This is because there are pi bonds involved in these molecules. For stable pi bonds to be formed between two atoms, the participating valence orbitals of the two atoms should have comparable sizes and comparable energies.Since Si is a bigger atom, there are more shielding effects by the electrons. Thus, the bond between Si and O is not so strong.....I.e by 3pπ-2pπ overlap, .......As down the group, size of atoms increase hence ability to form p-p multiple bonds decrease. ..

Explanation:
The Si=O molecule is unstable because of its electronic configuration and the overlapping of the atomic orbitals involved in the bond formation. The valence electron configuration of Si is 3s²3p², whereas oxygen has a valence electron configuration of 2s²2p⁴.

Electronic configuration:
The electronic configuration of Si=O is such that the silicon atom has an incomplete octet, and the oxygen atom has an expanded octet. This makes the molecule energetically unstable, and it tends to react with other compounds to attain a stable configuration.

Overlapping of atomic orbitals:
The formation of the Si=O bond involves the overlapping of the atomic orbitals of the silicon and oxygen atoms. The overlapping of the p orbitals of the silicon and oxygen atoms leads to the formation of a pi bond.

Types of overlapping:
There are different types of overlapping of atomic orbitals. The following are the types of overlapping that can occur in the Si=O molecule:

1. 3pPi -2pPi overlapping: This type of overlapping occurs when the 3p orbital of silicon overlaps with the 2p orbital of oxygen. This overlapping is not favorable because of the difference in energy levels of the orbitals involved.

2. 2pPi -2pPi overlapping: This type of overlapping occurs when the 2p orbital of silicon overlaps with the 2p orbital of oxygen. This overlapping is not favorable because the overlap is not strong enough to form a stable bond.

3. 3pPi -3pPi overlapping: This type of overlapping occurs when the 3p orbital of silicon overlaps with the 3p orbital of oxygen. This overlapping is not favorable because the energy levels of the orbitals involved are too high.

4. 3dPi -3dPi overlapping: This type of overlapping occurs when the 3d orbital of silicon overlaps with the 3d orbital of oxygen. This overlapping is not favorable because the 3d orbitals are not involved in the bond formation of the Si=O molecule.

Therefore, the Si=O molecule is unstable because of its electronic configuration and the unfavorable overlapping of the atomic orbitals involved in the bond formation.