Question

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Q.16
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Answer 1

(a).1.The valency of an element is determined by the number of valence electrons in the outermost shell of an atom of an element.
2. All the elements in a group have the same number of Valence Electrons. Therefore, elements in the same group should have the same valency. For example, the elements of group 1 contain only one valence electron; the valency of the elements of group 1 is one. Similarly, for group 2, the valency is two.

(b). 1.In the atoms of the inert gas ( zero group elements), all the electronic shells, including the outermost shell are completely filled.
2.The electronic configuration is stable, and these elements do not lose or accept electrons. These elements do not take part in chemical reactions. Hence, they are called as noble gases or zerovalent.

(c).1.In a period while going from left to right, atomic radius goes on decreasing and the atomic number increases one by one, that means positive charge on the nucleus increases by one unit at a time.
2.However, the additional electron gets added to the same outermost shell. Due to the increased nuclear charge the electrons are pulled towards the nucleus to a greater extent, as a result the size of atom decreases i.e., atomic radius decreases.

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Answer 2

(a) Elements in the same group have similar chemical properties because they have the same electronic configuration in the outermost shell i.e. they have the same number of valence electrons and so same valency.
(b) Group 18 consists of noble gases or rare gases such as Ne,Ra,Ar etc. which have completely filled valence shells . They do not require any electron as their octet(or duplet in case of He) is already complete.Hence they are stable and so are called
zerovalent elements.
(c)As we go through a period the atomic number increases by 1 in every successive element and due to this it's proton number also increases by 1.As the number of proton increases it exerts a more greater force to the electrons due to which the electrons are pulled towards the nucleus,thus decreasing the atomic radius of the atom.