What is the Hybridisation of Carbon in $H-C-H$ :

Question

Accepted Answer

The correct option is (3): $Sp^{2}$

To determine the hybridization of the carbon atom in the $H-C-H$ fragment (which implies a carbon atom bonded to two hydrogen atoms and likely involved in a double bond to satisfy carbon's valency, such as in an alkene like ethene, $\mathrm{CH_2=CH_2}$, or formaldehyde, $\mathrm{H_2C=O}$), we need to count the number of regions of electron density around the central carbon atom. Each single bond, double bond, triple bond, and lone pair counts as one region of electron density.

In a common scenario where a carbon atom is bonded to two hydrogen atoms and also involved in a double bond (e.g., in ethene, $\mathrm{H_2C=CH_2}$, focusing on one carbon), the carbon atom forms two single bonds with hydrogen atoms and one double bond with another carbon atom.  * Two single bonds (with hydrogen) = 2 regions of electron density
  * One double bond (with another carbon or oxygen, e.g., in $\mathrm{H\_2C=O}$) = 1 region of electron density
  * Total regions of electron density = $2 + 1 = 3$.
    When a central atom has three regions of electron density, it undergoes $sp^2$ hybridization. This results in three $sp^2$ hybrid orbitals and one unhybridized $p$ orbital. The $sp^2$ hybrid orbitals form sigma bonds, and the unhybridized $p$ orbital forms a pi bond. This leads to a trigonal planar geometry with bond angles of approximately $120^\circ$.The incorrect option is (1): $Sp^{3}$
$Sp^{3}$ hybridization occurs when a carbon atom has four regions of electron density, typically forming four single bonds (e.g., in methane, $\mathrm{CH\_4}$). This leads to a tetrahedral geometry, which is not the case for a carbon forming a double bond.

The incorrect option is (2): Sp
Sp hybridization occurs when a carbon atom has two regions of electron density, typically forming two double bonds or one single and one triple bond (e.g., in carbon dioxide, $\mathrm{CO\_2}$, or ethyne, $\mathrm{HC \equiv CH}$). This results in a linear geometry, which is not consistent with the $H-C-H$ fragment where carbon is connected to two hydrogens.

The incorrect option is (4): $Sp^{3}d$
$Sp^{3}d$ hybridization typically occurs for central atoms from the third period or below, as it requires the involvement of d-orbitals. Carbon is a second-period element and does not utilize d-orbitals for bonding, making this hybridization impossible for carbon under normal circumstances.