Electron Pairing in Ground-State Configurations

Steps:

• Identify atomic numbers: Ba (56), Br (35), S (16), Si (14).
• Determine electron configurations: Ba ([Xe] 6s²), Br ([Ar] 3d¹⁰ 4s² 4p⁵), S ([Ne] 3s² 3p⁴), Si ([Ne] 3s² 3p²).
• Examine orbital filling: Paired electrons fill orbitals completely (two per orbital); unpaired occur in partially filled p subshells.
• Identify atom with no unpaired electrons: Only Ba has all orbitals fully paired.

Why A is correct:

• Barium's even number of 56 electrons fills all orbitals in pairs per the Aufbau principle, resulting in a closed-shell configuration.

Why the others are wrong:

• Br: 4p⁵ subshell has one unpaired electron.
• S: 3p⁴ subshell has two unpaired electrons.
• Si: 3p² subshell has two unpaired electrons.

Final answer: A