Q10 of 61 Page 1

(a) Define the following terms:

(i) Valency; (ii) Atomic size


(b) How do the valency and' the atomic size of the elements vary while going from left to right along a period in the modern periodic table?

(a) (i) Valency is the combining capactity of an element and is determined by number of electrons lost or regained to achieve the nearest stable configuration.

(ii) Atomic size is defined as the distance between the nuclei centre and the outermost electron shell of the atom.


(b) When moving from left to the right in a period, the valency of the elements increases from 0 to 4 and then it decreases to 0.


As we move from left to right in a period, the nuclear charge on an element increases but the number of energy shells remains the same. Due to increase in the nuclear charge, the force of attraction between nucleus and electrons increases. As a result, the size of the atom decreases on moving from left to right in a period.


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8

Write the names and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen can take place. State the essential conditions required for an addition reaction to occur write the chemical equation giving the name of the reactant and the product of such a reaction.

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Based on the group valency of elements state the formula for the following giving justification for each:

(i) Oxides of 1st group elements,


(ii) Halides of the elements of group 13, and


(iii) Compounds formed when an element of group 2 combines with an element of group 16.

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