12.1    Physical Properties of  Group VII Elements

Introduction to Group VII elements

• Group VII elements(also called the ‘halogens‘) are p-block elements with a characteristic outer shell configuration of ns²np⁵.

• Some common physical properties of the halogens are as follow:

Note:

1. Bromine is a dark red liquid but forms reddish-brown gas
2. Iodine is a black solid but forms a purple vapour on gentle heating
3. The trend is the halogens get darker going down the Group
4. Iodine is insoluble in water but it dissolves in potassium iodide, KI solution due to the formation of I3⁻ ion
5. In organic solvents, halogens exist as free molecules, X2.

Variation in atomic radius

1) The atomic radius of halogens increases going down the Group. This is because going down the Group, each succeeding element has one more shell of electrons. The distance between nucleus and outer electrons are progressively further.

Variation in electronegativity

• The electronegativity of halogens decreases down the This is because going down the Group, the distance between the nucleus and bonding electrons increases. Therefore the electrons are attracted less strongly by the nucleus.
• Fluorine is the most electronegative element, and is assigned an electronegativity of 4.0 on the Pauling scale
• In other words, the oxidising power(and reactivity) decreases down the Group

Variation in volatility

• The volatility of halogens decreases down the This is because going down the Group, the number of electrons in the halogen molecules increases. More temporary dipoles can be set up and the strength of van der Waal’s forces increases.
• Hence the boiling point increases and the halogens become less volatile

Variation in bond  enthalpy

• Excluding fluorine, the bond enthalpy of halogens decreases down the This is because going down the Group, the distance between the nucleus and bonding pair of electrons increases. The bonding electrons are less attracted and as a result, the covalent bond gets weaker. Less energy is required to break the bond.
• The bond enthalpy is exceptionally low because F2 is a very small molecule with six lone pairs of electrons. The repulsion created between these electrons reduces the energy needed to break the covalent bond