The electronic configuration of three elements `X,Y` and `Z` are given below:
`X 2`
`Y 2,6`
`Z 2,8,2`
(i) Which element belongs to second period?
(ii) Which element belongs to second group?
(iii) Which element belongs to eighteenth group?

Explanation

(i) The element which has two electron shells in its atom should belong to second period. In this case the element `Y` has two electron shells (with electron configuration 2,5), therefore element `Y` belongs to second period.
(ii) The element which has 2 valence electrons (except the 2 electrons in first valence shell), should belong to the seond group. here the element `Z` has 2 valence electrons in its atom, therefore, element `ZS` belongs to second group.
(iii) The element having 2 electrons in its only shell (`K` shell) or 8 electrons in its valence shell should belong to eighteenth group of noble gases. In this case, element `X` has only one shell containig 2 electrons, so element `X` belongs to eighteenth group of the periodic table. It is actually the noble gas helium.