Reactive, soft, low melting points
Reactive: These metals are highly reactive because they only have one valence electron, which can be easily transferred.
Soft: Since they are in group one, they have a large atomic size and one valence electron. Their weak metallic bonding accounts for their softness.
Low Melting Points: "As a metal is heated to its melting point, the metallic bonds keeping the atoms in place weaken so that the atoms can move around, and the metallic bonds eventually break completely at the metal's boiling point." (https://en.wikipedia.org/wiki/Alkali_metal). Since alkali metals have weak bonds, they have low melting points.
Note: Reactivity increases as you move down a group - the valence electrons are farther and farther away from the nucleus, and electron shielding increases, ionization energy decreases. The melting points also decrease as you move down a group - the atomic radii increase.