Group 17 Elements

Group 17 includes Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). They are collectively known as the halogens (Greek halo means salt and genes born i.e., salt producers). The halogens are highly reactive non-metallic elements.
·       Electronic Configuration: All these elements have seven electrons in their outermost shell (ns2 np5) which is one electron short of the next noble gas.

·       Atomic and ionic radii: Halogens have the smallest atomic radii in their respective periods because of maximum effective nuclear charge. Atomic and ionic radii increase from fluorine to iodine due to increasing number of shells.

·       Ionisation Enthalpy: They have very high ionization enthalpy because of small size as compared to other groups. Due to increase in atomic size, ionisation enthalpy decreases down the group.

·       Electron Gain Enthalpy:
a) Halogens have maximum negative electron gain enthalpy because these elements have only one electron less than stable noble gas configuration.
b) Electron gain enthalpy becomes less negative down the group because atomic size increases down the group.
c) The negative electron gain enthalpy of fluorine is less than that of chlorine. It is because, in fluorine the incoming electron goes to the 2p subshell, but in Cl it enters in to the 3p subshell. Due to the compactness of 2p subshell compared to 3p subshell, the electron – electron repulsion is greater in fluorine than in chlorine. So, F does not easily gain electron.

·      Electronegativity: They have very high electronegativity. The electronegativity decreases down the group. Fluorine is the most electronegative element in the periodic table.

·       Bond Dissociation Enthalpy:
a) Bond dissociation enthalpy follows the order: Cl2 > Br2 > F2 > I2
b) This is because as the size increases bond length increases.
c) Bond dissociation enthalpy of Cl2 is more than F2 because there are large electronic repulsions of lone pairs present in F2.

·       Colour: All halogens are coloured because of absorption of radiations in visible region which results in the excitation of outer electrons to higher energy levels.

·       Oxidising power:
a) All halogens are strong oxidising agents because they have a strong tendency to accept electrons.
b) Order of oxidizing power is: F2 > Cl2 > Br2 > I2

·       Physical Properties: Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Their melting and boiling points steadily increase with atomic number.

·       Oxidation State:
a)     All the halogens exhibit –1 oxidation state. However, chlorine, bromine and iodine exhibit + 1, + 3, + 5 and + 7 oxidation states also.
b)     The higher oxidation states of chlorine, bromine and iodine are realized mainly when the halogens are in combination with the small and highly electronegative fluorine and oxygen atoms e.g., in interhalogens, oxides and oxoacids.
c)     The fluorine atom has no d orbitals in its valence shell and therefore cannot expand its octet. Being the most electronegative, it exhibits only – 1 oxidation state.
d)     The ready acceptance of an electron is the reason for the strong oxidising nature of halogens. F2 is the strongest oxidising halogen and it oxidises other halide ions in solution or even in the solid phase.
e)     Fluorine oxidises water to oxygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids.
2F2 (g) + 2H2O(l) ® 4H+ (aq) + 4F– (aq) + O2 (g)
X2 (g) + H2O (l) ® HX(aq) + HOX (aq) (where X = Cl or Br)
f)      More the value of the SRP, more powerful is the (algebraically) oxidising agent. Hence the order of oxidising power is F2 > Cl2 > Br2 > I2. Since Standard Reduction Potential is the highest for F2, it is a strongest oxidising agent.