Assertion : Formaldehyde is a planar molecule.

Reason : It contains sp2 hybridised carbon atom.



According to Molecular orbital theory (MOT), the carbon in the carbonyl groups (C=O) is actually sp2 hybridized (consisting one s and two p orbitals) and therefore it contributes 3 sp2 orbitals for ∂- bonds (sigma i.e.single bonds) and the 4th valence electron (as carbon has a valency of 4) remains in its (C) one unhybridized partially filled p-orbital.


This unhybridized p orbital forms a π bond (pi i.e. double bond) by collateral or sidewise overlapping with the partially free p-orbital of the oxygen atom.


Thus the carbonyl atom of formaldehyde and the other 3 atoms attached to it ( 2 H atoms and one O atoms through sigma bonding) lie in the same plane while the π electron cloud (pi bonds are not actual bonds, they are more like electronic attractions) lies above and below of this planar structure.



In addition the lone pairs left on the oxygen atom also lie in the same plane results in a planar structure for formaldehyde (HCHO) with bond angles of almost 120◦(angle for triagonal planar structures).

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