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General Characteristics of a Group

1. Metallic Character
In a group, on moving downwards, the metallic character gradually increases. For example, in the halogen family (group VIIA), fluorine, chlorine and bromine are typical non-metals but iodine, though a non-metal has a metallic lustre. This property is more marked in the elements of groups IVA and VA. These groups begin with a non-metal and end with metals.


2. Valency
In a group, all the elements have the same valency. The group number is, in fact, the valency with respect to oxygen.


3. Atomic Size
It is the distance between the centre of the nucleus and the outermost shell of an isolated atom. The atomic radius of hydrogen atom is 37pm. On moving downwards, in a group the size of the atom increases. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.

4. Physical Properties
There is a regular gradation in their physical properties like melting point, boiling point density, etc in a group. For example, in alkali metals (group IA), the melting point decreases on moving down the group.

5. Chemical Reactivity
Chemical reactivity also shows a definite gradation. For example, in halogens, the reactivity towards hydrogen and water decreases on moving downwards.

6. Electro Negativity
The tendency to gain an electron decreasing on moving down a group is called electronegativity. For example, fluorine has the greatest tendency to gain an electron and iodine has the least tendency to gain an electron.

In the table 5.3 some properties of the elements of the third period are summarised.
 

Table:5.3 Properties of the elements of third period

 

Property

In a group from top to bottom

In a period from left to right

Number of valence electrons 

Remains unchanged

Increases

Valency with respect to hydrogen

Remains uncharged

Increases from 1 to 4 then decreases to 1

Valency with respect to Oxygen

Remains unchanged

Increases

Metallic character

Increases

Decreases

Non-metallic character

Decreases

Increases

Electronegativity

Decreases

Increases

Electropositive character

Increases

Decreases

Atomic size or atomic radius

Increases

Decreases

 




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