What determines the hybridisation of an atom in a molecule?

The hybridisation of an atom in a molecule is determined by the number of atoms attached to it and lone pairs present.

In more detail, hybridisation is a concept in molecular chemistry that describes the combination of atomic orbitals to form new hybrid orbitals. These hybrid orbitals are suitable for the pairing of electrons to form chemical bonds in valence bond theory. The type of hybridisation that occurs is determined by the atom's electron configuration and the number of atoms attached to it, including any lone pairs of electrons.

For instance, if an atom is bonded to two other atoms and has no lone pairs, it will be sp hybridised. This is because it needs two orbitals for bonding, one s and one p orbital, hence the term 'sp'. If an atom is bonded to three other atoms and has no lone pairs, it will be sp2 hybridised. This is because it needs three orbitals for bonding, one s and two p orbitals, hence the term 'sp2'. If an atom is bonded to four other atoms and has no lone pairs, it will be sp3 hybridised. This is because it needs four orbitals for bonding, one s and three p orbitals, hence the term 'sp3'.

Lone pairs also play a crucial role in determining the hybridisation of an atom. For example, if an atom is bonded to two other atoms and has one lone pair, it will also be sp2 hybridised. This is because the lone pair will occupy one of the hybrid orbitals, leaving the other two for bonding. Similarly, if an atom is bonded to three other atoms and has one lone pair, it will be sp3 hybridised.

In summary, the hybridisation of an atom in a molecule is determined by the number of atoms it is bonded to and the number of lone pairs it has. This concept is fundamental in understanding the shape and reactivity of molecules in chemistry.