Question

which particle is not isoelectronic with the electron configuration given below?
$1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6$
br
rb⁺
se²⁻
kr

Explanation:

Step1: Count total electrons

Count electrons in given configuration:
$2+2+6+2+6+2+10+6 = 36$ electrons

Step2: Find electrons in each particle

• Br: Atomic number 35, neutral = 35 electrons
• $\text{Rb}^+$: Atomic number 37, loses 1 e⁻ = 36 electrons
• $\text{Se}^{2-}$: Atomic number 34, gains 2 e⁻ = 36 electrons
• Kr: Atomic number 36, neutral = 36 electrons

Step3: Identify non-isoelectronic particle

Isoelectronic = same electron count; Br has 35 ≠ 36.

Answer:

Br