Excellence Classes

The distance between the valence electron and the nucleus is less in sodium than potassium. So sodium has a greater electrostatic force between nucleus and valence electrons which doesn't let it lose electrons easily. Whereas, in potassium, due to the weaker electrostatic force than sodium, it can lose electrons easily. So as the atomic size increases down the alkali group, the electrostatic force becomes weaker and reactivity increases.

Physical Properties:
Metallic Character: As their size is large, its outermost electron is far from the nucleus and can be easily removed. As forces are weak, they will have low melting and boiling point. This group shows the metallic character due to its low ionisation energy, consequently making it more electropositive showing oxidation state of +1. Ionisation energy decreases as we move down this group. Due to its larger size, it also has a low density.
Soft: As the bond gets weaker due to its larger size, this group metals are softer.Softness increases as we move down the group.
Chemical Properties:
Reaction with halogens: Alkali metals burn in halogen which later forms halides.
Reaction with water: When alkali metals react with a water molecule, they give out hydrogen.
Reaction with oxygen: Alkali metals burn in oxygen, which later forms
oxides.