SeCN)2
Selenocyanogen
OCN
Cyanate ion
HOCN
Cyanic acid
NCN2–
Cyanamide ion
H2NCN
Cyanamide
N3
Azide ion
HN3
Hydrogen azide

 
The best known pseudohalide is CN–1. This resembles Cl–1, Br, and I in the following respects:
  • It forms an acid HCN.
  • It can be oxidized to form a molecule cyanogen (CN)2.
  • It forms insoluble salts with Ag+, Pb2+, and Hg+.
  • Interpseudohalogen compounds CICN, BrCN, and ICN can be formed.
  • AgCN is insoluble in water but soluble in ammonia, as is AgCl.
  • It forms a large number of complexes similar to halide complexes, e.g., [Cu(CN)4]2– and [CuCl4]2–, and [Co(CN)6]3– and [CoCl6]3–.




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