General Physical and Chemical Properties of the First Row of Transition Elements, Titanium to Copper

Full Notes

Colour, transition metals and d-orbital splitting have been covered in more detail here and here.
This page is just what you need to know for CIE A-level Chemistry :)

A transition element is defined as a d-block element that forms one or more stable ions with an incomplete d-subshell.

For Example:

• Fe²⁺ (3d⁶) and Fe³⁺ (3d⁵) both have partially filled d-orbitals, so Fe is a transition element.
• Zn is not a transition element because Zn²⁺ has a full d-subshell (3d¹⁰), despite being in the d-block.

Sketching 3d Orbitals

You need to be familiar with the shapes of d orbitals, especially:

• 3dxy orbital: lobes lie between the x and y axes in the x-y plane.
• 3dz² orbital: shaped like a doughnut around a dumbbell along the z-axis.

The shapes of d orbitals explain many of the properties shown by transition elements (see below).

Properties of Transition Elements

Variable Oxidation States

Transition elements can lose different numbers of d and 4s electrons, leading to a variety of stable oxidation states.

This is because the energy difference between 4s and 3d subshell is small, so both can be lost easily.

Example:

• Iron forms Fe²⁺ and Fe³⁺, manganese can go from Mn²⁺ to Mn⁷⁺.

Catalytic Behaviour

Transition metals act as catalysts because they:

• Can change oxidation state easily (e.g. Fe²⁺ ⇌ Fe³⁺).
• Can adsorb reactants onto their surface or form intermediate compounds.
• Can use empty d orbitals to form temporary bonds.

Examples:

• Iron in the Haber process
  (N₂ + 3H₂ ⇌ 2NH₃).
• Vanadium(V) oxide (V₂O₅) in the Contact process
  (SO₂ + O₂ → SO₃).

Formation of Coloured Compounds

Transition metal ions form coloured solutions and compounds due to:

• Electron transitions within the d-orbitals.
• When ligands bond to a metal ion, the d-orbitals split into different energy levels.

Electrons absorb light to move between these levels, observed colour is from unabsorbed wavelengths.

Formation of Complex Ions

Transition metals form complex ions by accepting lone pairs from ligands into vacant d orbitals.

• This is possible because their d orbitals are partially filled and energetically accessible.

Example:

• [Fe(H₂O)₆]³⁺ – iron(III) ion surrounded by six water molecules as ligands.

Why Transition Metals Have Variable Oxidation States

• The 4s and 3d subshells are very close in energy.
• Electrons can be lost from both sub-shells to give different oxidation states.
• This is unlike s- and p-block elements, where only the outermost electrons are easily removed.

Why Transition Metals Make Good Catalysts

• Can switch between oxidation states to transfer electrons in redox reactions.
• Have available d orbitals to form intermediates with reactants.
• This lowers activation energy and speeds up reactions.

Why They Form Complex Ions

• Transition metals have vacant d orbitals.
• These orbitals are at the right energy to accept lone pairs from ligands.
• The positive charge on the metal ion attracts the negative or lone-pair-rich ligands.

Summary

• Transition elements are d-block elements forming ions with incomplete d-subshells.
• Key properties: variable oxidation states, catalytic activity, coloured compounds, complex ion formation.
• These arise from the close energy of 3d and 4s orbitals and availability of vacant d orbitals.
• Zn is not a transition element because Zn²⁺ has a full 3d subshell.