Question
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.
Chemical reactivity increases with increase 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 increase in atomic number
down the group
C.
The reactivity decreases in the alkali metals but
increases in the halogens with increase in atomic
number down the group
D.
In both the alkali metals and the halogens the chemical
reactivity decreases with increase in atomic number
down the group
Answer :
In alkali metals the reactivity increases but in the
halogens it decreases with increase in atomic number
down the group
Solution :
The alkali metals are highly reactive because their first ionisation potential is very low and hence they have great tendency to loses electron to form unipositive
ion.
NOTE : On moving down group - $$I$$ 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 enthalpy of hydration of halide ion. However their reactivity decreases with increase in
atomic number.