Singlet and Triplet States

Singlet and triplet states can be explained by spin multiplicity (2S+1) value, where 'S' is total spin and is equal to n/2 where 'n' is number of unpaired electrons. If spin multiplicity value is one , the energy state is called Singlet (S) and If spin multiplicity value is three , the energy state is called Triplet (T) state.
A singlet state refers to a system in which all the electrons are paired. The net angular momentum of the particles in this type of system is zero. Therefore, we can say that the overall spin quantum number (s) is zero. If we take the spectrum of this system, it shows one spectral line that's why called 'singlet state'.
Triplet state refere to a system has two unpaired electrons. The net angular momentum of the particles in this type of system is one. Therefore, the spin quantum number is one. This allows three values of the angular momentum as -1, 0 and +1. Hence, the spectral lines obtained for this type of system split into three lines that's why called 'triplet state'.

Comparison Between Singlet and Triplet States

The energy state having only spin paired electrons are called singlet (S) states and those having only parallel spin electrons as triplet (T) states.
All singlet states are diamagnetic while all triplet states are paramagnetic.
The singlet state does not split but triplet state splits up into three different energy states in the applied magnetic field. Thus singlet states have only one way of interacting with magnetic field.
Triplet excited states have lower energy than singlet excited states.
Singlet states have a net zero angular momentum.
Electronic transition between S → S or T → T is allowed but S → T or T → S is forbidden. Forbidden transition may occur in few cases due to spin orbit coupling.

Descreption	Singlet State	Triplet State
Notations	So, S1, S2, ...	To, T1, T2, ...
(2S+1) Value	One	Three
No. of Unpaired Electrons	Zero	Two
Angular Momentum	Zero	One
Spin Quantum Number	Zero	One
Magnetic Property	Diamagnetic	Paramagnetic
Spectral Lines	Zero	Three
Energy of Excited State	High	Low