Question

Which of the following sets of quantum numbers could represent the last electron added to complete the electron configuration for a ground state atom of Br (Z = 35) according to the Aufbau principle, $n ; l ; m_l ; m_s$

• A. 4 ; 0 ; 0 ; $-\frac{1}{2}$
• B. 4 ; 1 ; 1 ; $-\frac{1}{2}$
• C. 3 ; 1 ; 1 ; $-\frac{1}{2}$
• D. 4 ; 1 ; 2 ; $+\frac{1}{2}$

Hint:

For p-orbitals: $l=1$ and $m_l = -1, 0, +1$.

Answer 1

The Correct Option is B

Concept: Quantum numbers define the state of an electron:
• $n$ = principal quantum number
• $l$ = azimuthal quantum number
• $m_l$ = magnetic quantum number
• $m_s$ = spin quantum number

Step 1: Write electronic configuration of Br (Z=35):
\[ Br: [Ar] 3d^{10} 4s^2 4p^5 \]

Step 2: Last electron enters 4p orbital $\Rightarrow n=4, l=1$.

Step 3: Possible $m_l$ values: $-1, 0, +1$ and last electron pairs up $\Rightarrow m_s = -\frac{1}{2}$.

Step 4: Valid set matching these values is (4,1,1,$-\frac{1}{2}$).
Conclusion:
Correct quantum numbers are (4,1,1,$-\frac{1}{2}$)