Question

Explain why actinoid contraction is greater from element to element than lanthanoid contraction.

Hint:

Just remember the order of shielding effect: \(s \gt p \gt d \gt f\). Since \(5f\) is larger and more diffused than \(4f\), its shielding is even worse, leading to greater contraction.

Answer 1

Step 1: Recall what causes the contraction in both series.
As we move across the lanthanoids or the actinoids, each new electron enters an f-orbital, and f-orbitals are known to be very diffuse and spread out in shape, so they shield the outer electrons from the nucleus rather poorly.
Step 2: Compare the diffuseness of 4f and 5f orbitals.
The $5f$ orbitals of the actinoids extend over a larger region of space than the $4f$ orbitals of the lanthanoids, which means the shielding provided by a $5f$ electron is even weaker than that provided by a $4f$ electron.
Step 3: Translate weaker shielding into a stronger pull on outer electrons.
Because the shielding is poorer in the actinoids, the outer electrons feel a greater effective nuclear charge as atomic number rises, and are consequently pulled in more strongly at every step along the series.
Step 4: State the conclusion.
This stronger, less shielded nuclear pull is why the drop in atomic and ionic size from one actinoid to the next is larger than the corresponding drop across the lanthanoids. \[ \boxed{\text{Poorer shielding by diffuse } 5f \text{ electrons causes greater actinoid contraction}} \]