d and f Block Elements MCQs
Multiple Choice Questions for NEET and IIT-JEE
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Transition elements have both (n-1)d and ns electrons available for bonding because the energy gap between these orbitals is very small. This allows them to lose or share different numbers of electrons, resulting in variable oxidation states.
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Manganese (Mn) has the electronic configuration [Ar] 3d5 4s2. By involving all 5 of its 3d electrons and both 4s electrons in bonding (as seen in compounds like KMnO4), it can exhibit a maximum oxidation state of +7, which is the highest in the 3d transition series.
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Across a period in the 3d transition series, as the atomic number increases, the effective nuclear charge increases because the shielding effect of d-electrons is poor. This extra nuclear pull causes a contraction in ionic size. Moving from Cr (Z=24) to Mn (Z=25) to Fe (Z=26) to Co (Z=27) in the +3 oxidation state, the size decreases. Cr3+ has the lowest atomic number among them and thus the largest ionic radius.
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The energy levels of the outermost ns orbital and the inner (n-1)d orbital are remarkably close. When transition metals form bonds, they lose electrons from the ns orbital first, and depending on the chemical environment, they can also readily involve varying numbers of electrons from the (n-1)d subshell.
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Transition metals make excellent catalysts because they can transition between different oxidation states to form unstable intermediate compounds with reactants. Their empty or partially filled d-orbitals also give them a high propensity to form complexes, lowering the activation energy of the reaction.
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This statement is false. Most transition metal ions are vividly colored due to d-d electronic transitions. When d-orbitals are partially filled, electrons can absorb visible light to jump to higher d-orbital energy levels, leaving behind a colored ion.
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The general electronic configuration for d-block transition elements features a progressively filled inner d-subshell and an outer s-subshell containing 1 or 2 electrons, which is expressed as (n-1)d1−10 ns1−2.
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The presence of unpaired electrons in the (n-1)d subshell allows for d-d transitions. When a ligand approaches, the d-orbitals split into different energy levels, and these unpaired electrons absorb specific wavelengths of visible light to transition between levels, causing the compound to appear colored.
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Let's evaluate the d-configurations:
• Fe2+: [Ar] 3d6 → 4 unpaired electrons.
• Cu: [Ar] 3d10 4s1 → 0 unpaired d-electrons.
• Zn: [Ar] 3d10 4s2 → 0 unpaired d-electrons.
• Ni3+: [Ar] 3d7 → 3 unpaired electrons.
Therefore, Fe2+ has the maximum number of unpaired d-electrons.
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Zinc has the electronic configuration [Ar] 3d10 4s2. Because its 3d subshell is completely filled and highly stable, zinc only loses its two 4s electrons to form Zn2+. It does not lose electrons from its stable 3d10 shell, preventing variable valencies.
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Strictly speaking, Lanthanum (Z=57, [Xe] 5d1 6s2) is a d-block element because its last electron enters a d-orbital. However, because its physical and chemical properties mirror those of the 14 lanthanides that follow it (Ce to Lu), it is traditionally studied alongside them and grouped in the lanthanide family.
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Transition elements show variable valencies/oxidation states because electrons from both the outer ns orbital and the inner (n-1)d orbital can participate in chemical bonding due to their close energy levels.
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Showing variable oxidation states is a defining feature of transition elements. They do form alloys easily due to similar atomic sizes, they are largely paramagnetic due to unpaired d-electrons, and the inert pair effect is a property of heavier p-block elements, not d-block elements.
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Neutral Chromium (Cr, Z=24) has an exceptional ground-state configuration of [Ar] 3d5 4s1. When forming a Cr2+ ion, it loses 2 electrons: first from the outermost 4s subshell, and then one from the 3d subshell. This leaves it with a valence configuration of 3d4 4s0.
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Let's check for unpaired electrons which are needed for d-d color transitions:
• Zn2+: 3d10 (0 unpaired electrons → colourless)
• Ni2+: 3d8 (2 unpaired electrons → coloured)
• Cr3+: 3d3 (3 unpaired electrons → coloured)
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Transition elements explicitly exhibit variable valencies due to the close energy levels of (n-1)d and ns electrons. Therefore, stating that they do not exhibit variable valency is incorrect.
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The first transition series (3d series) spans from Scandium (Z=21) to Zinc (Z=30). Iron (Fe), Copper (Cu), and Vanadium (V) all belong to this series. Silver (Ag, Z=47) belongs to the second transition series (4d series).
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Due to the extra stability associated with half-filled subshells (d5), an electron shifts from the 4s orbital to the 3d orbital. Thus, Chromium's valence structure is 3d5 4s1 rather than the expected 3d4 4s2.
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The 3d5 4s2 configuration corresponds to Manganese (Mn). It has 7 valence electrons available for bonding, allowing it to reach a maximum oxidation state of +7, which is higher than any of the other configurations given.
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Magnetic moment is calculated by the spin-only formula: μ = √[n(n+2)] Bohr Magnetons, where n is the number of unpaired electrons. A 3d5 configuration contains 5 unpaired electrons (the maximum possible in a d-subshell), thereby yielding the highest magnetic moment.
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Let's check the neutral atomic configurations:
• Cr: [Ar] 3d5 4s1 → 5 (d) + 1 (s) = 6 unpaired electrons.
• Fe: [Ar] 3d6 4s2 → 4 unpaired electrons.
• Sc: [Ar] 3d1 4s2 → 1 unpaired electron.
• V: [Ar] 3d3 4s2 → 3 unpaired electrons.
Chromium has the maximum total number of unpaired electrons.
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The strength of a metallic bond depends on the number of unpaired valence electrons available to contribute to the collective metallic lattice sea. Chromium ([Ar] 3d5 4s1) has 6 unpaired electrons, leading to exceptionally strong inter-atomic metallic bonding compared to V, Fe, or Sc.
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The spin-only magnetic moment increases directly with the number of unpaired electrons. A 3d5 configuration possesses 5 unpaired electrons, which is the maximum configuration capacity for a single d-subshell.
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As we move across the lanthanide series, electrons are added to the inner 4f subshell. The shape of f-orbitals is highly diffuse, meaning they shield outer electrons from the growing nuclear charge poorly. Consequently, the effective nuclear charge increases steadily, drawing the outer shells closer to the nucleus and causing a steady contraction in atomic/ionic size.
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Transition metals like Fe, Ni, and Co pack tightly into crystal lattices leaving empty spaces called interstices. Small non-metal atoms like H, B, C, and N can trap themselves inside these gaps without forming formal chemical bonds, generating interstitial compounds.
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Ozone reacts with Mercury (Hg) to form mercurous oxide (Hg2O). This reaction breaks the surface tension of mercury, causing it to lose its meniscus and stick to the glass walls of a vessel instead of rolling smoothly. This phenomenon is known as the "tailing of mercury" and serves as a classic qualitative test for ozone gas.
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Neutral Chromium configuration is 3d5 4s1. Removing 3 electrons to form Cr3+ yields a 3d3 configuration. Following Hund's rule, these 3 electrons sit in separate d-orbitals with parallel spins, giving 3 unpaired electrons.
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Zinc forms stable Zn2+ ions with a completely filled 3d10 configuration. Because there are no empty spots or unpaired electrons within the d-subshell, d-d electronic transitions cannot happen, making zinc compounds white/colourless.
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Let's check configurations:
• Fe3+: 3d5 (unpaired electrons → coloured)
• Cr3+: 3d3 (unpaired electrons → coloured)
• Cu2+: 3d9 (unpaired electrons → coloured)
• Cu+: 3d10 (completely filled d-shell → colourless)
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Let's examine the configurations:
• Cu+: 3d10 (no unpaired electrons → colourless)
• Cu2+: 3d9 (1 unpaired electron → coloured, typically blue)
• V5+: 3d0 (no d-electrons → colourless)
• Ti4+: 3d0 (no d-electrons → colourless)
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Cr3+ has a 3d3 outer configuration. Because it possesses partially filled d-orbitals with 3 unpaired electrons, it undergoes d-d transitions and imparts a distinctive violet/green color to solutions. Cu+ (3d10), Zn2+ (3d10), and Ti4+ (3d0) do not have unpaired d-electrons.
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While size decreases across any period, a pronounced, continuous, and steady reduction in atomic and ionic radii with increasing atomic number is a hallmark characteristic of f-block elements due to Lanthanide and Actinide contractions.
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Transition elements are characterized by an incompletely filled inner d-subshell. Configuration (b) shows a partially filled 3d2 subshell, identifying it as Titanium (Ti, Z=22), a classic transition element.
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Note: The question text contains a typo ("Cs" for Cesium instead of "Ce" for Cerium), but the context matches Cerium. Cerium (Ce, Z=58) is a lanthanide. Its core stable oxidation state is +3. However, it also readily shows a +4 oxidation state because losing 4 electrons yields a highly stable, empty noble-gas electron shell (f0 configuration).
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In the +7 oxidation state (like in MnO4−), Manganese is deeply electron-deficient and sits at its maximum possible oxidation state. Because it strongly tends to gain electrons and reduce its state back down to more stable forms like Mn2+, it acts as an exceptionally powerful oxidizing agent.
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Let's count unpaired electrons (n):
• Ti3+: 3d1 → n = 1
• Sc3+: 3d0 → n = 0
• Mn2+: 3d5 → n = 5
• Zn2+: 3d10 → n = 0
With 5 unpaired electrons, Mn2+ produces the largest magnetic moment (~5.92 BM).
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Statement (c) is incorrect. Cerium(IV) ions definitely exist in aqueous solutions and are widely utilized in analytical chemistry as strong oxidizing titrants (cerimetry).
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The configuration (n-1)d5 ns2 has a total of 7 valence electrons available for bonding (5 in the d-subshell and 2 in the s-subshell). This allows elements like Manganese (Mn) to achieve a maximum oxidation state of +7 (e.g., in KMnO4).
Maximum possible oxidation states for the choices:
- (a) +5
- (b) +6
- (c) +4
- (d) +7
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While Cobalt (Z = 27) comes before Nickel (Z = 28) in atomic number, it is an exception in the periodic table because its atomic mass is slightly higher due to its isotopic composition. Looking at their actual standard atomic weights:
- Cobalt (Co): 58.93 g/mol
- Nickel (Ni): 58.69 g/mol
- Iron (Fe): 55.85 g/mol
Therefore, the decreasing sequence of atomic weight is Co > Ni > Fe.
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Generally, ionization energy increases across a period from left to right due to an increase in effective nuclear charge. Furthermore, Manganese (Mn) has a highly stable, symmetric, half-filled d-subshell configuration ([Ar] 3d5 4s2).
Removing a shielding electron from this highly stable configuration requires significantly more energy compared to its neighboring elements (Ti, V, and Cr), resulting in the highest first ionization potential among them.
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This electronic configuration belongs to Copper (Cu, Z = 29). Elements are classified into blocks based on the orbital into which the last (differentiating) electron enters. Because the last electron goes into the inner 3d subshell to complete it, Copper belongs strictly to the d-block.
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Transition elements are defined as elements that possess a partially filled d-subshell in their atomic ground state or common oxidation states.
- Option (a) is Calcium (s-block).
- Option (b) is Germanium (p-block).
- Option (d) is Gallium (p-block).
- Option (c) is Titanium (Ti, Z = 22), which features a partially filled 3d2 subshell, making it a classic transition metal.
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Paramagnetism occurs when a substance contains one or more unpaired electrons. Unpaired electron spins generate an individual net magnetic dipole moment. When an external magnetic field is applied, these spins align with the field, weakly pulling the compound into it.
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Isoelectronic species are atoms or ions that contain the exact same number of total electrons:
- Mn2+: Atomic number 25 − 2 electrons = 23 electrons (3d5 configuration)
- Fe3+: Atomic number 26 − 3 electrons = 23 electrons (3d5 configuration)
Since both ions possess 23 electrons, they are isoelectronic to each other.
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Let's check the valence electron configurations to look for unpaired electrons:
- Cu+: 3d10 → All electrons are paired (Diamagnetic)
- Ni2+: 3d8 → Contains 2 unpaired electrons in the d-orbitals (Paramagnetic)
- Zn2+: 3d10 → All electrons are paired (Diamagnetic)
- Sc3+: 3d0 → No electrons in the d-subshell (Diamagnetic)
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Chromium (Z = 24) displays an exceptional electron configuration. Instead of the expected [Ar] 3d4 4s2, an electron from the 4s subshell shifts to the 3d subshell. This gives it a completely half-filled, symmetrical d-subshell (3d5 4s1), which provides extra thermodynamic stability.
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Due to the phenomenon of lanthanide contraction, the ionic radii of trivalent lanthanide ions (M3+) steadily shrink across the series from left to right as the atomic number grows. Because Lanthanum (La) is located at the very beginning of this sequence, its trivalent ion has the least nuclear pull on the outer shells and possesses the largest radius.
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In options (b), (c), and (d), the higher oxidation state is always more stable in an aqueous medium (e.g., Cr3+ is highly stable due to half-filled t2g3 crystal splitting, and Cu2+ is favored over Cu+ due to its high hydration enthalpy).
However, for Vanadium, the V2+ ion has a very stable half-filled t2g3 configuration, making it exceptionally resistant to oxidation in typical acid solutions relative to the oxovanadium ion (VO2+, where V is in the +4 oxidation state).
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Zirconium (Zr) belongs to the 4d series (5th period), and Hafnium (Hf) is positioned right beneath it in the 5d series (6th period). Normally, size increases down a group. However, the 14 lanthanides slip into the periodic table immediately before Hafnium.
The poor shielding of these intervening 4f electrons shrinks the atomic volume of Hf down so intensely that its radius becomes nearly identical to that of Zirconium (Zr ≈ 160 pm, Hf ≈ 159 pm). This makes them "chemical twins."
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The primary driver behind lanthanide contraction is the imperfect or poor shielding effect of the 4f-electrons. 4f orbitals have a highly diffused shape, meaning they fail to shield outer electrons from the growing positive nuclear charge. As the atomic number increases, the nucleus pulls the entire electron cloud closer, shrinking the atom.
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Let's map out the 3d orbital distributions according to Hund's Rule:
- Co2+: 3d7 → 3 unpaired electrons
- Fe2+: 3d6 → 4 unpaired electrons
- Fe3+: 3d5 → 5 unpaired electrons (Every single d-orbital has 1 electron)
- Co3+: 3d6 → 4 unpaired electrons
Thus, Fe3+ possesses the maximum possible number of unpaired electrons in the 3d series.
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Transition metals explicitly exhibit paramagnetic properties because of the presence of unpaired electrons in their incomplete (n-1)d subshells. Each unpaired electron acts like a tiny bar magnet; an external magnetic field forces these spins to align parallel to it, drawing the metal inward.
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Verdigris is the classic green patina or pigment that forms naturally on copper, brass, and bronze architectural structures over time due to weathering. Chemically, it is identified as a complex arrangement of basic copper acetate, written with the chemical formula [Cu(CH3COO)2 · CuO · 6H2O].
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For transition metal oxides, the acid-base character changes depending on the metal's oxidation state:
- Low oxidation state (+2): Predominantly basic (e.g., MnO)
- Intermediate oxidation state (+4): Amphoteric (e.g., MnO2 — reacts with both acids and strong bases)
- High oxidation state (+7): Strongly acidic (e.g., Mn2O7)
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According to Fajans' Rules, a lower positive charge on a metal ion minimizes its polarizing power, leading to a high degree of ionic character.
MnO involves Mn in a low +2 state, rendering it a highly ionic and basic oxide. Conversely, higher states (+6 in CrO3 and +7 in Mn2O7) polarize oxygen heavily, producing covalent properties.
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Calomel is the historical and common chemical name for Mercurous chloride. Because Mercury(I) does not exist as a single free ion but rather as a stable dimeric diatomic unit (Hg22+), its structural formula is written correctly as Hg2Cl2.
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Blue vitriol is the historical commercial name for Copper(II) sulfate pentahydrate (CuSO4 · 5H2O). In this crystalline complex, four water molecules bind directly via coordinate covalent links to the copper center, while a fifth water molecule remains locked in place via hydrogen bonding inside the crystal lattice.
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Mercury sits very low on the electrochemical activity series, meaning its affinity for oxygen is weak. Its metal-oxygen bond is thermally unstable. When heated, Mercuric oxide (HgO) readily undergoes thermal decomposition into pure liquid mercury metal and oxygen gas:
2HgO(s) + Δ → 2Hg(l) + O2(g)
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Ferrous ammonium sulfate, FeSO4 · (NH4)2SO4 · 6H2O, is universally known as Mohr's salt. It is classified as a double salt and is widely preferred in standard laboratory volumetric analyses (redox titrations) because solid Mohr's salt resists atmospheric oxidation to ferric forms much better than simple ferrous sulfate crystals.
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