Following statements regarding the periodic trends of chemical reactivity of the alkali metals and the halogens are given. Which of these statements gives the correct picture: A)In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group B)The reactivity decreases in the alkali metals but increases in the halogens with increases in atomic number down the group. C)In both the alkali metals and the halogen the chemical reactivity decreases with increases in atomic number down the group D)Chemical reactivity increases with increases in atomic number down the group in both the alkali metals and halogens.

Options

A. chemical reactivity increases with increases in atomic number down the group in both the alkali metals and halogens
B. in alkali metals the reactivity increases but in the halogens it decreases with increases in atomic number down the group
C. the reactivity decreases in the alkali metals but increases in the halogens with increases in atomic number
D. in both the alkali metals and halogens, the chemical reactivity decreases with increases in atomic number down the group.

Answer

B

Explanation

The alkali metals are highly reactive because their first ionisation potential is very low and hence they have great tendency to lose ns electron to form unipoisitive ion. On moving down a group from Li to Cs ionisation enthalpy decreases hence the reactivity increases. The halogens are most reactive elements due to their low bond dissociation energy, high electron affinity and high enthlpy of hydration of halide ion. on moving down the group their reactivity decreases with increases in atomic number due to following factors.
(i) X-X bonds have low dissociation energy.
(ii) The electronegativity decreases from F to I due to which bond between halogens and other elements become weaker and weaker.
(iii) As the size increases, the attraction for an additional electron by the nucleus becomes less, thus reactivity decreases.