What is the ground-state electron configuration of iron (Fe, atomic number 26) using noble-gas shorthand?

• A. $\left[Ar\right]4^{s}23^{d}8$
• B. $\left[Ne\right]3^{s}23^{p}64^{s}23^{d}6$
• C. $\left[Ar\right]4^{s}23^{d}6$
• D. $\left[Ar\right]4^{s}13^{d}7$

Answer: (C)

Verified step by step guidance

1. Identify the atomic number of iron (Fe), which is 26. This means iron has 26 electrons to arrange in its electron configuration.
2. Recall the order in which electron orbitals are filled according to the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, then 3d.
3. Write the electron configuration up to the nearest noble gas before iron, which is argon (Ar) with 18 electrons. This shorthand is written as \([\mathrm{Ar}]\) and represents the configuration \$1s^2 2s^2 2p^6 3s^2 3p^6$.
4. Distribute the remaining 8 electrons (26 total minus 18 in argon) into the 4s and 3d orbitals. According to the filling order, 4s fills before 3d, so place 2 electrons in 4s: \$4s^2$.
5. Place the remaining 6 electrons in the 3d orbital: \$3d^6\(. Therefore, the noble-gas shorthand electron configuration for iron is \)[\mathrm{Ar}] 4s^2 3d^6$.