(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 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 two 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?
(a) An element's valency is determined by the number of electrons in its outer shell. Hence, the number of valence electrons obtained from the electronic configuration of the element gives the valency i.e. the number of electrons lost, gained or shared by the element to attain the noble gas configuration.
The valency of an element of atomic number 9 (Electronic configuration: 2,7) would be 1 since the number of valence electrons in its outermost shell is 7 so it needs only one electron to attain the noble gas configuration.
(b) (i) Element D (19) has one electron in its outermost shell. Its electronic configuration is 2, 8, 8, 1.
(ii) Elements A (4) and E (20) have two electrons in their outermost shells. Electronic configuration of A: 2, 2
Electronic configuration of E: 2, 8, 8, 2
As both of them have a valency of two, hence, they belong to group 2 of the periodic table.
(iii) Elements A (4) and B (9) belong to the second period, and elements D (19) and E (20) belong to the fourth period of the periodic table.
As the effective nuclear charge that pulls the outermost electron closer to the nucleus increases from left to right in a period, the atomic radii of the elements decrease.
Therefore, A (4) has a bigger atomic radius than B (9) and D (19) has a bigger atomic radius than E (20).
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