Which statement correctly identifies molybdenum’s electron configuration as an exception?

Which statement correctly identifies molybdenum’s electron configuration as an exception?

• A. [Kr] 5s1 4d5
• B. [Kr] 5s2 4d4
• C. [Kr] 5s2 4d5
• D. [Kr] 4s2 3d10

Answer: (A)

An important idea is that electron configurations sometimes deviate from the simple filling order because certain arrangements give extra stability. For transition metals, a half-filled or fully filled d subshell often lowers energy due to exchange interactions and reduced repulsion. Molybdenum has atomic number 42, and following a naïve Aufbau fill would suggest 4d^4 5s^2. But the actual ground-state configuration is 4d^5 5s^1. Keeping the 4d subshell half-filled (d^5) while keeping just one electron in 5s creates greater stability, so this arrangement is energetically favored over the expected one. This is the kind of exception seen when a subshell achieves a half-filled stability in the transition metals. So the configuration that matches molybdenum’s real electron arrangement is [Kr] 5s^1 4d^5.

An important idea is that electron configurations sometimes deviate from the simple filling order because certain arrangements give extra stability. For transition metals, a half-filled or fully filled d subshell often lowers energy due to exchange interactions and reduced repulsion.

Molybdenum has atomic number 42, and following a naïve Aufbau fill would suggest 4d^4 5s^2. But the actual ground-state configuration is 4d^5 5s^1. Keeping the 4d subshell half-filled (d^5) while keeping just one electron in 5s creates greater stability, so this arrangement is energetically favored over the expected one. This is the kind of exception seen when a subshell achieves a half-filled stability in the transition metals.

So the configuration that matches molybdenum’s real electron arrangement is [Kr] 5s^1 4d^5.