a. If two group(or ligands) attached to an atom are switched resulting in a new stereoisomer, the atom is called a stereocentre. If the new stereoisomer is an enantiomer, the stereocentre is a chiral centre. All stereocentres are not chiral centres, but all chiral centres are stereocentre.
b. `sp^(2)`- Hybridised atoms have flat geometry and thus possess a plane of symmerty. So they cannot be chiral centres. Carbocation, `(R_(1),R_(2)R_(3)C^(o+))`, free radicals `(R_(1),R_(2),R_(3)C^(*)), (C=C), (C=O)`, acid, acid derivatives, `(C=N)` in imines and oximes, all have `sp^(2)`-hybridises `C` atom and thus cannot be chiral.
(c ) `sp^(3)`-hybrised `Si, N,P,S` can be chiral.
For example
i. i. `R_(2)-underset(R_(3))underset(|)overset(R_(1))overset(|)(.^(**)SI)-R_(4)` , ii. `R_(2)-underset(R_(3))underset(|)overset(R_(1))overset(|)(.^(o+)N^(**))-R_(4)`
iii. `R_(2)overset(o+)(-)underset(R_(3))underset(|)overset(R_(1))overset(|)(P^(**))-R_(4)` , iv. `R_(2)-underset(O^(Θ))underset(|)overset(R_(1))overset(|)(.^(o+)N^(**))-R_(3)`
Trivalent `S` and `P` compounds, with lone pair of `vec(e )'s` can be chiral.
v. `underset("Sulphonium salt")([R_(1)-underset(R_(3))underset(|)overset(o+)overset(..)(S)-R_(2)]X^(Θ))` , vi. `underset("Sulphoxide")(R_(1)-underset(O)underset(||)overset(..)(S)-R_(2))`
viii. `underset("Trialkyl phosphine")(R_(1)-underset(R_(3))underset(|)overset(..)(P)-R_(2))`
