There are 14 elements in actinoid series. Which of the following elements does not belong to this series? (i)

NCERT Exemplar · Q10

Q. There are 14 elements in actinoid series. Which of the following elements does not belong to this series?

(i) U

(ii) Np

(iii) Tm

(iv) Fm

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Concept understanding — Actinoid Contraction Greater

The Intuition First

Imagine you're holding a rope that runs through a series of rings. As you pull the rope tighter, the rings get squeezed closer together. Something similar happens inside the atoms of the actinoid elements — but instead of a rope, it's the pull of the nucleus, and instead of rings, it's the electron cloud.

The actinoid series runs from thorium ( $Z=90$ ) to lawrencium ( $Z=103$ ). As you go from one element to the next, you add one proton to the nucleus and one electron to the 5f subshell. That extra proton yanks harder on all the electrons. But here's the key: the 5f orbitals are shaped like little dumbbells that poke inside the atom, very close to the nucleus. They don't shield the outer electrons well from the growing nuclear charge.

So the outer electrons feel a stronger and stronger pull as you move right across the series. The entire electron cloud shrinks. The atomic radius decreases — steadily, noticeably. That's the actinoid contraction.

The Precise Statement

Actinoid contraction is the steady decrease in atomic (and ionic) radii across the actinoid series (from Th to Lr), caused by the poor shielding ability of 5f electrons.

The contraction is greater than what you see in the lanthanoid series (the 4f elements). Why? Two reasons:

The 5f orbitals are more diffuse and penetrate less than 4f orbitals. They sit further from the nucleus on average, so they shield even more poorly. Each added 5f electron does almost nothing to cancel the pull of the new proton.
Relativistic effects become significant for heavy nuclei (Z > 90). The 5f and 6d electrons move at speeds close to the speed of light, which contracts their orbitals further. This is a subtle but real effect that amplifies the contraction.

The result: the total decrease in ionic radius across the actinoid series is about 0.03–0.04 Å — roughly 10–15% larger than the lanthanoid contraction.

Why It Matters …

Why this formula?

Actinoid Contraction: Why It Is Greater Than Lanthanoid Contraction

Let’s build this from the ground up — understanding the why before the what.

1. What Is Actinoid Contraction?

Actinoid contraction is the steady decrease in atomic and ionic radii across the actinoid series (from Th to Lr, atomic numbers 90–103).

It is greater in magnitude than the analogous lanthanoid contraction (across the lanthanoids, Ce–Lu).

Key result: The contraction per element is larger in actinoids (~2–3 pm per element) than in lanthanoids (~1–2 pm per element).

2. The Core Reason: Poorer Shielding by 5f Electrons

The Shielding Effect

Electrons in inner shells shield the outer electrons from the full nuclear charge.
The effective nuclear charge ( $Z_{\text{eff}}$ ) felt by an electron is:

Z_{\text{eff}} = Z - S

where $Z$ = atomic number, $S$ = shielding constant.

Why 5f Shielding Is Weaker Than 4f

4f orbitals (lanthanoids) are more penetrating — they have a small but significant probability near the nucleus. This gives them better shielding ability.
5f orbitals (actinoids) are more diffuse and less penetrating. They are spread farther from the nucleus, so they shield the outer electrons less effectively.

Result: For the same increase in $Z$ , the $Z_{\text{eff}}$ increases more in actinoids than in lanthanoids.

A larger $Z_{\text{eff}}$ pulls the electron cloud inward more strongly → greater contraction.

3. Mathematical Expression of the Trend

The contraction is described by the change in radius per added proton:

For lanthanoids:

\Delta r_{\text{4f}} \approx -1 \text{ to } -2 \text{ pm per element}

For actinoids:

\Delta r_{\text{5f}} \approx -2 \text{ to } -3 \text{ pm per element}

The ratio of contraction is roughly:

\frac{\Delta r_{\text{5f}}}{\Delta r_{\text{4f}}} \approx 1.5 \text{ to } 2

4. Why the Difference in Shielding? — Orbital Shape Matters

Property	4f orbital (lanthanoids)	5f orbital (actinoids)
Principal quantum number ( $n$ )	4	5
Radial extent	More compact	More diffuse
Penetration near nucleus	Higher (has a small lobe near nucleus)	Lower (less probability near nucleus)
Shielding efficiency	Better	Poorer

The radial distribution function shows that 4f electrons have a secondary maximum close to the nucleus, while 5f electrons lack this — they are more "spread out."

5. The Chain of Reasoning (Step-by-Step)

Add a proton → nuclear charge $Z$ increases by 1.
Add a 5f electron → it shields poorly because of its diffuse shape.
Net effect: $Z_{\text{eff}}$ increases more than it would for a 4f electron. …

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The actinoid series consists of elements from atomic number 89 (Ac) to 102 (No). Thulium (Tm) has atomic number 69, placing it in the lanthanoid series, not the actinoids. The correct answer is (iii) Tm.

The actinoid series is the set of 14 elements that follow actinium (Ac, Z=89) in the periodic table, where the 5f subshell is progressively filled. These elements span atomic numbers 90 through 103, but the series is traditionally defined as the 14 elements from thorium (Th, Z=90) to lawrencium (Lr, Z=103), inclusive. The question states "14 elements in actinoid series," which matches this range.

Let’s check each option:

U (Uranium, Z=92) – Uranium is the fourth element in the actinoid series, following Th, Pa, and Np. It belongs here.
Np (Neptunium, Z=93) – Neptunium is the fifth actinoid, directly after uranium. It belongs.
Tm (Thulium, Z=69) – Thulium is in the lanthanoid series (the 4f block), not the actinoids. Its atomic number is far below 89. This is the odd one out.
Fm (Fermium, Z=100) – Fermium is the 11th actinoid, a synthetic element in the series. It belongs. …

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Common Mistakes: Actinoid Contraction & Series Membership

✗ Mistake 1: Confusing Actinoids with Lanthanoids

Students often mix up Tm (Thulium) — a lanthanoid — with an actinoid element.

Why it happens: Both series are f-block elements with similar properties (contraction, variable oxidation states). The names sound alike (actinoid vs lanthanoid), and Tm is rarely encountered in problems.

How to avoid: Memorise the first and last elements of each series:

Actinoids: Th (Z=90) to Lr (Z=103)
Lanthanoids: La (Z=57) to Lu (Z=71)

Tm (Z=69) lies within the lanthanoid series, not the actinoid series.

✗ Mistake 2: Forgetting the Atomic Number Range

Students rely on memory without checking atomic numbers.

Why it happens: The series names (U, Np, Fm) sound similar, and Tm is a less common symbol.

How to avoid: Always map symbol → atomic number quickly:

Symbol	Element	Z	Series
U	Uranium	92	Actinoid
Np	Neptunium	93	Actinoid
Tm	Thulium	69	Lanthanoid
Fm	Fermium	100	Actinoid

Key check: Z = 90–103 → actinoid. Z = 57–71 → lanthanoid.

✗ Mistake 3: Misinterpreting "Actinoid Contraction Greater"

Some students think "greater contraction" means all actinoids are more reactive or have unusual properties — leading them to wrongly exclude elements like Fm.

Why it happens: The phrase "actinoid contraction is greater than lanthanoid contraction" is often memorised without understanding. …

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Q.There are 14 elements in actinoid series. Which of the following elements does not belong to this series?

Concept understanding — Actinoid Contraction Greater

The Intuition First

The Precise Statement

Why It Matters …

Why this formula?

Actinoid Contraction: Why It Is Greater Than Lanthanoid Contraction

1. What Is Actinoid Contraction?

2. The Core Reason: Poorer Shielding by 5f Electrons

The Shielding Effect

Why 5f Shielding Is Weaker Than 4f

3. Mathematical Expression of the Trend

4. Why the Difference in Shielding? — Orbital Shape Matters

5. The Chain of Reasoning (Step-by-Step)

Method: Electronic Configuration Check for 5f-Series Elements

Why This Method Works

Steps

Common Mistakes: Actinoid Contraction & Series Membership

✗ Mistake 1: Confusing Actinoids with Lanthanoids

✗ Mistake 2: Forgetting the Atomic Number Range

✗ Mistake 3: Misinterpreting "Actinoid Contraction Greater"

Q.There are 14 elements in actinoid series. Which of the following elements does not belong to this series?

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