Which of the following statements are correct? (A) The electronic configuration of Cr is [Ar]\,3d5\,4s1. (B) The…

🧠 The Atomic Ground Rules Building an atom requires following three key principles: Aufbau (order of energy), Pauli (no shared addresses), and Hund (maximum spin).

🗺️ The Fact Check

• (A): CORRECT. Chromium ($Z=24$) is a famous exception. It adopts $[\text{Ar}] 4s^1 3d^5$ to achieve a half-filled, symmetric d-subshell.
• (B): CORRECT. Magnetic quantum number ($m_l$) ranges from $-l$ to $+l$. For $l=1$ (p), $m_l$ can be $-1$.
• (C): CORRECT. The Aufbau Principle states that electrons occupy the lowest available energy orbital first.
• (D): FALSE. The total number of nodes (radial + angular) is given by $(n-1)$. The formula $(n-2)$ has no basis in the standard model.

Most appropriate answer: (A), (B) & (C).

⚡ The $d^5$ Stability Chromium and Copper ($d^{10}$) are the two "thieves" of the periodic table—they steal one electron from the 4s to complete their d-subshell stability.

⚠️ Common Traps Forgetting the $(n-1)$ node rule. For a 1s orbital, $n=1$, nodes $= 0$. For 2s, $n=2$, nodes $= 1$. The count is always one less than the shell number.

$\boxed{\text{Answer: (d)}}$