D And F Block Elements – NEET UG ChemistryPractice Questions & PYQs

The aqueous solubility of ionic substances is governed by the balance between their lattice energies and hydration energies, alongside the specific solubility product constants of sparingly soluble salts. Highly soluble alkali metal salts like $\mathrm{Na}_2\mathrm{S}$ dissolve readily, while transition metal sulfides show minimal solubility governed by their very small solubility products. Comparing the metal sulfides, $\mathrm{ZnS}$ has a larger solubility product than $\mathrm{CuS}$, establishing the correct descending solubility order.

Alloys are metallic materials created by combining a base metal with other metallic or non-metallic elements to modify their mechanical properties. While traditional alloys like invar, bell metal, and bronze consist strictly of metallic mixtures, steel is uniquely formulated by combining iron with the non-metal carbon. This incorporation of carbon gives steel its distinct properties and identifies it as the alloy containing a non-metal constituent.

Melting points among transition metals are heavily influenced by the strength of metallic bonding, which correlates with the extent of d-electron delocalization in the crystal lattice. While a general upward trend exists across parts of the series, manganese possesses half-filled d-subshells that bind its electrons tightly and restrict their participation in metallic bonding. Consequently, manganese exhibits an anomalously low melting point that falls well below titanium and chromium, invalidating the proposed sequential order.

Transition metals function as efficient catalysts by offering alternative reaction pathways with reduced activation energies through temporary intermediate complex formation. This capability arises directly from their ability to shift smoothly between multiple oxidation states during a reaction cycle. By accepting and donating electrons flexibly, metal ions facilitate bond breaking and formation before returning to their original oxidation state when the reaction concludes.

Primary standards in volumetric analysis require substances that are exceptionally stable, non-deliquescent, and capable of being weighed with high precision. Potassium dichromate fulfills these requirements perfectly because it does not absorb atmospheric moisture and maintains a constant composition over time. Sodium dichromate, conversely, is highly deliquescent, meaning it readily takes up water from the air and prevents accurate preparation of standard solutions. This fundamental physical difference is why potassium dichromate is universally preferred in titrimetric estimations despite both sharing similar chromium chemistry.

Actinide elements exhibit a wide range of oxidation states due to the comparable energies of their 5f, 6d, and 7s orbitals, which allow varying numbers of electrons to participate in bonding. Actinium, however, features a valence electronic configuration that readily loses all three outer electrons to achieve the exceptionally stable noble gas configuration of radon. Because its 5f orbitals remain completely empty and unengaged in bonding, actinium is strictly restricted to exhibiting only a single positive oxidation state.

Standard reduction potential values integrate the energy changes of sublimation, ionization, and hydration required to convert a metal ion into a solid atom. Most first-row transition elements display negative potentials because the energy required to remove valence electrons exceeds the energy released during hydration. Copper uniquely overcomes this trend due to its exceptionally high hydration enthalpy and large atomization energy combined with a high second ionization enthalpy, which together make the overall reduction process energetically favorable. This balance results in a positive standard potential for copper while neighboring metals retain negative values.

The stability of a specific oxidation state among transition metals is strongly enhanced by symmetrical half-filled or completely filled electronic subshells due to high exchange energy. Manganese in its $+2$ oxidation state achieves a half-filled $3d^5$ configuration, conferring maximum thermodynamic stability that outranks its neighbors. As electrons are added or removed from this optimal half-filled state, exchange energy decreases, causing the relative stability to drop across the series. Consequently, the descending order of stability for these divalent ions peaks dramatically at manganese and follows the sequence $\mathrm{Mn} > \mathrm{Fe} > \mathrm{Cr} > \mathrm{Co}$.

Acidified potassium dichromate is a powerful oxidizing agent that converts sulfite ions into sulfate ions while undergoing reduction itself. During this redox reaction, the orange-red dichromate ion containing chromium in the $+6$ oxidation state is reduced to the chromium(III) ion. Because chromium(III) subsequently combines with the sulfate ions present in the acidified reaction mixture, it yields chromium(III) sulfate, $\mathrm{Cr}_2(\mathrm{SO}_4)_3$. The characteristic emerald green appearance of this specific salt dissolved in the solution accounts for the observed color change.

Industrial chemistry relies on specific large-scale synthesis routes tailored to the thermodynamic and kinetic requirements of each substance. Sulphuric acid is produced via the catalytic oxidation of sulphur dioxide in the Contact process, while ammonia is synthesized directly from elemental nitrogen and hydrogen under high pressure using the Haber process. In metallurgy, molten pig iron is converted into steel by removing impurities through oxidation in the Bessemer process, and early historical production of sodium hydroxide famously utilized the Leblanc process.

Transition metal atoms lose electrons from their outermost $4s$ orbital before the inner $3d$ orbitals when forming positively charged ions. Cobalt has an atomic number of 27, corresponding to a neutral ground-state configuration of $[Ar]4s^2 3d^7$. Removing three electrons involves taking two from the $4s$ shell and one from the $3d$ shell, leaving behind precisely six electrons in the $3d$ subshell to form $\mathrm{Co}^{3+}$. Other ions such as $\mathrm{Mn}^{3+}$, $\mathrm{Fe}^{3+}$, and $\mathrm{Ni}^{3+}$ result in different residual $d$-electron counts.

The chemistry of lanthanoids is overwhelmingly dominated by the loss of three electrons, which removes the two outer s-electrons and one from the inner d or f orbitals to achieve a noble-gas-like core stability. Although specific elements can display alternate oxidation states like two or four, these are strictly localized instances occurring only when empty, half-filled, or completely filled $f$-subshells are formed. Across the entire series, the removal of three electrons requires a balanced ionization energy profile that makes the resulting tripositive ion the thermodynamic norm for all these rare-earth metals.

Elements belonging to the same group in the second and third transition series normally experience an increase in atomic radius down the column, but the intervention of the lanthanide series neutralizes this trend. The filling of inner $4f$ orbitals provides very poor shielding of nuclear charge, causing a progressive shrinkage known as the lanthanide contraction. This cumulative pulling effect counteracts the expected size increase, making zirconium and hafnium ions virtually identical in ionic radius. Therefore, despite their different periods, both $\mathrm{Zr}^{4+}$ and $\mathrm{Hf}^{4+}$ exhibit matching dimensions in aqueous chemistry.

Transition metal ions exhibit color in solution when they possess unpaired electrons in their $d$-subshell, allowing $d-d$ electronic transitions upon absorbing visible light. Titanium with atomic number 22 has a neutral electronic configuration of $4s^23d^2$, meaning the tripositive ion $\mathrm{Ti}^{3+}$ retains a single unpaired electron in its $3d$ orbital. In contrast, ions like $\mathrm{Sc}^{3+}$, $\mathrm{La}^{3+}$, and $\mathrm{Lu}^{3+}$ have completely empty or fully filled subshells, preventing any such optical transitions. Thus, only this specific transition metal ion fulfills the electronic requirement for absorption and coloration.

Transition elements display variable oxidation states because both their outermost s-electrons and penultimate d-electrons can participate in bonding. The maximum possible oxidation state corresponds directly to the total sum of electrons residing in the valence $ns$ and $(n-1)d$ orbitals. Among the given options, the configuration with five d-electrons and two s-electrons possesses a total of seven valence electrons available for sharing or loss, allowing it to exhibit up to seven distinct oxidation states.

Removing a second electron from transition metals generally becomes harder across a period due to an increasing nuclear charge, which normally places manganese ahead of chromium. However, the first ionization of chromium leaves behind a stable half-filled $3d^5$ configuration, creating an unusually high energetic barrier to remove a second electron. Because of this extra stability, the second ionization enthalpy of chromium spikes dramatically, pushing it above manganese while the remaining elements follow the expected periodic trend.

The stability of transition metal ions in aqueous solution is heavily dictated by hydration energetics and crystal field stabilization, particularly the symmetrical half-filling of the $t_{2g}$ orbital set in octahedral aqua complexes. Chromium in the tripositive state adopts a robust $3d^3$ configuration, which completely fills the lower $t_{2g}$ sublevel and yields a remarkably high crystal field stabilization energy that resists change in aqueous media. Other listed ions either lack this ideal symmetrical stabilization or readily undergo side reactions, making the chromium tripositive ion uniquely persistent and stable in water.

The progressive filling of 4f orbitals across the lanthanoid series results in poor shielding of the nuclear charge, causing a gradual contraction in atomic and ionic radii known as lanthanoid contraction. Because of this phenomenon, the atomic radii of the second and third transition series elements become nearly identical, which actually enhances the chemical similarities between these two series rather than eliminating them. Consequently, claiming that the 4d and 5d series have no similarities contradicts the periodic consequences of lanthanoid contraction.

The availability of a wider range of oxidation states in actinoids stems from the very small energy gap between their outermost valence orbitals, specifically the five-f and six-d subshells. Because these orbitals lie at comparable energy levels, electrons can be easily promoted from one to the other during bond formation without requiring excessive energy input. In contrast, the four-f orbitals in lanthanoids are buried deep within the atom and possess a significantly larger energy separation from the five-d orbitals, restricting their participation in multiple oxidation states.

Coloration in transition metal ions arises from d-d electronic transitions, which require the presence of at least one unpaired d electron in the valence shell. Scandium ion has a noble gas configuration with a completely empty d-subshell, making it colourless, whereas titanium ion possesses a single unpaired d electron and exhibits colour. Nickel ion contains eight d electrons with two unpaired electrons, ensuring it is also intensely coloured in aqueous medium alongside titanium.