The d-block elements (groups 3-12) are positioned in the large middle section of the periodic table, flanked between the s- and p-blocks. The f-block elements, also known as the inner transition elements, are placed in a separate panel at the bottom of the periodic table.
According to IUPAC, a transition metal is defined as an element which has an incomplete d-subshell either in its neutral atom or in its common oxidation states. Group 12 elements ($Zn$, $Cd$, and $Hg$) have a completely filled $d^{10}$ configuration in their ground state as well as in their common oxidation states, and are therefore not strictly regarded as transition metals. However, their chemistry is studied alongside transition metals due to periodic trends.
The general electronic configuration of the outer orbitals of d-block elements is $(n-1)d^{1-10} ns^{1-2}$ (except for Palladium, which has a unique configuration of $4d^{10} 5s^0$).
The energy difference between the $(n-1)d$ and $ns$ orbitals is very small. Consequently, half-filled ($d^5$) and completely filled ($d^{10}$) configurations acquire extra stability. This is reflected in the anomalous configurations of certain elements:
Transition metals exhibit typical metallic properties including high tensile strength, ductility, malleability, high thermal and electrical conductivity, and metallic lustre. They are generally very hard and have low volatility, resulting in high melting and boiling points. The high melting points are attributed to the involvement of greater numbers of electrons from $(n-1)d$ in addition to the $ns$ electrons in interatomic metallic bonding.
In general, ions of the same charge in a given series show a progressive decrease in radius with increasing atomic number. This is due to the poor shielding effect of d-electrons, which fails to compensate for the increasing nuclear charge, pulling the outer electrons closer.
An important phenomenon is the Lanthanoid Contraction: the filling of $4f$ orbitals before the $5d$ series results in a regular decrease in atomic radii. This contraction essentially compensates for the expected increase in size down the group, making the atomic radii of the second ($4d$) and third ($5d$) transition series nearly identical (e.g., $Zr$ and $Hf$).
There is a general increase in ionisation enthalpy along each series from left to right due to the increase in nuclear charge. However, the trend is irregular because the removal of one electron alters the relative energies of the $4s$ and $3d$ orbitals. The second and third ionisation enthalpies show breaks in steady increases at stable configurations like $d^5$ and $d^{10}$.
One of the most notable features of transition elements is their great variety of oxidation states. This variability arises from the incomplete filling of d-orbitals, allowing oxidation states to differ by units of one (e.g., $V^{II}$, $V^{III}$, $V^{IV}$, $V^{V}$).
Transition metal ions frequently exhibit magnetic behavior, classified into diamagnetism (repelled by magnetic fields) and paramagnetism (attracted by magnetic fields). Paramagnetism arises from the presence of unpaired electrons.
The magnetic moment ($\mu$) is calculated using the "spin-only" formula:
where $n$ is the number of unpaired electrons and the unit is the Bohr Magneton (BM).
When an electron from a lower-energy d-orbital is excited to a higher-energy d-orbital (d-d transition), it absorbs light of a specific frequency in the visible region. The observed colour of the transition metal ion corresponds to the complementary colour of the light absorbed.
Transition metals and their compounds are highly effective catalysts due to their ability to adopt multiple oxidation states and form complexes. Examples include:
These are formed when small atoms like $H$, $C$, or $N$ are trapped inside the crystal lattices of transition metals. They are typically non-stoichiometric (e.g., $TiC$, $Fe_3H$) and exhibit high melting points, extreme hardness, and metallic conductivity while remaining chemically inert.
Because transition metals have similar metallic radii (within 15% of each other), atoms of one metal can easily replace atoms of another in the crystal lattice, readily forming homogeneous solid solutions called alloys (e.g., brass, bronze, and stainless steel).
An important chemical used in the leather industry and as an oxidant. It is prepared from chromite ore ($FeCr_2O_4$) via the following steps:
Chromate ($CrO_4^{2-}$) and dichromate ($Cr_2O_7^{2-}$) ions are interconvertible in aqueous solution depending on pH: chromate is stable in alkaline solution, while dichromate is stable in acidic solution.
Prepared by the fusion of pyrolusite ore ($MnO_2$) with an alkali metal hydroxide ($KOH$) and an oxidising agent like $KNO_3$ or oxygen, producing dark green potassium manganate ($K_2MnO_4$), which then disproportionates in neutral or acidic solution to yield permanganate:
In acidic solutions, $KMnO_4$ acts as a powerful oxidising agent, oxidising iodides to iodine, iron(II) to iron(III), oxalates to carbon dioxide, and nitrites to nitrates.
The f-block consists of two series: Lanthanoids (the 14 elements following Lanthanum, $Ce$ to $Lu$) and Actinoids (the 14 elements following Actinium, $Th$ to $Lr$).