The electron configuration of $_{29}$ Cu is

Step 1: Analyze the question We need the ground-state electron configuration of copper (Z = 29), so it has 29 electrons.

Step 2: The concept Electrons fill orbitals in this order (Aufbau principle): 1s → 2s → 2p → 3s → 3p → 4s → 3d. Each subshell holds a limited number of electrons: s = 2, p = 6, d = 10. Copper is a well-known exception. A completely filled 3d subshell (d¹⁰) is extra stable, so one electron moves from 4s to 3d.

Step 3: Solution

• Expected: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹
• Actual (the exception): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰

Check the total: 2 + 2 + 6 + 2 + 6 + 1 + 10 = 29 ✓

Step 4: Check the options

• A: 2+2+6+2+6+1+10 = 29 ✓ and it matches copper's configuration.
• B: 4s³ is impossible, because an s subshell holds a maximum of 2 electrons. ✗
• C: 2p⁵ gives a total of 2+2+5+2+6+2+10 = 29, but 2p is not filled before 3s begins, so it is not the ground state. ✗
• D: 3p² gives a total of 2+2+6+2+2+1+10 = 25, not 29. ✗

Answer: A

Exam tip: Remember the two famous exceptions, Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹). Half-filled and fully filled d subshells are extra stable. Also, always add up the superscripts to check they equal the atomic number, which quickly eliminates wrong options.