How to find a valency of the given element if the atomic number is given?

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Solution

Use the formula $2 n^{2}$ where n is the no. of the shell K,L,M etc. this formula is used to find no. of electrons that can be accommodated in a shell
For ex, if you want valency of element with atomic no.34
Then using the formula
For 1st shell $2 \times 1 \times 1 = 2$
For 2nd shell $2 \times 2 \times 2 = 8$
For 3rd shell $2 \times 3 \times 3 = 18$
Here's the trick adding the electrons in these shells.=28 And the no. left is 6
So as we know we need 8 electrons in last shell so, 2 electrons are less so the valency is -2
If the atomic no.were 36 it would be a noble gas valency = 0, 37 then a valency = +1

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