Three elements A, B and C have 3,4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the modern periodic table. Also, give their valencies.

Explanation

The group number for elements can be identified as 10 + number of valence electrons. Element A with 3 valence electrons must show valency as 3 and should belongs to group 13. The elements of group 13 are B, Al, Ga, In or TI
Element B contains 4 valence electrons so it must be from group 14 ( 10 + 4) and valency should be 4. The element B can be C, Si, Ge, Sn or Pb. Similarly Element C has two valence electrons hence must be from group 2. So element C can be Be, Mg, Ca, Sr, Ba or Ra.