Alkali metals are placed at the extreme left of the periodic table. They belongs to group number I or IA. Alkali metals have large atomic size with only one valence electron. Therefore, they have weak metallic bonding between the atoms. Due to the presence of weak metallic bonding alkali metals are soft and can be cut with a knife.
Lithium, sodium and potassium are all soft metals that are easily cut with a scalpel or knife.