Ch 1 · General physical and chemical properties of the first row of transition elements · AS & A Level Chemistry 9701
Topic 1 of 5 in Chemistry of transition elements — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Every d-block element from scandium to zinc has electrons entering 3d orbitals, but not every d-block element behaves as a "transition element" in the chemical sense examined at A2. This lesson defines the term precisely, introduces the shapes of the d orbitals, and explains the four characteristic properties that follow directly from the close energies and availability of the 3d and 4s sub-shells.
Key ideas — Defining a transition element
A transition element is a d-block element that forms one or more stable ions with an incomplete d subshell. First-row atoms have the configuration mathrm[Ar]3d^x4s^2, with two exceptions: chromium, mathrm[Ar]3d^54s^1, and copper, mathrm[Ar]3d^104s^1, both gaining extra stability from a half-filled or fully filled 3d subshell. When these atoms form ions, the 4s electrons are always removed before any 3d electrons, since once occupied the 3d orbitals fall below 4s in energy. Scandium and zinc are d-block elements but not transition elements: Sc forms only mathrmSc^3+ (mathrm[Ar]3d^0, no d electrons at all), and Zn forms only mathrmZn^2+ (mathrm[Ar]3d^10, a completely full d subshell); neither ion has an incomplete d subshell.
🧑🏫 Teacher's explanation
Think of transition elements like a versatile multi-tool. While a standard metal might only have one way to react, transition metals have 'internal flexibility' because their 3d electrons are at a very similar energy level to their 4s electrons. This allows them to lose different numbers of electrons depending on the situation, leading to multiple oxidation states.
📖 Definitions
🔍 Worked example — Easy
Determine the electron configuration of Fe^2+ and Fe^3+ ions, given that the atomic number of Iron is 26.
🔍 Worked example — Standard
Explain why Zn^2+ is not classified as a transition element according to the Cambridge definition.
🔍 Worked example — Exam-style
A transition metal M has the configuration [Ar] 3d^5 4s^1. It forms an ion M^2+. Write the configuration of M^2+.
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🎯 Exam question
Key ideas — Shapes of the 3d orbitals
The five 3d orbitals of a free atom are degenerate (equal in energy). The 3d_xy orbital has a four-lobed, cloverleaf shape lying in the xy-plane, with its lobes pointing between the x and y axes. The 3d_z^2 orbital has a distinctive shape: two lobes pointing along the z-axis, plus a torus (ring) of electron density around the nucleus in the xy-plane.
🧑🏫 Teacher's explanation
Imagine you are trying to place five different-shaped pieces of clay into a single box. In a free atom, all these 'clay pieces' (the 3d orbitals) are exactly the same weight and cost the same energy to make; we call this being 'degenerate'. However, once you bring other atoms or ions near the transition metal, the shape of these orbitals matters immensely.
📖 Definitions
🔍 Worked example — Easy
A transition metal ion has a 3d_xy orbital. Describe the orientation of its lobes relative to the x and y axes.
🔍 Worked example — Standard
Sketch the components of a 3d_z^2 orbital.
🔍 Worked example — Exam-style
Explain why the five 3d orbitals in a free atom are considered degenerate.
⚠️ Common errors
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🎯 Exam question
(a) Describe the shape of the 3d_z^2 orbital. [2 marks] (b) Explain why the 3d_xy orbital is considered degenerate with the 3d_z^2 orbital in a free atom. [1 mark] [3 marks]
Key ideas — Characteristic properties of transition elements
Transition elements share four characteristic properties: variable oxidation states (for example mathrmFe^2+/mathrmFe^3+, mathrmCu^+/mathrmCu^2+, and manganese spanning +2 to +7); catalytic behaviour, both as elements (iron in the Haber process) and as ions in solution (Fe^2+/Fe^3+ in the iodide–peroxodisulfate reaction); complex ion formation with ligands bonded through dative covalent bonds; and coloured compounds, since most transition metal ions in solution or in solid compounds are coloured, unlike the white or colourless compounds of mathrmSc^3+ and mathrmZn^2+.
🧑🏫 Teacher's explanation
Imagine a transition metal as a multi-story building where the floors (energy levels) are very close together. Because the 4s and 3d floors are so similar in height, it is very easy for an electron to jump from one to the other or for the building to lose electrons from different levels. This is why they can have many different 'oxidation states'—they aren't stuck with just one way of being charged.
📖 Definitions
🔍 Worked example — Easy
Explain why Scandium (Sc^3+) is typically colorless, whereas Copper (Cu^2+) is blue.
🔍 Worked example — Standard
A transition metal ion M^n+ has a configuration of [Ar] 3d^5. It can lose electrons to form M^(n+1)+ with a configuration of [Ar] 3d^4. Explain why this transition is possible.
🔍 Worked example — Exam-style
Calculate the mass of FeCl_3 required to react with 2.00 moles of Na_2S_2O_8 in a redox reaction where Fe^2+ is oxidized to Fe^3+. (Assume 1:1 stoichiometry for the metal ion change).
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🎯 Exam question
Key ideas — Explaining variable oxidation states
The 3d and 4s sub-shells of the first-row transition elements are very close in energy. Successive ionisation energies therefore increase only gradually as electrons are removed from first 4s and then 3d, without the very large jump seen after removing the outer electrons of an s- or p-block element. Because no single oxidation state is far more energetically favourable than its neighbours, transition elements can lose different numbers of electrons under different conditions, giving rise to several stable oxidation states for the same element.
🧑🏫 Teacher's explanation
Imagine you are a person trying to decide how many heavy coats to carry while hiking. If all the coats weigh almost exactly the same, you might choose to carry one, two, or three depending on how cold it gets. In transition metals, the 'coats' are the electrons in the 4s and 3d sub-shells. Because these sub-shells have very similar energy levels, the energy 'cost' to remove an electron from 4s versus 3d is nearly the same.
📖 Definitions
🔍 Worked example — Easy
Explain why Manganese (Mn) can exhibit oxidation states from +2 to +7.
🔍 Worked example — Standard
Compare the ionization energy trends of Magnesium (Group 2) and Scandium (Transition Metal) when moving from the first to the second ionization energy.
🔍 Worked example — Exam-style
A transition metal ion X^n+ has a configuration of [Ar] 3d^5. If it loses one more electron to become X^(n+1)+, explain why this is energetically feasible.
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Key ideas — Explaining catalytic behaviour and complex formation
Transition elements act as catalysts because they can interconvert between accessible oxidation states, accepting and donating electrons in redox catalysis (as mathrmFe^2+/mathrmFe^3+ do), and because their energetically accessible d orbitals allow reactant molecules to bond to a metal surface, weakening their bonds and lowering the activation energy of the reaction. The same accessible, energetically low-lying vacant d orbitals explain complex formation: a ligand's lone pair can be donated into an empty d (or hybridised d/s/p) orbital of the metal ion to form a dative covalent bond, something the ions of typical s- and p-block metals, which lack suitably accessible empty orbitals, do not readily do.
🧑🏫 Teacher's explanation
Think of transition metals like a Swiss Army knife. While a standard s-block metal (like Sodium) is like a single-purpose blade, transition metals have multiple 'tools' (oxidation states) and 'attachments' (ligands) they can use. They are chemically flexible because their 3d and 4s electrons are very close in energy, allowing them to lose different numbers of electrons without a massive energy penalty.
📖 Definitions
🔍 Worked example — Easy
Explain why transition metals exhibit variable oxidation states, whereas Group 2 metals do not.
🔍 Worked example — Standard
A transition metal ion acts as a catalyst in a redox reaction. Explain the role of its d orbitals in this process.
🔍 Worked example — Exam-style
Explain why transition metal ions form complex ions while Mg^2+ ions typically do not.
⚠️ Common errors
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🎯 Exam question
(a) Explain how the presence of vacant d orbitals allows a transition metal to act as a heterogeneous catalyst. [2] (b) Explain why the ability to change oxidation states allows a transition metal to act as a homogeneous catalyst. [2] [4 marks]
Worked example — Model answer
Question. (a) Define the term transition element, and use it to explain why zinc is not classified as a transition element. [2] (b) Give the full electron configuration of a chromium atom (Z = 24), and explain why it differs from the pattern mathrm[Ar]3d^44s^2 predicted by filling orbitals in order. [3] (c) Explain, in terms of atomic orbitals, why mathrmFe^3+ readily forms complex ions with ligands such as water, while mathrmCa^2+ does not. [3]
🔍 Worked example — Easy
State the electron configuration of Cr (Z=24) and Cr^3+ ions.
🔍 Worked example — Standard
Explain why Cu^2+ is coloured while Zn^2+ is colourless.
🔍 Worked example — Exam-style
Explain why transition metals act as catalysts in many industrial processes.
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