What is the number of 3s electrons in Mg?

There are two 3s electrons in Mg. You can tell this easily, by looking at the periodic table. Mg is in the third row (3) and second column (s=2.)

This trick works in the p-block as well, so if you look at Oxygen, it’s got the 2s^2 ( because it’s in the 2nd row and it has passed the 2s column), and then it’s the fourth element in on the p-block (p=4.)

So, oxygen’s noble gas configuration is [He] 2s^2 2p^4

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