• 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 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 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 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 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
• Because of this, Roman numerals are used to indicate the oxidation state on 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 oxidation state of Cr is +3) is?dark green
  • Whereas the colour of the [Cr(NH₃)₆]³⁺?complex (oxidation state of Cr is still +3) is?purple

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