The Transition Elements: Titanium to Copper (CIE A Level Chemistry)

• Transition elements are d-block elements which form one or more stable ions with an incomplete d subshell
• They are all metals found in the d-block of the Periodic Table, between Groups 2 and 13
  • Sometimes they are referred to as transition metals
• Not all d-block elements are classed as transition elements: scandium and zinc, for example, are not classed as transition elements, despite being in the d-block
• Scandium is not classed as a transition element because:
  • It only forms one ion, Sc³⁺
  • The Sc³⁺ion has no electrons in its 3d subshell; it has the electronic configuration of [Ar]
• Zinc is also not classed as a transition element because:
  • It also forms only one ion, Zn²⁺
  • The Zn²⁺ ion has a complete 3d subshell; it has the electronic configuration [Ar]3d¹⁰ 

The transition elements on the periodic table

The transition elements are in the central d-block on the periodic table

• The transition elements all have incomplete d subshells
• There are five orbitals in a d subshell. Some of these orbitals may have similar shapes but different orientations, whereas others may have completely different shapes
• The five orbitals are
  • 3d_yz
  • 3d_xz
  • 3d_xy
  • 3d_{x² - y²}
  • 3d_z²
• Note that students are required to sketch the shapes of the 3d_xy and 3d_z² orbitals only

Shapes of the 3d orbitals

• The 3d_yz, 3d_xz, and 3d_xy orbitals are orbitals which lie in the y-z, x-z and x-y plane respectively
  • They all have four lobes that point between the two axes

The 3d_yz, 3d_xz, and 3d_xy orbitals

The 3d_yz, 3d_xz, and 3d_xy orbitals all have four lobes which are similar in shape but point between different axes

• The 3d_{x2 - y2} orbital looks like the 3d_yz, 3d_xz, and 3d_xy orbitals, as it also consists of four lobes
• The difference is that these lobes point along the x and y axes and not between them

The 3d_x²-y² orbital

The four lobes in a 3d_x²-y² orbital point along the axes

• The 3d_z² orbital is different from the other orbitals, as there are two main lobes which form a dumbbell shape
• The two main lobes point along the z-axis and there is a “doughnut” ring around the centre

The 3d_z² orbital 

The 3d_z² orbital has a dumbbell shape with a ring around the centre

• Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
  • Variable oxidation states
  • Behave as catalysts
  • Form complex ions
  • Form coloured compounds

Ions of transition metals

• Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
• However, unlike other metals, transition elements can form more than one positive ion
  • They are said to have variable oxidation states
• Due to this, Roman numerals are used to indicate the oxidation state of the metal ion
  • For example, the metal sodium (Na) will only form Na⁺ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
  • The transition metal iron (Fe) can form Fe²⁺ (Fe(II)) and Fe³⁺ (Fe(III)) ions
• The table below shows the most common oxidation states of a few transition metals

Oxidation states of transition elements table

Coloured complex

• Another characteristic property of transition elements is that their compounds are often coloured
  • For example, the colour of the [Cr(OH)₆]^3- complex (where the oxidation number of Cr is +3) is dark green
  • Whereas the colour of the [Cr(NH₃)₆]³⁺ complex (where the oxidation number of Cr is still +3) is purple 

Colours of common transition metal complexes

Examples of some transition metal ions and their coloured complexes

Transition elements as catalysts

• Since transition elements can have variable oxidation states, they make excellent catalysts
• During catalysis, the transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction
  • For example, iron (Fe) is commonly used as a catalyst in the Haber Process, switching between the +2 and +3 oxidation states
• Substances can also be adsorbed onto their surface and activated in the process

Complex ions

• Another property of transition elements caused by their ability to form variable oxidation states is their ability to form complex ions
• A complex ion is a molecule or ion, consisting of a central metal atom or ion, with a number of molecules or ions surrounding it
• The molecules or ions surrounding the central metal atom or ion are called ligands
• Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element
  • For example, the chromium(III) ion can form [Cr(NH₃)₆]³⁺, [Cr(OH)₆]^3- and [Cr(H₂O)₆]³⁺ complex ions