Why are Alkali metals soft?

Hint: The elements present in group \[1\] (IA) of the periodic table are called alkali metals. These all elements are strong bases which are capable of neutralizing strong acids. The elements consist of Sodium (Na), Lithium (Li), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr).

Complete answer:
Alkali metals are present in the group \[1\] (IA) occupying the leftmost side of the periodic table. These metals are called alkalies because they react with water and form alkalies. They are strong bases which have the capability of neutralizing strong acids. Alkali metals belong to the s- block elements. The alkali metals consist of chemical elements such as Sodium (Na), Lithium (Li), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr). Alkali metals are the most reactive elements on earth as they readily lose their electrons.
Alkali metals have only a single electron in their valence shell which can be easily donated. Due to the presence of this single electron in their valence shell, alkali metals are soft in nature. The metallic bonding between the atoms in alkali metals are weak. So, they have weak binding energy in their lattice. Hence, alkali metals are found very soft. They are so soft that they can be cut with a knife.
Therefore, the alkali metals are soft due to the presence of a single electron in its outermost shell and they have weak binding energy in their crystal lattice.

Note:
Sodium (Na) and Potassium (K) are the sixth and seventh most abundant elements present in the Earth’s crust. The other alkali metals are very rare such Lithium (Li), Rubidium (Rb) and cesium (Cs) whereas Francium (Fr) is a natural radioactive isotope and it is found very rare.