Apart from tetrahedral geometry, another possible geometry for CH₄ is square planar with the four H atoms at the corners of the square and the C atom at its centre. Explain why CH₄ is not square planar?
In the excited state, the electronic configuration of carbon is . To form four equivalent C-H bonds, one and three orbitals of carbon undergo hybridisation.
This mixing forms four hybrid orbitals of equivalent energies and shape, which are directed towards the four corners of a tetrahedron. The angle between these hybrid orbitals is 109.5°.
If CH were square planar, the bond angles would be 90° and 180°, which contradicts the spatial arrangement of the four hybrid orbitals. Furthermore, with four bonding electron pairs and no lone pairs on the central carbon atom, the electron pairs arrange themselves tetrahedrally, resulting in a tetrahedral geometry rather than square planar.
Explanation
The textbook explains that CH4 involves sp3 hybridisation of one s and three p orbitals, creating four equivalent hybrid orbitals directed towards the corners of a tetrahedron at 109.5° angles. A square planar geometry would not accommodate these equivalent orbitals at 109.5°, and VSEPR theory (Table 4.6) dictates that four electron pairs around a central atom with no lone pairs adopt a tetrahedral arrangement.
Solution Steps
Step 1: Identify the hybridisation of carbon in CH4 as sp3, mixing one 2s and three 2p orbitals.
Step 2: State that the four sp3 hybrid orbitals are directed towards the four corners of a tetrahedron with a bond angle of 109.5°.
Step 3: Conclude that a square planar geometry (90° and 180° angles) does not match the tetrahedral arrangement of the four equivalent sp3 hybrid orbitals and electron pairs.