The d- and f-Block Elements

The d-block elements, or transition elements, occupy the middle of the periodic table (groups 3–12) and are defined as elements whose atoms or stable ions have partly filled d orbitals. Their general electronic configuration is $(n-1)d^{1-10}\,ns^{1-2}$, where the penultimate $d$ shell fills up across each series. By this strict definition zinc, cadmium and mercury ($d^{10}$) are not true transition elements, although they are studied with the block — a classic NEET point.

Two configurations break the expected order because half-filled and fully-filled $d$ sub-shells are extra stable: chromium is $3d^5\,4s^1$ (not $3d^4\,4s^2$) and copper is $3d^{10}\,4s^1$ (not $3d^9\,4s^2$). These anomalies are frequently tested, so commit them to memory.

The transition metals share characteristic physical trends. They have high melting and boiling points and high densities, owing to strong metallic bonding involving both $d$ and $s$ electrons. Their atomic radii decrease at first across a series, then stay almost constant, and rise slightly at the end — a much smaller variation than in the main groups, because added $d$ electrons shield the increasing nuclear charge fairly well.

Most transition metals also form alloys readily (their similar atomic sizes let one metal replace another in the lattice), display a range of oxidation states, and behave as good catalysts — properties explored in the next topic. The first transition series (Sc to Zn, filling $3d$) is the most important for NEET, and being fluent with writing the configuration of any first-series atom or ion (remembering to remove $4s$ electrons before $3d$ when forming cations) underpins the whole chapter.

The most distinctive chemistry of the transition metals follows from their $d$ electrons. Because the $(n-1)d$ and $ns$ orbitals are close in energy, electrons from both can be used in bonding, giving variable oxidation states. Manganese is the classic example, showing every state from $+2$ to $+7$; the highest states appear in oxides and oxoanions (e.g. $\text{MnO}_4^-$). The maximum oxidation state usually rises to the middle of a series and then falls. This variability is a guaranteed NEET topic.

Transition metal ions are typically coloured, because a partly filled $d$ sub-shell allows d–d electronic transitions: the ion absorbs one wavelength of visible light to promote a $d$ electron and transmits the complementary colour. Ions with empty ($\text{Sc}^{3+}$, $d^0$) or completely filled ($\text{Zn}^{2+}$, $d^{10}$) $d$ orbitals have no such transition and are colourless — a popular NEET discriminator.

Their unpaired $d$ electrons make most transition metal ions paramagnetic. The (spin-only) magnetic moment is $\mu = \sqrt{n(n+2)}\ \text{BM}$, where $n$ is the number of unpaired electrons; more unpaired electrons give a larger moment. NEET often asks you to count unpaired electrons in an ion and compute $\mu$.

Transition metals and their compounds are excellent catalysts (using variable oxidation states to form intermediates, e.g. Fe in the Haber process, $\text{V}_2\text{O}_5$ in the Contact process), form interstitial compounds (small atoms like H, C, N occupy lattice holes, making the metal harder), and readily form complex (coordination) compounds. Two compounds are studied in detail: potassium permanganate ($\text{KMnO}_4$) and potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$), both powerful oxidising agents whose preparation, structure and reactions are regular NEET fare.

The f-block elements, or inner transition elements, are placed separately at the bottom of the periodic table and are characterised by the filling of $f$ orbitals. The first series, the lanthanoids (cerium to lutetium, often taken with lanthanum), involves the progressive filling of the $4f$ subshell; their general configuration is $[\text{Xe}]\,4f^{1-14}\,5d^{0-1}\,6s^2$.

The lanthanoids are remarkably similar to one another in chemistry. Their most common and stable oxidation state is $+3$, although a few show $+2$ or $+4$ when these give a stable $f^0$, $f^7$ or $f^{14}$ configuration (e.g. $\text{Ce}^{4+}$ from $f^0$, $\text{Eu}^{2+}$ from $f^7$). Most lanthanoid ions are coloured and paramagnetic owing to unpaired $4f$ electrons, and the metals are reactive, silvery, and used in alloys such as misch metal (in lighter flints) and as magnets.

The defining feature of the series is the lanthanoid contraction: a steady, small decrease in atomic and ionic radius from one lanthanoid to the next across the series. It occurs because the $4f$ electrons being added shield the increasing nuclear charge very poorly (the $4f$ orbitals are diffuse and inner), so the effective nuclear charge felt by the outer electrons rises and pulls them in. NEET regularly tests both the cause and the consequences.

The consequences of the lanthanoid contraction are far-reaching: the elements of the second and third transition series become almost identical in size (for example zirconium and hafnium, or niobium and tantalum), which makes these pairs very hard to separate; it also accounts for the similar properties of the lanthanoids themselves and influences the basicity of their hydroxides (which decreases as size decreases). Understanding this single contraction, its cause and its size-related consequences is the highest-yield idea from the f-block for NEET.

The second f-block series, the actinoids (thorium to lawrencium, taken with actinium), involves the filling of the $5f$ subshell, with the general configuration $[\text{Rn}]\,5f^{1-14}\,6d^{0-1}\,7s^2$. Unlike the lanthanoids, all actinoids are radioactive, and the elements beyond uranium (the transuranium elements) are synthetic, made artificially in nuclear reactions.

A major difference from the lanthanoids is that the actinoids show a much greater range of oxidation states. While $+3$ is common, states up to $+6$ and $+7$ are seen in the early actinoids (for example uranium commonly shows $+6$ in $\text{UO}_2^{2+}$). This wider variability arises because the $5f$, $6d$ and $7s$ orbitals are close in energy and the $5f$ electrons are less tightly held and more available for bonding than the $4f$ electrons of the lanthanoids.

There is an analogous actinoid contraction — a steady decrease in size across the series — which, like the lanthanoid contraction, is due to the poor shielding of the nuclear charge, here by $5f$ electrons. The actinoid contraction is slightly greater from element to element than the lanthanoid contraction because $5f$ electrons shield even more poorly.

For NEET, the key comparison points to remember are: both series show a $+3$ state and a contraction caused by poor f-orbital shielding; but actinoids are all radioactive, show more and higher oxidation states, and have $5f$ electrons that are more chemically available than the lanthanoids' $4f$ electrons. Actinoids also have important applications — uranium and plutonium as nuclear fuels, americium in smoke detectors — which NEET sometimes references. Holding these similarities and differences clearly, together with the lanthanoid contraction from the previous topic, covers the f-block questions you are likely to face.