Question

If the spin of the electrons in the excited state are parallel, it is called as:

• A. Doublet state
• B. Triplet state
• C. Singlet state
• D. Parallel state

Hint:

If the excited-state electron spins are parallel, the system is in a triplet state. If they are paired (opposite), the system is in a singlet state.

Answer 1

The Correct Option is B

Step 1: Understanding electronic spin states. When an electron in a molecule or atom gets excited, it moves to a higher energy level. The total spin of the system determines whether the state is singlet, triplet, or other multiplicities. 

Step 2: Characteristics of a triplet state. - In a singlet state, the electron spins are paired (opposite), meaning the total spin quantum number \( S = 0 \). 
- In a triplet state, the electron spins are parallel, meaning the total spin quantum number \( S = 1 \). 
- The triplet state is lower in energy than the singlet excited state because parallel spins reduce electron repulsion. 

Step 3: Why other options are incorrect. - (A) Doublet state: Applies to systems with an unpaired electron, such as free radicals. 
- (C) Singlet state: Electrons have paired spins (\( S = 0 \)), not parallel. 
- (D) Parallel state: Not a standard term in quantum chemistry.

Answer 2

The question asks what we call an excited state where the two electrons involved have parallel spins. Let's check each term against the spin arrangement it describes.

1. Doublet state: A doublet arises when a system has exactly one unpaired electron, giving it two possible spin orientations. This term applies to species like free radicals, not to a pair of parallel-spin electrons in an excited molecule.
2. Triplet state: When both electrons in an excited pair have parallel spins, their spins add up to give a total spin quantum number of 1, and this state can exist in three possible spin orientations, which is why it is called a triplet.
3. Singlet state: In a singlet state, the two electrons are still paired with opposite spins, giving a total spin of 0, so it is exactly the opposite arrangement to what the question describes.
4. Parallel state: This is not a defined term in quantum chemistry or spectroscopy. No electronic state is formally labeled this way, even though it describes the spin orientation in plain words.

An excited state with parallel electron spins is, by definition and by the number of spin orientations it allows, called a triplet state.

So the correct answer is Triplet state.