Question

in electronic configuration , for an example calcium's (atomic no.= 20) configuration why we write as 2,8,8,2 and why not 2,8,10 as 3rd shell contains 18 electrons.

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Answer 1

See, you've to remember the magic number: 2.8.8.8.......

The first will always have 2 electrons because that's how many electrons it can hold. And after that 2nd, 3rd..... shell can have maximum 8 electrons. (This is also called valency btw)

Anyways as you said Calcium's atomic number is 20

So you would figure the electronic configuration by adding:  2+8+8 = 18

You need 2 more so you write 2 for the fourth shell.

2.8.8.2 (=20)

In the configuration 2.8.10, the 3rd shell has more than 8 electrons and that is not possible. It can only be 8 or less than 8