in the electronic configuration say for example oxygen has 6 electrons in its valence shell while there are 2 electrons in its inner shell. if you add the 6 electrons + 2 electrons you get 8 which is the atomic number of oxygen

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there are 8 electrons in the outermost shell of neon while the inner layer has 2 electrons add 2+8=10.10 is the atomic number
suppose the group number of magnesium is 2 and period number is 3 then find its atomic number?
there are 10 electrons in the valence shell and 2 in the inner shell. so add 2+10=12. 12 is the atomic number.
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We can say easily for example lithum configuration is 1s^2 2s^1 now aad the powers of it 2 +1=3 and its atomic number is 3 ny this we can also say it's position in perodic table