Question

How can the valency of an element be determine if its electronic configuration is known ? What will be the valency of an element of atomic no 9?

Answer 1

Valency of an element is the numbers of electrons that can be lost or gained by that element to complete it's last orbit.
For example lithium (atomic number =3)
electronic configuration will be = 2,1
For the completion of the last shell lithium should have 8 electrons but it has only 1 electron.
So it's will try to gain 7 electrons or it will lose 1 electron and will become inert as it's previous shell will be completed .
During this case gaining 7 electrons is quit difficult in front of losing 1 electron.
So it will lose 1 electron
Hence it's valency is 1.
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Now :;:;,
Valency of element with atomic number =9
Electronic configuration=2,7
It will gain 1 electron and will complete the orbit. So, it's valency is 1.
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Hope you get it.