CUET UG Chemistry Booster Test – 2 General Properties

According to the IUPAC definition, a transition element is one whose atom or one of its common oxidation states possesses an incomplete d-subshell. Zinc has the electronic configuration [Ar]3d¹⁰4s². When it forms its most common ion, Zn²⁺, the two 4s electrons are removed, giving the configuration [Ar]3d¹⁰. Since both the atom and the common ion possess a completely filled d-subshell, Zinc does not satisfy the IUPAC definition of a transition element. Consequently, although it belongs to the d-block, it is not regarded as a true transition metal.

Transition metals often form interstitial compounds by trapping small atoms such as hydrogen, carbon or nitrogen within the spaces of their crystal lattice. Although these compounds become harder and often possess higher melting points, the metallic framework remains largely unchanged. As a result, they continue to exhibit metallic properties such as electrical conductivity. This characteristic distinguishes interstitial compounds from many ionic and covalent solids. Therefore, transition metals retain their metallic conductivity even after forming interstitial compounds.

Most transition metals, except Zinc, Cadmium and Mercury, are hard because of strong metallic bonding involving both ns and (n−1)d electrons. Therefore, Statement 3 is correct. Some borides of transition metals possess extremely high hardness values and may approach diamond in hardness, making Statement 2 correct. Statement 1 is incorrect because interstitial compounds are generally very hard rather than soft. Statement 4 is also incorrect because transition-metal alloys are usually strong, hard and possess high melting points. Therefore, only Statements 2 and 3 are correct.

Transition metals possess strong metallic bonding because both the outer ns electrons and several (n−1)d electrons participate in interatomic bonding. The resulting metallic lattice contains delocalized electrons that hold the metal ions together while still allowing layers of atoms to slide past one another. This characteristic gives rise to malleability and ductility. The extensive participation of d-electrons also contributes to the strength and hardness of transition metals. Therefore, strong metallic bonding involving both ns and d-electrons is responsible for these properties.

The passage states that the melting points of transition metals generally increase and reach a maximum near the middle of the series where the d⁵ configuration occurs. At d⁵, each d-orbital contains one unpaired electron, maximizing the number of electrons available for strong metallic bonding. This produces particularly strong interatomic interactions and consequently very high enthalpies of atomisation. Since melting point is directly related to the strength of metallic bonding, the maximum melting point is observed near the d⁵ configuration, except for a few anomalous cases such as manganese and technetium.

The passage clearly states that transition metals possess high enthalpies of atomisation because a greater number of (n−1)d and ns electrons participate in metallic bonding. Strong metallic bonding requires a large amount of energy to separate atoms completely into the gaseous state. Consequently, elements with high enthalpies of atomisation generally exhibit high melting and boiling points. Thus, the strong interatomic interactions responsible for these thermal properties are directly reflected in the high values of enthalpy of atomisation.

The metallic radii of the given elements are approximately: Sc = 164 pm, V = 135 pm, Fe = 126 pm and Cu = 128 pm. These values illustrate the general decrease in metallic radius across the first transition series due to increasing effective nuclear charge. Therefore, Sc matches 164 pm, V matches 135 pm, Fe matches 126 pm and Cu matches 128 pm. Hence the correct matching is 1-d, 2-c, 3-a and 4-b.

The ionic radii provided are Cr²⁺ = 82 pm, V²⁺ = 79 pm, Cu²⁺ = 73 pm and Ni²⁺ = 70 pm. To arrange them in decreasing order, the ion with the largest radius is placed first and the smallest last. Therefore, the correct order is Cr²⁺ > V²⁺ > Cu²⁺ > Ni²⁺. Converting this into the numbering scheme gives 2, 1, 4 and 3.

As we move across the first transition series, the nuclear charge increases steadily. At the same time, additional electrons enter the 3d orbitals. These 3d electrons provide partial shielding to the outer 4s electrons, reducing the full effect of the increasing nuclear charge. Consequently, atomic radii decrease, but the decrease is much less pronounced than expected. This partial shielding effect is responsible for the characteristic size trend observed across transition-metal series.

The shielding effect of f-electrons is significantly weaker than that of d-electrons. Because 4f electrons are diffuse and penetrate poorly, they do not effectively counterbalance the increasing nuclear charge across the lanthanoid series. This poor shielding leads to lanthanoid contraction, a gradual decrease in atomic and ionic radii. Since d-electrons provide somewhat better shielding than f-electrons, the correct statement is that one 4f electron shields less effectively than one d electron.

Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii across the lanthanoid series due to the poor shielding effect of 4f electrons. Because the 4f electrons do not effectively shield the increasing nuclear charge, the outer electrons are pulled closer to the nucleus. As a result, the atomic sizes of the 5d transition elements become much smaller than expected. Consequently, pairs of elements such as Zirconium (4d series) and Hafnium (5d series) possess almost identical radii and show very similar chemical properties. Therefore, lanthanoid contraction is the phenomenon responsible for this observation.

Lanthanoid contraction causes Hafnium and Zirconium to have almost identical atomic and ionic radii. Since chemical behaviour depends strongly on size and electronic structure, these two elements exhibit very similar physical and chemical properties. Consequently, conventional separation techniques become ineffective because both elements behave nearly identically in chemical reactions. This creates significant industrial difficulty in obtaining pure Zirconium and Hafnium from naturally occurring ores. Therefore, the most important consequence is the difficulty of separating these elements.

Density is defined as mass per unit volume. Across the first transition series from Titanium to Copper, atomic mass increases steadily because additional protons and neutrons are added to the nucleus. Simultaneously, metallic radii decrease slightly due to increasing effective nuclear charge. The decrease in size reduces atomic volume while the increase in mass raises the numerator of the density relationship. As a result, density increases significantly across the series. Therefore, the combined effect of increasing atomic mass and decreasing metallic radius accounts for the observed trend.

The densities of transition metals generally increase from Titanium to Copper because atomic masses increase while metallic radii decrease slightly. Among the listed metals, Nickel and Copper both possess densities of approximately 8.9 g cm⁻³ according to standard NCERT data. These values are among the highest in the first transition series and illustrate the strong metallic bonding and compact atomic packing present in these elements. Therefore, Nickel and Copper form the correct pair.

Zinc has the electronic configuration [Ar]3d¹⁰4s². The completely filled d-subshell contributes relatively little to metallic bonding compared with partially filled d-subshells found in most transition metals. Consequently, metallic bonding in Zinc is weaker than in neighbouring elements. This leads to a much lower enthalpy of atomisation, approximately 126 kJ mol⁻¹, which is the lowest value in the first transition series. Therefore, Zinc exhibits the minimum enthalpy of atomisation among the given elements.

The second (4d) and third (5d) transition series exhibit stronger metallic bonding than the first series. One important reason is the greater tendency of these heavier transition metals to form metal-metal bonds. Such bonding increases the overall strength of the metallic lattice and enhances interatomic interactions. Consequently, a larger amount of energy is required to separate the atoms completely into the gaseous state. This is reflected in their higher enthalpies of atomisation. Therefore, the frequent occurrence of metal-metal bonding contributes significantly to the greater enthalpies of atomisation observed in these series.

In transition elements, the energies of the 4s and 3d orbitals are very close to one another. When the first electron is removed during ionisation, the relative energies of these orbitals change. This change affects the stability of the resulting ion and produces irregularities in the trend of first ionisation enthalpies across the series. The effect is particularly noticeable in elements with half-filled and completely filled d-subshells. Therefore, the irregular trend is explained by changes in the relative energies of the 4s and 3d orbitals.

Zinc has the electronic configuration [Ar]3d¹⁰4s². During formation of Zn²⁺, the two outermost 4s electrons are removed, resulting in the highly stable configuration [Ar]3d¹⁰. Since the d¹⁰ configuration is exceptionally stable, the formation of Zn²⁺ is energetically favourable compared with many other transition-metal ions. Consequently, the sum of the first and second ionisation enthalpies is relatively low. This stability of the resulting ion plays a major role in the chemistry of Zinc.

Enthalpy of hydration and ionisation enthalpy are thermodynamic quantities that represent energy changes occurring during hydration and ionisation processes, respectively. Since these changes are measured for one mole of species, the standard unit used is kilojoule per mole (kJ mol⁻¹). This unit allows direct comparison of energetic trends among transition metals and their ions. Therefore, kJ mol⁻¹ is the correct unit for both quantities.

Mn²⁺ possesses the highly stable half-filled configuration 3d⁵, while Zn²⁺ possesses the highly stable completely filled configuration 3d¹⁰. The third ionisation process requires removing an electron from these exceptionally stable arrangements. Since such removal destroys the extra stability associated with half-filled and fully filled d-subshells, a large amount of energy is required. Consequently, the third ionisation enthalpies of manganese and zinc are significantly higher than those of neighbouring elements. This behaviour is a classic example of the stabilising effect of special electronic configurations.