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

Electronic configurations are determined by distributing an element's total complement of electrons into subshells in order of increasing energy according to the Aufbau principle. A neutral iron atom possesses twenty-six electrons, which fill the inner core orbitals sequentially before occupying the outer $4s$ and $3d$ levels. After completely filling the orbitals up through the $4s$ level with twenty electrons, the remaining six electrons enter the $3d$ subshell to complete the electron distribution.

Titanium with an atomic number of 22 normally exhibits an argon core followed by a $4s^2 3d^2$ configuration, which reduces to a $3d^2$ structure upon the loss of two electrons to form the divalent ion. According to Hund's rule, these two electrons occupy separate degenerate orbitals, yielding two unpaired electrons for the species. Substituting this value into the magnetic moment expression $\mu = \sqrt{n(n+2)}$ results in $\sqrt{2(2+2)}$, which simplifies to the square root of eight, yielding approximately 2.84 Bohr Magnetons.

Actinides exhibit a wider range of oxidation states compared to lanthanides because their $5f$ orbitals extend further from the nucleus and experience lower ionization energies. Among the given elements, americium displays multiple oxidation states ranging from $+2$ to $+6$ due to this larger atomic size and smaller energy gap between competing valence orbitals. In contrast, lanthanides like lanthanum are restricted to a single $+3$ state, while europium and gadolinium are limited to $+2$ and $+3$ states owing to the special stability of half-filled configurations.

Metals that occupy a higher position in the electrochemical series possess a greater tendency to undergo oxidation and can readily displace metals below them from their salt solutions. Aluminum sits above zinc in this reactivity series, meaning it easily displaces zinc from a $\mathrm{ZnCl}_{2}$ solution to form aluminum chloride. Consequently, storing this solution in an aluminum container triggers a spontaneous redox reaction that depletes the zinc chloride concentration.

In an alkaline medium, dichromate ions undergo an instantaneous acid-base equilibrium shift rather than a redox change, converting the bridging dichromate structure into monomeric units. Specifically, the addition of hydroxide ions neutralizes hydrogen ions and drives the equilibrium toward the formation of stable chromate ions along with water. This interconversion is represented by the ionic equation $\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} + 2\mathrm{OH}^{-} \rightarrow 2\mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O}$.

Biological oxidation of water into molecular oxygen during photosynthesis is driven by a specialized metalloenzyme cluster located within Photosystem II. This oxygen-evolving complex relies heavily on transition metal ions that cycle through multiple oxidation states to facilitate the extraction of electrons from water molecules. Manganese serves as the core catalytic metal within this active site, making it indispensable for splitting water and releasing oxygen gas.

The progressive filling of $4f$ orbitals prior to the transition elements of the third series introduces a poor shielding effect, which fails to offset the increasing nuclear charge. This phenomenon, known as lanthanide contraction, results in a steady decrease in atomic radii that almost perfectly compensates for the expected size increase down a group. As a result, zirconium and hafnium end up with nearly identical atomic radii despite belonging to different periods.

During the electrolysis of aqueous copper sulfate, $\mathrm{Cu}^{2+}$ ions migrate toward the cathode where they gain electrons to deposit as solid copper metal according to the reduction half-reaction $\mathrm{Cu}^{2+} + 2e^{-} \rightarrow \mathrm{Cu}$. Simultaneously, water molecules are oxidized at the inert platinum anode rather than sulfate ions, generating oxygen gas via the half-reaction $2\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{O}_{2} + 4\mathrm{H}^{+} + 4e^{-}$. Consequently, copper is liberated at the cathode while oxygen gas evolves at the anode.

Subshells that are exactly half-filled possess an extra measure of exchange energy and spherical symmetry, granting the atom heightened stability that can alter standard orbital filling sequences. Gadolinium possesses an atomic number of 64, placing ten valence electrons outside of its Xenon noble gas core across the $4f$, $5d$, and $6s$ levels. To maintain the profound stability of a symmetrical half-filled subshell, the valence electrons settle into the configuration $[\mathrm{Xe}] 4f^7 5d^1 6s^2$ rather than filling the $f$-orbitals exclusively.

In photographic processing, sodium thiosulfate acts as a fixing agent to eliminate residual light-sensitive silver salts from the emulsion after the image has been developed. Unexposed $\mathrm{AgBr}$ remains trapped in the film and would eventually darken when exposed to light if not cleared away. Introducing hypo converts this insoluble silver halide into a stable, water-soluble complex that rinses out completely.

Aqueous transition metal ions display distinct colors when their $d$-orbitals are partially filled, allowing electronic transitions to absorb specific wavelengths of visible light. The chromium three-plus ion possesses an outer electronic configuration of $3d^3$ with three unpaired electrons, facilitating these light-absorbing transitions to impart a characteristic green color to its solutions. In contrast, ions like zinc two-plus, copper one-plus, and titanium four-plus have either completely empty or fully filled $d$-subshells, resulting in colorless solutions.

Silver halides undergo a rapid photochemical reduction reaction when exposed to light, making them exceptionally photosensitive materials. Among these compounds, silver bromide exhibits the ideal sensitivity threshold required to capture latent images upon photon impact. Because of this unique property, it serves as the foundational active ingredient coated onto traditional photographic films and plates.

Transitioning to the trivalent lanthanide state involves losing two $6s$ electrons followed by a $4f$ electron, leaving the remaining $4f$ electrons to populate the available spatial orbitals singly before pairing begins. For holmium with atomic number 67, the neutral configuration contains eleven $4f$ electrons, which reduces to ten in the $\mathrm{Ho}^{3+}$ ion. Distributing these ten electrons across seven degenerate orbitals leaves exactly four orbitals singly occupied, matching the requirement for four unpaired electrons.

The oxidation state of an element in a neutral compound is determined by setting the algebraic sum of the oxidation numbers of all constituent atoms equal to zero, assigning standard values of $+1$ for potassium and $-2$ for oxygen. For $\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}$, this principle translates to the equation $2(1) + 2x + 7(-2) = 0$, where $x$ represents the unknown oxidation state of chromium. Simplifying the expression yields $2 + 2x - 14 = 0$, which reorganizes to $2x = 12$. Solving this equation results in $x = 6$.

Colour in transition metal complexes typically arises from $d-d$ electronic transitions made possible by partially filled $d$-orbitals. In both titanium in the plus four oxidation state and copper in the plus one oxidation state, the metal ions possess electronic configurations with completely empty or completely filled $d$-subshells respectively. Consequently, the absence of unpaired electrons and vacant spaces within the $d$-subshell prevents any such electronic transitions from occurring, rendering these specific species completely colourless.

While the trivalent state is standard across the lanthanide series due to typical energy trends, cerium possesses a neutral outer configuration containing a single $4f$ electron. By ejecting four total electrons, it sheds this final remaining $4f$ electron and achieves a completely vacant $4f$ subshell. This empty $4f^0$ state provides a high degree of core-like thermodynamic stability that compensates for the ionization energy required to remove the fourth electron.

The atomic number of a transition metal equals the total number of electrons in its neutral, uncharged ground state. For an ion with a $+3$ charge, three electrons have been removed, meaning the neutral atom possesses three more electrons than its given ionic configuration of $[\mathrm{Ar}] 3d^{4}$. Summing the electrons from the argon core, the $3d$ subshell, and the three regained electrons yields $18 + 4 + 3$, which results in an atomic number of 25.

The periodic classification of elements groups atoms based on the outermost sublevel that receives their differentiating electron. Elements whose last electron enters an inner $d$-subshell belong to the transition metals of the $d$-block, sharing similar metallic properties and variable oxidation states. In contrast, an element whose configuration ends in the $p$-subshell belongs to the main group $p$-block elements, placing it in a completely different chemical family.

Transition metals exhibit exceptional thermal stability and high melting points because their metallic lattices are held together by unusually strong cohesive forces. These robust bonds are formed by the extensive participation of inner-shell $3d$ electrons alongside the outer $4s$ electrons in interatomic orbital overlapping. Having a larger pool of bonding electrons increases the lattice enthalpy significantly, which directly accounts for why these metals require much higher temperatures to melt.

Bell metal is a specialized bronze alloy formulated by combining copper and tin in proportions that maximize acoustic resonance and structural durability. This distinct metallic blend typically consists of roughly eighty percent copper and twenty percent tin, creating a hard material traditionally cast for bells and cymbals. Other binary combinations such as copper with zinc or nickel yield entirely different industrial materials like brass and cupronickel.