CUET UG Chemistry Booster Test – 3 General Properties

The passage explicitly states that transition metals generally possess one or more typical metallic structures at normal temperatures. However, four elements are identified as exceptions: Zinc, Cadmium, Mercury and Manganese. These elements exhibit deviations from the structural behaviour commonly observed in transition metals. Therefore, the only option containing all four exceptions is Zn, Cd, Hg and Mn. Since the question directly asks for the complete set of exceptions, Option B is the correct answer.

Brass and bronze are examples of substitutional alloys in which atoms of one metal replace some atoms of another within the metallic lattice. The alloy generally forms a homogeneous solid solution where atoms are randomly distributed while maintaining metallic bonding. Because the sea of delocalised electrons remains present, these alloys continue to exhibit metallic properties such as electrical and thermal conductivity. Therefore, the conductivity is maintained through the formation of homogeneous metallic solid solutions.

Among the lanthanoids, hardness generally increases with increasing atomic number due to stronger metallic bonding and decreasing atomic size. NCERT specifically mentions Samarium as being "steel hard," highlighting its exceptional hardness compared with several other lanthanoids. This property illustrates the gradual strengthening of metallic interactions across the series. Therefore, Samarium is the lanthanoid identified with this distinctive description.

Zinc possesses the electronic configuration [Ar]3d¹⁰4s². Because the 3d subshell is completely filled, the d-electrons contribute minimally to metallic bonding. In most transition metals, partially filled d-orbitals participate significantly in metallic bonding, increasing hardness and strength. The lack of such participation in Zinc results in weaker metallic bonding and causes several physical properties to differ from those of typical transition metals. Therefore, the filled d¹⁰ configuration is responsible for Zinc's anomalous behaviour.

Melting points of transition metals depend primarily on the strength of metallic bonding, which in turn is related to the number of unpaired d-electrons participating in bonding and the enthalpy of atomisation. Tungsten possesses one of the highest melting points among all metals because of extremely strong metallic bonding. Molybdenum also exhibits very strong metallic interactions and therefore has a very high melting point, though slightly lower than Tungsten. Chromium follows next with a high melting point due to significant d-electron participation in bonding. Zinc, having a completely filled d¹⁰ configuration, contributes fewer electrons to metallic bonding and therefore exhibits a much lower melting point. Hence, the correct decreasing order is W > Mo > Cr > Zn.

Transition metals possessing very high enthalpies of atomisation have exceptionally strong metallic bonding. A large amount of energy is required to separate atoms from the metallic lattice and convert them into ions. Consequently, these metals are less likely to undergo oxidation and often exhibit low chemical reactivity. This tendency is reflected in their electrode potentials and their resistance to corrosion and chemical attack. Therefore, metals with very high boiling points and atomisation enthalpies generally behave in a noble manner in chemical reactions.

Across the first transition series, the nuclear charge increases steadily, which tends to decrease atomic size. However, additional electrons are simultaneously added to the 3d orbitals. These electrons contribute to shielding and increase electron-electron repulsion within the d-subshell. Towards the end of the series, these effects increasingly counterbalance the attraction caused by the growing nuclear charge. As a result, the decrease in atomic radius becomes very small and the radii remain nearly constant. Therefore, the observed constancy arises from the balance between increasing nuclear attraction and increasing shielding/repulsion effects.

The ionic radii of these M³⁺ ions decrease progressively across the transition series because the nuclear charge increases while shielding does not increase proportionally. The NCERT values are approximately Ti³⁺ = 67 pm, V³⁺ = 64 pm, Cr³⁺ = 62 pm and Fe³⁺ = 65 pm. Therefore, Ti³⁺ matches 67 pm, V³⁺ matches 64 pm, Cr³⁺ matches 62 pm and Fe³⁺ matches 65 pm. Hence, the correct matching is 1-c, 2-a, 3-d and 4-b.

The actinoid contraction refers to the gradual decrease in atomic and ionic radii across the actinoid series. The 5f electrons are even less effective at shielding the increasing nuclear charge than the 4f electrons of the lanthanoids. Consequently, the effective nuclear attraction experienced by outer electrons increases more strongly across the series. This causes a greater decrease in size from one element to the next. Therefore, actinoid contraction is generally greater than lanthanoid contraction.

Statement 2 is correct because imperfect shielding combined with increasing nuclear charge produces a gradual decrease in atomic and ionic sizes across transition and inner-transition series. Statement 3 is correct because one 4f electron shields another less effectively than d-electrons shield each other. Statement 4 is also correct because 5f electrons provide even poorer shielding than 4f electrons, contributing to the actinoid contraction. Statement 1 is incorrect because d-electrons do not provide perfect shielding. Therefore, Statements 2, 3 and 4 are correct.

Lanthanoid contraction arises because the 4f electrons do not effectively shield one another from the increasing nuclear charge across the lanthanoid series. Although 4f orbitals are inner orbitals, their complex shapes and poor penetration result in weak shielding. As additional protons are added to the nucleus, the effective nuclear charge experienced by outer electrons increases continuously. Consequently, the electrons are pulled closer to the nucleus, producing a gradual decrease in atomic and ionic radii. This phenomenon is known as lanthanoid contraction and is one of the most important trends associated with f-block chemistry.

Because of lanthanoid contraction, the atomic radii of third-transition-series elements are much smaller than expected. At the same time, these elements possess considerably larger atomic masses than their second-series counterparts. The combination of high mass and relatively small atomic volume results in exceptionally high densities. Elements such as Osmium and Iridium are among the densest known metals because of this effect. Therefore, density is one of the physical properties most strongly influenced by lanthanoid contraction.

Density is determined by mass per unit volume. Moving from Titanium toward Copper, atomic masses increase substantially while atomic radii decrease slightly because of increasing effective nuclear charge. The increased mass and reduced volume combine to produce a significant rise in density. Consequently, Copper possesses a much higher density than early transition metals such as Scandium. This trend is a characteristic feature of the first transition series and is clearly reflected in NCERT data.

The densities of the listed elements approximately follow the order: Scandium (3.0 g cm⁻³) < Vanadium (6.0 g cm⁻³) < Iron (7.9 g cm⁻³) < Nickel (8.9 g cm⁻³). As we move across the first transition series, atomic mass increases significantly while atomic volume does not increase proportionately. This causes a gradual increase in density. Therefore, when the elements are rearranged as Iron, Scandium, Nickel and Vanadium, the increasing order of density becomes Scandium → Vanadium → Iron → Nickel, corresponding to 2, 4, 1 and 3.

Enthalpy of atomisation measures the energy required to convert one mole of a metallic element into gaseous atoms. A higher value indicates stronger metallic bonding. Among the listed 3d-series elements, Vanadium possesses the highest enthalpy of atomisation, approximately 515 kJ mol⁻¹. This reflects the strong contribution of both ns and d-electrons to metallic bonding. The high atomisation enthalpy also explains Vanadium's relatively high melting point and strong metallic character. Therefore, Vanadium is the correct answer.

The enthalpy of atomisation measures the strength of metallic bonding within a metal. Heavy transition metals of the second and third series possess exceptionally high enthalpies of atomisation because many d-electrons participate in bonding. These strong interactions often extend into their compounds, leading to the formation of direct metal-metal bonds. Such bonding is considerably more common among heavier transition metals than among first-series transition metals. Therefore, high atomisation enthalpies are directly associated with the frequent occurrence of metal-metal bonding in their compounds.

Standard electrode potential (E°) is a measure of the tendency of a species to undergo reduction or oxidation under standard conditions. Since it represents an electrical potential difference, its unit is the volt (V). Although enthalpy of atomisation and ionisation enthalpy are measured in kJ mol⁻¹, electrode potentials are measured separately in volts. These thermodynamic quantities together help explain the stability and reactivity of transition metals and their ions.

Copper has the electronic configuration [Ar]3d¹⁰4s¹. When one electron is removed, Cu⁺ acquires the highly stable configuration [Ar]3d¹⁰. To form Cu²⁺, an electron must then be removed from this completely filled d-subshell. Since d¹⁰ configurations possess exceptional stability, a large amount of energy is required. Consequently, Copper exhibits an unusually high second ionisation enthalpy. This observation indicates the extra stability associated with the Cu⁺ ion.

The Cr⁺ ion possesses the exceptionally stable 3d⁵ configuration. This arrangement contains the maximum number of parallel spins in degenerate orbitals and therefore benefits from significant exchange-energy stabilization. Removal of an electron from Cr⁺ destroys this highly stable arrangement and requires a large amount of energy. In contrast, Mn⁺ still contains a 4s electron that can be removed more easily. Consequently, the ionisation enthalpy of Mn⁺ is lower than that of Cr⁺.

Copper possesses relatively high enthalpies of atomisation and ionisation. Considerable energy is required to convert metallic copper into Cu²⁺ ions. Although hydration of Cu²⁺ releases energy, this hydration enthalpy is insufficient to completely compensate for the energy consumed during atomisation and ionisation. As a result, the overall reduction potential of Cu²⁺/Cu becomes positive (+0.34 V). This explains why Copper behaves more nobly than many other transition metals and does not readily liberate hydrogen from dilute non-oxidising acids.