With four unpaired valence electrons, how can carbon bond to only three adjacent atoms?
The correct answer and explanation is:
Carbon, which has four unpaired valence electrons, typically forms four covalent bonds. However, when carbon bonds to only three adjacent atoms, it often involves an exception where one of the bonds is a double or triple bond, reducing the number of atoms directly bonded to the carbon.
In certain cases, such as with compounds like carbon monoxide (CO) or formaldehyde (CH2O), carbon can bond to just three atoms. This is possible because carbon is capable of forming double or triple bonds. A double bond consists of two pairs of shared electrons, and a triple bond consists of three pairs of shared electrons. In these bonds, the carbon atom is still involved in four electron pairs, but they are shared between fewer atoms.
For instance, in formaldehyde (CH2O), carbon forms two single bonds with two hydrogen atoms and one double bond with an oxygen atom. The double bond to oxygen involves two pairs of electrons shared between carbon and oxygen, leaving carbon bonded to three atoms in total. Even though carbon has four valence electrons, one pair is used in the double bond with oxygen, and the remaining two single bonds complete its four bonds in total.
In carbon monoxide (CO), carbon forms a triple bond with oxygen. The triple bond consists of three shared pairs of electrons, allowing carbon to bond to just one atom (oxygen), but carbon still adheres to the octet rule by having a full set of electrons (eight) in its outer shell. The triple bond ensures that carbon has enough electrons to form a stable bond while still bonding to only one atom.
These variations occur due to the flexibility of carbon in bonding, and the ability of its orbitals to hybridize in such a way that it can form bonds with fewer atoms while still maintaining a stable electronic configuration.