Question

what is formula of finding out valency of a compound

Answer 1

Valency is only for elements.If you were to dind out the valency of an oxygen.The valency will be 6.for O:2,6.Here 6 is the last .so 6 is valency.Its like 2,8,8,18.So donot manipulate the formulae and whatever it is..do not increase from 2 to 3 and 8 to 9 or 7.

Answer 2

//Helloo//

When you are looking at the Periodic Table, the horizontal rows (Periods) will tell you how many shells are required to the electrons. 

I'm taking an example of An element found in the 4th Period.
It means it requires 4 shells for its electrons. The vertical columns, excluding the transition elements Groups will tell how many electrons are in the valence shell. This only works using Groups 1A through 8A, not with the B columns. An element in Group 5A will have five electrons in its valence shell.

So, Carbon in Period 2 will require 2 electron shells.
it is in Group 4A there will be 4 electrons in the valence shell
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hope it helps!!