Question

Why nitrogen valency is 3? Why it couldn't be 5?

Answer 1

because it gains 3 electrons to obtain noble gas configuration
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Answer 2

Nitrogen atomic number = 7

Electronic configuration of Nitrogen =
1S2 2S2 3P3

so, now Nitrogen still need 3electron to be stable and to get octet configuration

Hence valency = 3

And 5 is not the valency it is valency electrons in the last orbit

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