An atom of an element has one electron in the valence shell and the two inner shells have $8$ electrons each. Find the atomic number of that element.

Electronic configuration of an element is $2^n$ where n is the number of orbit hence

1st orbit can not have more than 2 electrons $2^1=2$

2nd orbit can not have more than 8 electrons $2^2=8$

Electronic configuration of an element can be $2,8,8,1=19$