An Introduction
Transition elements are elements found on the periodic table in Groups 3-12 (old groups IIA-IIB) The term refers to the fact that the d sublevel that is being filled is at a lower principal energy level than the s sublevel that came before it. Scandium, the first transition element, has an electron configuration of [Ar]3d14s2. Remember that the configuration is the opposite of the fill order, with the 4 s filling before the 3 d begins. The transition elements are frequently referred to as transition metals because they are all metals. They exhibit typical metallic properties as a group and are less reactive than the metals in Groups 1 and 2.
Some of the more well-known ones are so unreactive that they can be found in nature in their free, or uncombined, form. Platinum, gold, and silver are examples. The transition elements are often referred to as "d -block" elements due to their unique filling order. Compounds containing a variety of transition elements are distinguished by their ability to be widely and vividly coloured. The d-orbitals absorb light of various energies as visible light passes through a transition metal compound dissolved in water. Visible light of a given energy level that is not absorbed results in a clear coloured solution.
Properties of Transition Elements
The transition elements' general properties are as follows:
They are typically metals with a high melting point.
They have a variety of oxidation states.
They usually combine to form coloured compounds.
They are frequently paramagnetic.
They have a high charge/radius ratio.
High density and hardness.
The boiling and melting points are both very high.
Construct paramagnetic compounds.
Variable oxidation states are displayed.
Coloured compounds and ions are common.
Create catalytically active compounds.
Create stable complexes
Oxidation States
The number of electrons that an atom loses, gains, or appears to use when joining with another atom in a compound is related to its oxidation state. It also determines an atom's ability to oxidise (lose electrons) or reduce (gain electrons) other atoms or species. Almost all transition metals have multiple oxidation states that have been experimentally observed. Ions are formed by adding or subtracting negative charges from an atom. Keeping the atomic orbitals in mind when assigning oxidation numbers aids in understanding that transition metals are a special case, but not an exception to this convenient method.
An atom with an oxidation number of -1 accepts an electron to achieve a more stable configuration. The electron donation is then +1. When a transition metal loses electrons, it usually loses s orbital electrons first, followed by d orbital electrons. See Formation of coordination complexes for a more detailed discussion of how these compounds form. Most transition metals have multiple oxidation states because transition metals lose electron(s) more easily than alkali metals and alkaline earth metals. The valence s-orbital of alkali metals contains one electron, and their ions almost always have oxidation states of +1. (from losing a single electron). Similarly, alkaline earth metals have two electrons in their valence s-orbitals, resulting in +2 oxidation state ions (from losing both). Transition metals, on the other hand, are more complex and exhibit a variety of observable oxidation states, owing primarily to the removal of d-orbital electrons.
FAQs on Transition Elements- Classification, Properties and Oxidation States
1. What are transition elements and where are they located on the periodic table?
Transition elements, also known as d-block elements, are metallic elements that have an incomplete d subshell in their atoms or ions. According to the IUPAC definition, a transition element has a partially filled d orbital. They are located in Groups 3 to 12 of the modern periodic table, bridging the highly reactive s-block metals on the left and the p-block elements on the right.
2. How are the transition elements classified into different series?
The transition elements are classified into four main series based on which d-orbital is being filled:
- First Transition Series (3d series): This series includes elements from Scandium (Sc, Z=21) to Zinc (Zn, Z=30), where the 3d orbital is progressively filled.
- Second Transition Series (4d series): This includes elements from Yttrium (Y, Z=39) to Cadmium (Cd, Z=48), corresponding to the filling of the 4d orbital.
- Third Transition Series (5d series): This series contains elements from Lanthanum (La, Z=57) and then from Hafnium (Hf, Z=72) to Mercury (Hg, Z=80), where the 5d orbital is filled.
- Fourth Transition Series (6d series): This series starts with Actinium (Ac, Z=89) followed by elements from Rutherfordium (Rf, Z=104) onwards, corresponding to the filling of the 6d orbital. This series is incomplete.
3. Why do transition elements exhibit variable oxidation states?
Transition elements show variable oxidation states because the energy difference between their ultimate shell (ns) and penultimate shell (n-1)d orbitals is very small. Consequently, electrons from both the ns and (n-1)d orbitals can participate in chemical bonding. For example, Manganese (Mn) can exhibit oxidation states from +2 (by losing two 4s electrons) up to +7 (by losing two 4s and five 3d electrons).
4. What makes most compounds of transition elements coloured?
The colour of transition metal compounds is due to a phenomenon called d-d transition. In the presence of ligands, the d-orbitals of the transition metal ion split into two sets with different energy levels. When light falls on the compound, an electron from a lower energy d-orbital absorbs energy and gets excited to a higher energy d-orbital. The compound displays the complementary colour of the light absorbed. For this to occur, the metal ion must have a partially filled d-orbital, which is why compounds of Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are typically colourless.
5. What are the key physical properties that characterise transition elements?
Transition elements share several characteristic physical properties:
- High Melting and Boiling Points: Due to the involvement of (n-1)d electrons in addition to ns electrons in strong metallic bonding.
- High Density: Their densities are generally high because of their relatively small atomic radii and high atomic masses.
- Metallic Character: They are all metals, exhibiting properties like high tensile strength, ductility, malleability, and good thermal and electrical conductivity.
- High Enthalpy of Atomisation: This is a result of the strong interatomic bonding arising from a large number of unpaired d-electrons.
6. How does the electronic configuration of transition elements explain their magnetic properties?
The magnetic properties of transition elements are directly linked to their electronic configuration, specifically the presence of unpaired electrons in their d-orbitals.
- Paramagnetism: Most transition elements are paramagnetic because they contain one or more unpaired electrons. These unpaired electrons create a magnetic moment, causing the substance to be weakly attracted to an external magnetic field.
- Diamagnetism: Substances with no unpaired electrons (like those with d⁰ or d¹⁰ configuration, e.g., Zn²⁺) are weakly repelled by a magnetic field and are called diamagnetic.
7. Why are many transition metals and their compounds used as catalysts?
Transition metals and their compounds function as excellent catalysts primarily for two reasons:
- Variable Oxidation States: Their ability to exist in multiple oxidation states allows them to form unstable intermediate compounds with reactants. This provides an alternative reaction pathway with a lower activation energy, thereby increasing the reaction rate.
- Large Surface Area: In their finely divided state, they provide a large surface area where reactant molecules can be adsorbed, increasing the concentration of reactants and weakening their bonds.
A classic example is the use of Iron (Fe) in the Haber-Bosch process for ammonia synthesis.
8. What is the fundamental difference between transition elements and inner transition elements?
The primary difference lies in the orbital where the last electron enters. In transition elements (d-block), the differentiating electron enters the penultimate (n-1)d orbital. In contrast, for inner transition elements (f-block), the differentiating electron enters the anti-penultimate (n-2)f orbital. Inner transition elements, which include the Lanthanoids and Actinoids, are placed separately at the bottom of the periodic table.
9. Why do transition metals form interstitial compounds?
Transition metals form interstitial compounds because their crystal lattices have empty spaces, or 'interstices', between the metal atoms. Small non-metal atoms like hydrogen, carbon, or nitrogen can fit into these spaces without distorting the metal lattice. This trapping of small atoms leads to compounds that are typically non-stoichiometric, very hard, and have high melting points. For example, steel is an interstitial compound of iron and carbon.
10. Why does the atomic radius not change significantly across a transition series?
Across a transition series, as the atomic number increases, the nuclear charge also increases, which tends to pull the electron cloud closer and decrease the atomic radius. However, simultaneously, an electron is added to the (n-1)d orbital. These d-electrons provide a poor shielding effect. The increased nuclear charge is partially counteracted by the increased shielding effect of the d-electrons. These two opposing effects—increased nuclear charge and increased shielding—largely balance each other out, resulting in a relatively small and gradual change in atomic radii across the series.
