Question

Give Reason: Sodium is placed in group 1A and period 3.

Answer 1

Sodium (2,8,1) has 3 shells
No.of shells = it's Period number.
Therefore it is placed in the 3rd period.

Sodium has 1 valence electrons
Therefore it is placed in group 1

VALENCE ELECTRONS(n) GROUP
1 1
2. 2
3 and above. n+10


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Answer 2

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It has the electronic configuration 1s2 2s2 2p6 3s1. The group 1 elements all contain elements with one valence electron.

The chemical properties are a function of the number of valence electrons an element has. In group 1, all elements have one valence electron and thus they have similar chemical properties (highly reactive, low melting and boiling point).

That is why it is kept in the 1st group.