Arrange the elements given below in the increasing order of their atomic size. Element A 1s22s22p6 
Element B 1s22s2
 Element C 1s22s22p63s1
 Element D 1s22s22p3

The answer to your question is: B, D, A, C

Explanation:

Atomic size is the distance from the nucleus to the outermost shell or valence shell. Then, we can know the atomic size for the number of electrons in an element.

In this exercise, there are 4 elements and their number of electrons, so we must know this number for each element.

Element A 1s22s22p6 has 8 electrons

Element B 1s22s2 has 4 electrons

Element C 1s22s22p63s1 has 11 electrons

Element D 1s22s22p3 has 7 electrons

The smallest atomic size is for Element B, then Element D, Element A and the biggest atomic size is for element C.