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Thursday, August 20, 2026

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)?

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)? +
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.
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 ↑
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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What is the primary cause of Lanthanoid Contraction?

What is the primary cause of Lanthanoid Contraction? +
Correct Answer
Poor shielding effect of the 4f-electrons
Detailed Explanation
Lanthanoid contraction is the gradual decrease in atomic and ionic radii of the lanthanoid elements from La to Lu with increasing atomic number. The main reason is the poor shielding ability of the 4f-electrons. As we move from La to Lu:
  • The nuclear charge increases because the atomic number increases.
  • Electrons are progressively added to the 4f subshell.
  • The 4f-electrons shield the nuclear charge ineffectively.
  • Therefore, the effective nuclear charge experienced by the outer electrons increases.
  • The electron cloud is pulled closer to the nucleus.
  • Hence, atomic and ionic radii gradually decrease.
Increasing nuclear charge + poor 4f shielding → increasing Zeff → decreasing radius
Why is 4f Shielding Poor?
The shielding effectiveness generally follows:
s > p > d > f
Since 4f orbitals have poor penetration and are relatively diffuse, their shielding of the nuclear charge is weak. Therefore, the increasing nuclear charge is not effectively cancelled by the added 4f-electrons.
Major Consequences of Lanthanoid Contraction
1. Similarity between 4d and 5d elements
For example, Zr and Hf have very similar atomic radii and chemical properties.

2. Decrease in basicity of lanthanoid hydroxides
Basicity generally decreases from La(OH)3 to Lu(OH)3.

3. Difficulty in separation of lanthanoids
Their chemical properties become increasingly similar.
JEE / NEET Quick Trick
Lanthanoid Contraction = Poor shielding by 4f-electrons

4f shielding ↓ → Zeff ↑ → Radius ↓
30 Related MCQs with Solutions
1. The primary cause of lanthanoid contraction is:
A. Increase in atomic mass
B. Poor shielding by 4f-electrons
C. Increase in number of shells
D. Strong shielding by 4f-electrons
Answer: B. Poor shielding by 4f-electrons
2. Lanthanoid contraction refers to:
A. Increase in atomic radius across lanthanoids
B. Decrease in atomic/ionic radii across lanthanoids
C. Increase in ionic charge
D. Decrease in atomic number
Answer: B
3. Which electrons are mainly responsible for lanthanoid contraction?
A. 2p
B. 3d
C. 4f
D. 5d
Answer: C. 4f
4. The shielding efficiency of 4f-electrons is:
A. Very high
B. Moderate
C. Poor
D. Perfect
Answer: C. Poor
5. Across the lanthanoid series, effective nuclear charge generally:
A. Decreases
B. Remains constant
C. Increases
D. Becomes zero
Answer: C. Increases
6. The gradual decrease in ionic radii from La3+ to Lu3+ is called:
A. Inert pair effect
B. Lanthanoid contraction
C. Diagonal relationship
D. Shielding effect
Answer: B
7. Which orbital has the poorest shielding ability?
A. s
B. p
C. d
D. f
Answer: D. f
8. Correct order of shielding effectiveness is:
A. f > d > p > s
B. s > p > d > f
C. p > s > f > d
D. d > f > s > p
Answer: B. s > p > d > f
9. Due to lanthanoid contraction, the size of Ln3+ ions:
A. Increases
B. Decreases
C. Remains unchanged
D. First increases then decreases
Answer: B. Decreases
10. Lanthanoid contraction results mainly from increasing:
A. Atomic volume
B. Effective nuclear charge
C. Shielding effect
D. Number of shells
Answer: B. Effective nuclear charge
11. Which pair has unusually similar atomic radii because of lanthanoid contraction?
A. Na and K
B. Zr and Hf
C. Li and Na
D. Mg and Ca
Answer: B. Zr and Hf
12. The 4f-electrons are added progressively in:
A. Alkali metals
B. Lanthanoids
C. Halogens
D. Noble gases
Answer: B. Lanthanoids
13. Lanthanoid contraction causes the basicity of Ln(OH)3 to generally:
A. Increase across the series
B. Decrease across the series
C. Remain constant
D. Become zero
Answer: B. Decrease
14. Which statement is correct?
A. 4f-electrons shield very effectively
B. 4f-electrons shield poorly
C. 4f-electrons are absent in lanthanoids
D. 4f-electrons increase atomic radius
Answer: B
15. The effective nuclear charge increases because:
A. Nuclear charge increases faster than shielding
B. Nuclear charge decreases
C. 4f shielding is perfect
D. Electrons disappear
Answer: A
16. Which series shows lanthanoid contraction?
A. Li to Ne
B. Sc to Zn
C. La to Lu
D. Na to Ar
Answer: C. La to Lu
17. Which of the following is NOT a consequence of lanthanoid contraction?
A. Similarity of Zr and Hf
B. Decrease in basicity of hydroxides
C. Gradual decrease in Ln3+ radii
D. Large increase in atomic radius from La to Lu
Answer: D
18. Poor shielding is mainly due to the nature of:
A. s-orbitals
B. p-orbitals
C. d-orbitals
D. f-orbitals
Answer: D. f-orbitals
19. As atomic number increases across lanthanoids, nuclear charge:
A. Decreases
B. Increases
C. Remains constant
D. Becomes negative
Answer: B. Increases
20. Lanthanoid contraction is most directly associated with:
A. 4f filling
B. 3d filling
C. 5p filling
D. 6s filling
Answer: A. 4f filling
21. Which statement best explains lanthanoid contraction?
A. Added 4f electrons completely cancel added nuclear charge
B. Added 4f electrons poorly shield the increasing nuclear charge
C. Nuclear charge decreases
D. Number of shells decreases
Answer: B
22. The radius of Hf is unexpectedly similar to that of:
A. Ti
B. Zr
C. Sc
D. Y
Answer: B. Zr
23. Which electrons have the least penetrating power among s, p, d and f?
A. s
B. p
C. d
D. f
Answer: D. f
24. Lanthanoid contraction is a result of competition between:
A. Nuclear charge and shielding
B. Mass and volume
C. Ionization and electron affinity only
D. Melting point and boiling point
Answer: A
25. Which ion has a smaller radius: La3+ or Lu3+?
A. La3+
B. Lu3+
C. Both are equal
D. Cannot be predicted
Answer: B. Lu3+
26. Which property generally decreases from La(OH)3 to Lu(OH)3?
A. Basicity
B. Atomic number
C. Nuclear charge
D. Number of electrons
Answer: A. Basicity
27. Which effect explains the similarity of the second and third transition series?
A. Inert pair effect
B. Lanthanoid contraction
C. Diagonal effect
D. Photoelectric effect
Answer: B. Lanthanoid contraction
28. Which is the correct sequence of shielding effectiveness?
A. f > d > p > s
B. s > p > d > f
C. s > d > p > f
D. p > s > d > f
Answer: B
29. The net effect of lanthanoid contraction is:
A. Increasing size
B. Decreasing size
C. No change in size
D. Random change in size
Answer: B. Decreasing size
30. The best one-line explanation of lanthanoid contraction is:
A. Strong shielding by 4f-electrons
B. Poor shielding by 4f-electrons causing increasing effective nuclear charge
C. Decreasing nuclear charge
D. Addition of new shells
Answer: B
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Which of the following statements regarding interstitial compounds is incorrect?

Which of the following statements regarding interstitial compounds is incorrect? +
Correct Answer
Incorrect statement: Interstitial compounds are always non-stoichiometric.
Detailed Explanation
Interstitial compounds are formed when small atoms such as H, B, C or N occupy the interstitial spaces (holes) in the crystal lattice of transition metals. Common examples include:
Fe3C   |   TiC   |   VN   |   Fe4N
Their important characteristics are:
  • They are generally hard and rigid.
  • They often have high melting points.
  • They generally retain the metallic conductivity of the parent metal.
  • They are usually formed by small atoms occupying interstitial sites in a metal lattice.
  • They may be non-stoichiometric, but they are not necessarily always non-stoichiometric.
Therefore, saying that all interstitial compounds are necessarily non-stoichiometric is incorrect.
Why are they called Interstitial Compounds?
The small atoms do not normally replace metal atoms in the crystal lattice. Instead, they occupy the empty spaces between the metal atoms.
Metal lattice + small atom in interstitial site → Interstitial compound
For example, carbon atoms can occupy the spaces in the iron lattice to form iron carbide.
JEE / NEET Quick Revision
Interstitial compounds → small atoms in lattice holes → hard + high m.p. + metallic conductivity → often non-stoichiometric, but not always.
30 Related MCQs with Solutions
1. Interstitial compounds are formed when small atoms occupy:
A. Substitutional sites only
B. Interstitial sites
C. Nuclei of metal atoms
D. Valence shells only
Answer: B. Interstitial sites
2. Which of the following atoms commonly forms interstitial compounds?
A. Na
B. K
C. C
D. Cs
Answer: C. Carbon
3. Interstitial compounds are mainly associated with:
A. Alkali metals
B. Transition metals
C. Noble gases
D. Halogens
Answer: B. Transition metals
4. Which is an example of an interstitial compound?
A. NaCl
B. TiC
C. H2O
D. CO2
Answer: B. TiC
5. Interstitial compounds are generally:
A. Soft
B. Hard
C. Gaseous
D. Volatile liquids
Answer: B. Hard
6. Interstitial compounds generally have:
A. Very low melting points
B. High melting points
C. No melting point
D. Melting points below room temperature
Answer: B. High melting points
7. Which property is usually retained by interstitial compounds?
A. Metallic conductivity
B. Molecular volatility
C. Ionic conductivity only
D. Gas-like behaviour
Answer: A. Metallic conductivity
8. Which statement is incorrect?
A. Small atoms occupy lattice holes
B. They are generally hard
C. They always have fixed stoichiometry
D. They can retain metallic character
Answer: C.
Interstitial compounds can often show variable composition.
9. Which element can occupy interstitial positions in transition-metal lattices?
A. Hydrogen
B. Carbon
C. Nitrogen
D. All of these
Answer: D. All of these
10. The small atoms in interstitial compounds are generally:
A. Larger than metal atoms
B. Smaller than metal atoms
C. Equal in size to metal atoms
D. Always noble gases
Answer: B. Smaller than metal atoms
11. Which compound is associated with iron and carbon?
A. FeCl3
B. Fe3C
C. FeSO4
D. Fe(NO3)3
Answer: B. Fe3C
12. Which of the following is a carbide-type interstitial compound?
A. TiC
B. NaCl
C. MgO
D. CaCO3
Answer: A. TiC
13. Interstitial compounds are generally formed by:
A. Large non-metals entering lattice sites
B. Small atoms entering lattice holes
C. Complete replacement of metal atoms
D. Noble gases entering the nucleus
Answer: B
14. Which is NOT a typical property of interstitial compounds?
A. High hardness
B. High melting point
C. Metallic conductivity
D. High volatility
Answer: D. High volatility
15. Interstitial compounds may be:
A. Only stoichiometric
B. Only non-stoichiometric
C. Stoichiometric or non-stoichiometric
D. Always molecular
Answer: C
16. The term "interstitial" refers to:
A. Surface of a crystal
B. Space between atoms in a lattice
C. Nucleus of an atom
D. Electron cloud
Answer: B
17. Which element is particularly small and can enter metal lattice holes?
A. H
B. Cs
C. Rb
D. Ba
Answer: A. Hydrogen
18. Interstitial compounds are usually:
A. Non-metallic insulators
B. Metallic or metal-like in conductivity
C. Molecular gases
D. Ionic liquids
Answer: B
19. Which statement about stoichiometry is correct?
A. All interstitial compounds must be non-stoichiometric
B. Some interstitial compounds have definite stoichiometry
C. None have definite composition
D. Stoichiometry is impossible
Answer: B
20. Which pair contains interstitial compounds?
A. TiC and VN
B. NaCl and KBr
C. H2O and NH3
D. CO2 and SO2
Answer: A. TiC and VN
21. Interstitial atoms mainly cause:
A. Weakening of the metallic lattice
B. Distortion and strengthening of the lattice
C. Complete destruction of lattice
D. Formation of a gas
Answer: B
22. Which is an important industrial feature of many interstitial compounds?
A. High hardness
B. Extreme volatility
C. Low thermal stability
D. Low melting point
Answer: A. High hardness
23. Carbon in iron lattice can form:
A. Fe3C
B. FeCl3
C. FeO4
D. Fe(NO3)2
Answer: A. Fe3C
24. Which small atoms are commonly involved in interstitial compounds?
A. H, B, C, N
B. Na, K, Rb, Cs
C. Cl, Br, I, At
D. Xe, Kr, Ar, Ne
Answer: A
25. Which statement is true?
A. Interstitial compounds are always ionic
B. Interstitial compounds are generally associated with metallic lattices
C. Interstitial compounds are always molecular
D. They contain no metals
Answer: B
26. Which property results from strong interaction between the interstitial atom and metal lattice?
A. Increased hardness
B. Increased volatility
C. Gas formation
D. Loss of all conductivity
Answer: A
27. Which statement is incorrect regarding interstitial compounds?
A. Small atoms occupy lattice holes
B. They are generally hard
C. They are always non-stoichiometric
D. They may have high melting points
Answer: C. They are always non-stoichiometric
28. VN is an example of an interstitial compound involving:
A. Vanadium and nitrogen
B. Vanadium and sodium
C. Vanadium and neon
D. Vanadium and chlorine
Answer: A. Vanadium and nitrogen
29. The electrical conductivity of many interstitial compounds is:
A. Metallic
B. Always zero
C. Only ionic
D. Molecular
Answer: A. Metallic
30. The best description of interstitial compounds is:
A. Small atoms occupy interstitial sites of a metal lattice
B. Large atoms replace all metal atoms
C. Only non-metals form them
D. They are always gaseous
Answer: A
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