Q.General electronic configuration of actinoids is $(n-2)f^{1-14}(n-1)d^{0-2}ns^2$. Which of the following actinoids have one electron in 6d orbital?

The key idea is that the actinoid contraction (poor shielding by 5f electrons) makes the 6d orbital slightly higher in energy, so electrons fill the 5f subshell first. For the early actinoids, the 6d orbital gets one electron before 5f filling begins.

Reasoning:

1. The general configuration is $[\text{Rn}] 5f^{1-14} 6d^{0-2} 7s^2$. For atomic numbers 90–95, the 5f and 6d levels are very close.
2. Thorium (Z=90) is $6d^2 7s^2$; protactinium (Z=91) is $5f^2 6d^1 7s^2$. From uranium onward, the 5f subshell starts filling preferentially. …

Ground-state configurations of the early actinoids:

• U (Z = 92): $[Rn]5f^3\,6d^1\,7s^2$ — one 6d electron.
• Np (Z = 93): $[Rn]5f^4\,6d^1\,7s^2$ — one 6d electron.
• Pu (Z = 94): $[Rn]5f^6\,7s^2$ — no 6d electron.
• Am (Z = 95): $[Rn]5f^7\,7s^2$ — no 6d electron. …

Method: Electronic Configuration Writing Using Aufbau Principle

This method uses the Aufbau principle (filling orbitals in order of increasing energy) along with the general electronic configuration of actinoids to determine which elements have a 6d¹ configuration.

Steps:

Step 1: Recall the general configuration pattern

Actinoids follow:

For actinoids, $n = 7$, so:

Step 2: Write the expected configuration for each element

Start from the base: Radon ($Z = 86$) has configuration $[\text{Rn}]$. Then add electrons in order:

• First fill $7s^2$
• Then $5f$ orbitals
• Then $6d$ orbitals (if any electrons remain)

Step 3: Apply to each element

• Uranium (Z = 92):

  $92 - 86 = 6$ electrons beyond Rn

  $7s^2$ (2 e⁻) + $5f^3$ (3 e⁻) + $6d^1$ (1 e⁻) = 6 e⁻

  → Has 6d¹ ✓

• Neptunium (Z = 93):

  $93 - 86 = 7$ electrons beyond Rn …

Common Mistakes Students Make on This Question

Mistake 1: Forgetting the Order of Filling (Aufbau Principle)

The error: Students often write the electronic configuration by simply filling the 5f orbitals first, without considering that the 6d orbital can be occupied before 5f in some cases.

How to avoid: Remember the Aufbau order:

For actinoids, the general sequence is:

$1s \to 2s \to 2p \to 3s \to 3p \to 4s \to 3d \to 4p \to 5s \to 4d \to 5p \to 6s \to 4f \to 5d \to 6p \to 7s \to 5f \to 6d$

So 7s fills first, then 5f, then 6d — but only after 5f is partially filled.

Mistake 2: Assuming All Actinoids Have Zero 6d Electrons

The error: Many students think that because the general configuration shows $(n-1)d^{0-2}$, the 6d orbital is always empty for early actinoids.

The truth: For Uranium (Z=92) and Neptunium (Z=93), one electron actually occupies the 6d orbital before the 5f subshell is fully populated.

How to avoid: Write the actual configurations step-by-step:

• U (92):

  $[Rn] 5f^3 6d^1 7s^2$ — one electron in 6d ✓

• Np (93):

  $[Rn] 5f^4 6d^1 7s^2$ — one electron in 6d ✓

• Pu (94):

  $[Rn] 5f^6 7s^2$ — no 6d electron ✗

• Am (95):

  $[Rn] 5f^7 7s^2$ — no 6d electron ✗

Mistake 3: Confusing with Lanthanoid Contraction Analogy

The error: Students incorrectly apply the lanthanoid pattern (where 4f fills before 5d) to actinoids, assuming 5f always fills before 6d.

The key difference:

In actinoids, the 5f and 6d orbitals are closer in energy than 4f and 5d in lanthanoids. This causes early actinoids (U, Np) to have a 6d electron. …