(a) How is the valency of an element determined if its electronic configuration is known? Determine the valency of an element of atomic no. 9
(b) Given below are some elements of the Modern Periodic Table. Atomic numbers of the elements are given in the parentheses:
A(4), B(9), C(14), D(19), E(20)
(i) With the help of the electronic configuration, find out which one of the above elements will have one electron in its outermost shell?
(ii) Which rwo elements belong to the same group? Give reasons for your answer.
(iii) Which one of the above elements belonging to the fourth period has bigger atomic radius and why?.

Explanation

(a) Eleronic configuration is the distribution of electrons in the shells of the atom The number of electrons in the outermost shell will decide the valency of the element. Valency of the element of atomic no. 9 is 1.
Elements `(9) rArr 2,7`
As it gains 1 clectron to fill its shell to attain an octet, its valency is 1.
(b) `A(4) rArr 2,2" "B(9)rArr 2,7," "C(14) rArr 2,8,4" "D(19) rArr 2,8,8,1" "E(20) rArr 2,8,8,2`
(i) Element 'D' will have one electron in its outermost shell.
(ii) 'A' and 'E' belong to the same group as both of them have two electrons in the outermost shell and their valency is 2. Same group elements have four shells .
(iii) Elements 'D' and 'E' belong to the fourth period as both of them have four shells . As we move from left to right along a period, the atomic radius decrease due to the increase in nuclear charge which tends, to pull the electrons closer to the nucleus and reduces the size of the atom. Thus elements D has bigger atomic radius that elements E.