Four elements A ,B ,C and D along with their electonic configuration are given below :
Elements : A B C D
Electronic configuration : 2,1 2,8 2,8,1 2,8,8
Now answer the following questions :
(i) Which two elements belong to the same period ?
(ii) Which two elements belong to the same group ?
(iii) Which two elements belong to the 18th group ?
(iv) Which element out of A and C is more reactive and why ?
(v) Which element out of A and B forms the maximum number to compounds ?

Explanation

(i) Elements in a period have same number of shells . Thus elements A and B each with two shells belong to second period while elements C and D which have three shells each belong to third period.
(ii) elements in a group have same number of valence electrons . thus elements A and C belong to the same group .
(iii) Elements having all shells completely filled belong to group 18 (noble gases. ) Thus elements B and D belong to 18th group .
(iv) Because of bigger size and weaker forces of attraction of the nucleus on the valence electron element C (2,8,1) is more reactive than element A.
(v) B being a noble gas does not form compounds . But element A has only electron in the valence shell which it can lose very easily. therefore, element A forms maximum number of compounds.