Question

An element A has 2 element in its fourth shell.state its atomic number

Answer 1

Its electronic configuration of the first 3 shells will be
2 , 8 , 8
and 4th shell has 2 electrons
Total electrons = 2 + 8 + 8 + 2 = 20
So it will have 20 Protons as well because Atoms are neutral .
So Atomic number of this element is 20.
The element is Calcium .

Answer 2

in Frist cell we put 2 and in second we put 8 and in third we put 18 . and the question say in fourth cell 2 element . so atomic number is 2+8+18+2=30