Question

Q. Five elements A, B, C, D and E have atomic number 2, 9, 11, 12 and 20 respectively: -
(i) Which pair of element belongs to the same group?
(ii) Which pair of element belongs to same period?

Answer 1

Dear Student


Atomic No. is the no. of protons in an element or no. of electrons in an element in its ground gaseous state.
We fill electron in an element (in it,s ground gasoues state) as 2,8,8,18.18.36 and so on. Thus atomic no. 1,2 belong to period 1, 3 to 10 (2+8) belong to period 3 and 11 to 18 (10+8) belong to period 3. Similarly, for same group there is addition of 1 electron from group 1 to 2, 8 electron from group 2 to 3 and group 3 to 4, and addition of 18 electron for group4 to 5 and 5 to 6 and so on.
thus, accounting to above details,
a) D and E
b) C and D