Characteristics of a Group:
In a group all the elements have the same valency. For example, group I elements: Li, Na, K etc., all have a valency of +1. Elements of group 17: CI, Br, I etc., all have valency of -1.
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(ii) Size of atoms:
Atomic radii or the size of atoms of the element increase on going down a group this is due to the addition of new electronic shells.
(iii) Metallic character:
On descending a group the metallic character increases. For example, groups 14 and 15 begin with carbon and nitrogen which are non-metals.
These groups end with lead and bismuth respectively which are metals. The oxides of the elements also become increasingly basic.
(iv) Melting and boiling points:
The melting and boiling points of metals decrease on going down in a group for example, the alkali metals of group 1 decrease gradually on going down the group. Li (m. pt. 180.5°C, b. pt. 1347°C), K (m. pt. 63.4°C, b. pt. 774°C).
The melting points and boiling points of non-metals increase on going down in a group. For example, the melting points of the group 17 increase gradually on going down in the group. F (m. pt. -219.6°C), I2 (m. pt. 113.6°C)
(v) Ionisation energy:
Ionisation energy decreases on moving down a group with the increase in atomic number. This is because the size of the atom increases and the valency electrons are farther removed from the attractive effect of the nucleus.
(vi) Chemical reactivity:
The chemical reactivity of metals increases on going down a group. For example, in group 1 of alkali metals, the chemical reactivity increases from lithium to francium.
On the other hand, the chemical reactivity of non-metals decreases on going down in a group. For example, in group 17 of halogen elements (non-metals), flourine is most reactive and iodine is least reactive.