Concept:Alkali metals show periodic trends down the group; identify which two properties both decrease.
Explanation:Down group 1, the atomic size increases as new electron shells are added.
This increases the distance between the nucleus and the outermost electron, so the first ionization energy decreases down the group.
Alkali metals have weak metallic bonding due to their large atomic size.
Therefore, their melting points also decrease down the group.
Reactivity of alkali metals increases down the group because ionization energy decreases.
Electronegativity decreases down the group as well, but reactivity increases.
Thus, the pair of properties that both decrease down the group is first ionization energy and melting point.
Answer:D. First ionization energy and melting point