Question

The atomic numbers of three elements A, B and C are given below:

Element	Atomic number
A	5
B	7
C	10

(i) Which element belongs to group 18?
(ii) Which element belongs to group 15?
(iii) Which element belongs to group 13?
(iv) To which period/periods do these elements belong?

Answer 1

(i) Element C belongs to group 18.
Electronic configuration of element C (10) = 2,8
Therefore, group number = valence shell + 10
                                      = 8+10 = 18.

(ii) Element B belongs to group 15.
Electronic configuration of element B (15) = 2,8,5
Therefore, group number = valence shell + 10
                                      = 5+10 = 15.

(iii) Element A belongs to group 13.
Electronic configuration of element A (5) = 2,3
Therefore, group number = valence shell + 10
                                      = 3+10 = 13.

(iv) The elements A, B and C all belong to period 2 because all three have the same valence shell, i.e, L.