Question
The ionisation energy of alkali metals decreases from $$Li$$ to $$Cs$$ because
A.
the atomic size increases from $$Li$$ to $$Cs$$
B.
the distance between nucleus and outermost orbital decreases from $$Li$$ to $$Cs$$
C.
electropositive character decreases down the group
D.
melting point decreases from $$Li$$ to $$Cs$$
Answer :
the atomic size increases from $$Li$$ to $$Cs$$
Solution :
Down the group from $$Li$$ to $$Cs,$$ the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is well screened from the nuclear charge.