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. Terefore, 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.

Due to the presence of weak metallic bonding ,alkali metals are soft and can be cut with a knife..


Ex-Sodium.

Explanation:

Alkali metals are a group of chemical elements that belong to Group 1 of the periodic table. This group includes elements such as lithium, sodium, potassium, rubidium, cesium, and francium. One of the unique properties of alkali metals is their softness, which allows them to be cut with a knife. This property can be explained by the following reasons:

1. Metallic Bonding:
Alkali metals have a single valence electron in their outermost energy level. This electron is loosely held by the positively charged atomic nucleus. Due to the low ionization energy of alkali metals, this valence electron is easily lost, resulting in the formation of a positive ion. The positive ions then arrange themselves in a regular pattern, creating a metallic lattice structure. Metallic bonding, which is the electrostatic attraction between the positive ions and the delocalized electrons, is responsible for the characteristic properties of metals, including their malleability and ductility.

2. Weak Metallic Bonding:
The metallic bond in alkali metals is relatively weak compared to other metals. This is because the alkali metals have larger atomic radii and lower effective nuclear charges. The larger atomic radii lead to a greater distance between the positive ions and the delocalized electrons, reducing the strength of the metallic bond. As a result, the alkali metals are more easily deformed.

3. Low Melting and Boiling Points:
Alkali metals have low melting and boiling points compared to other metals. This is due to the weak metallic bonding and the relatively small forces required to disrupt the metallic lattice structure. The low melting and boiling points contribute to the softness of alkali metals, allowing them to be easily cut with a knife.

4. Reactivity with Air and Moisture:
Alkali metals are highly reactive, particularly with air and moisture. When exposed to these substances, alkali metals can rapidly oxidize and form metal oxides or hydroxides. The formation of these compounds further weakens the metallic lattice structure, making the alkali metals even softer and more susceptible to being cut with a knife.

In conclusion, the softness of alkali metals can be attributed to their weak metallic bonding, low melting and boiling points, and their reactivity with air and moisture. These properties make alkali metals unique and distinguish them from other metals that cannot be easily cut with a knife.