Why is the density of 5d transition series elements (e.g., Hf, Ta, W) almost double that of their corresponding 4d series elements (e.g., Zr, Nb, Mo)?
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Correct Answer
Due to lanthanoid contraction, the atomic volumes of 5d elements become very small while their atomic masses are much higher than those of corresponding 4d elements.
Detailed Explanation
Density is given by:
ρ = Mass / Volume
The 5d transition elements such as Hf, Ta and W have considerably larger atomic masses than their corresponding 4d elements Zr, Nb and Mo.
At the same time, the lanthanoid contraction causes the atomic and metallic radii of the 5d elements to be unexpectedly close to those of the corresponding 4d elements.
For example:
Zr ≈ Hf | Nb ≈ Ta | Mo ≈ W
Thus, their atomic volumes are relatively similar, but the 5d elements contain much heavier atoms.
Therefore:
Higher mass + nearly similar volume → much higher density
This is why the densities of many 5d elements are approximately twice those of their corresponding 4d elements.
Role of Lanthanoid Contraction
Between the 4d and 5d series, the lanthanoids are filled with electrons in the 4f subshell.
The 4f-electrons shield the nuclear charge poorly. Consequently, effective nuclear charge increases and pulls the outer electrons inward.
This produces lanthanoid contraction.
Poor 4f shielding → Zeff ↑ → atomic radius ↓
As a result, the size of 5d elements is much closer to that of their 4d counterparts than would normally be expected.
Example: Zr vs Hf
Zr (4d series): atomic mass ≈ 91.2 u
Hf (5d series): atomic mass ≈ 178.5 u
Hf has almost twice the atomic mass of Zr, while their atomic sizes are surprisingly similar because of lanthanoid contraction. Therefore, Hf has a much higher density.
Hf (5d series): atomic mass ≈ 178.5 u
Hf has almost twice the atomic mass of Zr, while their atomic sizes are surprisingly similar because of lanthanoid contraction. Therefore, Hf has a much higher density.
ρ(Hf) ≫ ρ(Zr)
The same general idea applies to pairs such as:
Nb → Ta and Mo → W
JEE / NEET Quick Trick
5d density ↑↑ because Mass ↑ greatly, but Volume remains nearly similar due to Lanthanoid Contraction.
Mass ↑ + Volume ≈ constant → Density ↑
Mass ↑ + Volume ≈ constant → Density ↑
30 Related MCQs with Solutions
1. The density of many 5d transition elements is much higher than that of corresponding 4d elements mainly because:
A. Their atomic masses are much higher while atomic volumes are similar
B. Their atomic masses are lower
C. Their atomic radii are much larger
D. They have fewer electrons
Answer: A
2. Which phenomenon makes the sizes of Hf and Zr very similar?
A. Inert pair effect
B. Lanthanoid contraction
C. Diagonal relationship
D. Relativistic effect only
Answer: B. Lanthanoid contraction
3. Density is defined as:
A. Volume/Mass
B. Mass/Volume
C. Mass × Volume
D. Mass + Volume
Answer: B. Mass/Volume
4. Which pair belongs to corresponding 4d and 5d series?
A. Zr and Hf
B. Na and K
C. Ca and Sr
D. Sc and Y
Answer: A. Zr and Hf
5. The atomic mass of Hf is approximately:
A. 91 u
B. 120 u
C. 178.5 u
D. 207 u
Answer: C. 178.5 u
6. The atomic mass of Zr is approximately:
A. 40 u
B. 91 u
C. 178 u
D. 238 u
Answer: B. 91 u
7. Which statement about Zr and Hf is correct?
A. Hf is much larger than Zr
B. Their radii are surprisingly similar
C. Zr is much heavier than Hf
D. They belong to the same period
Answer: B
8. Lanthanoid contraction is primarily caused by:
A. Excellent shielding by 4f-electrons
B. Poor shielding by 4f-electrons
C. Absence of 4f-electrons
D. 5d shielding
Answer: B
9. Which 5d element corresponds to Nb?
A. Ta
B. W
C. Hf
D. Re
Answer: A. Ta
10. Which 5d element corresponds to Mo?
A. Hf
B. Ta
C. W
D. Re
Answer: C. W
11. Which 5d element corresponds to Zr?
A. Hf
B. Ta
C. W
D. Os
Answer: A. Hf
12. The high density of W compared with Mo is primarily related to:
A. Lower mass of W
B. Higher mass of W and relatively small atomic volume
C. Larger volume of W only
D. Absence of d-electrons
Answer: B
13. Which electrons show poor shielding in lanthanoids?
A. 1s
B. 2p
C. 3d
D. 4f
Answer: D. 4f
14. Due to lanthanoid contraction, the 5d elements have:
A. Much larger radii than 4d elements
B. Radii comparable to corresponding 4d elements
C. Zero atomic radius
D. Radii smaller than all 3d elements
Answer: B
15. The density of an element increases when:
A. Mass decreases and volume increases
B. Mass increases while volume remains similar
C. Both mass and volume decrease equally
D. Mass becomes zero
Answer: B
16. Which pair has nearly similar atomic radii because of lanthanoid contraction?
A. Zr and Hf
B. Na and K
C. Li and Cs
D. Mg and Ba
Answer: A
17. The 5d series follows the:
A. 3d series
B. 4d series
C. 6d series
D. 4f series only
Answer: B. 4d series
18. Which of the following has higher density?
A. Zr
B. Hf
C. Both have exactly equal density
D. Cannot be predicted
Answer: B. Hf
19. The unexpectedly similar size of Zr and Hf is due to:
A. Poor shielding by 4f-electrons
B. Excellent shielding by 5d-electrons
C. Inert pair effect
D. Hydrogen bonding
Answer: A
20. Which transition series contains Hf, Ta and W?
A. 3d
B. 4d
C. 5d
D. 6d
Answer: C. 5d
21. Which transition series contains Zr, Nb and Mo?
A. 3d
B. 4d
C. 5d
D. 6d
Answer: B. 4d
22. The main factor increasing density from 4d to 5d counterparts is:
A. Higher atomic mass
B. Lower nuclear charge
C. Larger atomic volume
D. Fewer protons
Answer: A. Higher atomic mass
23. The relatively small atomic volume of 5d elements is associated with:
A. Lanthanoid contraction
B. Alkali contraction
C. Hydrogen contraction
D. Inert gas expansion
Answer: A. Lanthanoid contraction
24. Which quantity is in the denominator of density?
A. Mass
B. Volume
C. Atomic number
D. Nuclear charge
Answer: B. Volume
25. Which of the following is a consequence of lanthanoid contraction?
A. Hf and Zr have very different radii
B. Hf and Zr have similar radii
C. Hf has no d-electrons
D. Zr becomes a lanthanoid
Answer: B
26. Which statement is correct regarding Ta and Nb?
A. Ta has much lower atomic mass
B. Ta has higher atomic mass and much higher density
C. Nb is a 5d element
D. Both are lanthanoids
Answer: B
27. Which element is the densest among the following?
A. Zr
B. Hf
C. Nb
D. Ta
Answer: D. Ta
28. Which element has a density close to that expected from a much heavier atom because of its compact atomic volume?
A. Hf
B. Na
C. K
D. Li
Answer: A. Hf
29. The correct relationship is:
A. ρ = V/M
B. ρ = M/V
C. ρ = M×V
D. ρ = M+V
Answer: B. ρ = M/V
30. The best explanation for the unusually high density of 5d transition metals is:
A. Higher atomic mass combined with lanthanoid-contraction-induced small atomic volume
B. Lower atomic mass and larger volume
C. Absence of f-electrons
D. Low nuclear charge
Answer: A
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