Specific heat capacity is a function of the forces between the molecules of a substance. As we go down the group the packing efficiency of the substance decreases due to increased size.Thus decreased force binding two atoms is observed and atoms have freedom. So, it decreases down the group.

Reason for Decrease in Specific Heat in Alkali Metals

1. Size of the Atoms
- As we move down the group in alkali metals, the size of the atoms increases due to the addition of new energy levels. This increase in size leads to a decrease in the density of the atoms.
- The decrease in density means that there are fewer atoms per unit volume, resulting in weaker interatomic forces and less resistance to the transfer of heat energy.

2. Increase in Number of Energy Levels
- With the addition of new energy levels as we move down the group, the number of electrons and electron shells also increases.
- This increase in the number of energy levels results in a greater distance between the outermost electron and the nucleus, leading to a decrease in the effective nuclear charge experienced by the outermost electron.
- The decrease in effective nuclear charge weakens the attraction between the outermost electron and the nucleus, making it easier for the electrons to move and transfer heat energy.

3. Electron Configuration
- The alkali metals have a single electron in their outermost shell, which is easily lost to form positive ions.
- This loose outer electron configuration allows for greater mobility of electrons, making it easier for the atoms to transfer heat energy.

4. Metallic Bonding
- Alkali metals exhibit metallic bonding where the outer electrons are free to move throughout the structure.
- This delocalization of electrons allows for efficient heat conduction within the metal structure, resulting in a lower specific heat capacity.
Overall, the combination of larger atom size, increased number of energy levels, electron configuration, and metallic bonding contribute to the decrease in specific heat as we move down the group in alkali metals.