Transition elements are d-block elements that form one or more stable ions with a partially full d subshell. However, not all d-block elements are transition elements by definition. For example, Sc and Zn are d-block elements but they do not fit the definition of transition elements because:
- Scandium forms only one ion (Sc3+) which has no electrons in the 3d subshell. The electronic configuration of Sc3+ is [Ar] 3d0 4s0.
- Zinc forms only one ion (Zn2+) which has a complete 3d subshell. The electronic configuration of Zn2+ is [Ar] 3d10 4s0.
By definition, the transition elements in the first row of the d-block are titanium (Ti) through to copper (Cu).
Electronic configurations of transition elements
| Element | Electronic configuration |
|---|---|
| titanium (Ti) | 1s2 2s2 2p6 3s2 3p6 3d2 4s2 |
| vanadium (V) | 1s2 2s2 2p6 3s2 3p6 3d3 4s2 |
| chromium (Cr) | 1s2 2s2 2p6 3s2 3p6 3d5 4s1 |
| manganese (Mn) | 1s2 2s2 2p6 3s2 3p6 3d5 4s2 |
| iron (Fe) | 1s2 2s2 2p6 3s2 3p6 3d6 4s2 |
| cobalt (Co) | 1s2 2s2 2p6 3s2 3p6 3d7 4s2 |
| nickel (Ni) | 1s2 2s2 2p6 3s2 3p6 3d8 4s2 |
| copper (Cu) | 1s2 2s2 2p6 3s2 3p6 3d10 4s1 |
The table above shows the electronic configurations of the atoms in the first row of the transition elements. As you can see from the table, the 4s subshell of transition elements is normally filled and the rest of the electrons occupy orbitals in the 3d subshell. Chromium and copper are the exceptions.
- Chromium atoms have just one electron each in the 4s subshell and the rest five electrons are arranged in the 3d subshell such that each of the 5 orbitals is occupied by a single electron.
- Copper atoms have just one electron each in the 4s subshell and the rest ten electrons are arranged in the 3d subshell such that each orbital is filled by two electrons.
Common Oxidation states of transition elements
Transition elements are all metals and as such, their atoms have a tendency to lose electrons and form positively charged ions. However, transition metals have variable oxidation states which means that each transition metal has the ability to form more than one ion. For example, copper can form the ions Cu+ and Cu2+.
The table below shows the oxidation states of the first row of the transition elements.
| Element | Common Oxidation states |
|---|---|
| titanium (Ti) | +3, +4 |
| vanadium (V) | +2, +3, +4, +5 |
| chromium (Cr) | +3, +6 |
| manganese (Mn) | +2, +4, +6, +7 |
| iron (Fe) | +2, +3 |
| cobalt (Co) | +2, +3 |
| nickel (Ni) | +2 |
| copper (Cu) | +1, +2 |
Due to the existence of variable oxidation states the names of compounds of transition elements should have their oxidation number included, e.g. manganese(IV) oxide, cobalt(II) chloride, iron(III) oxide, copper(II) sulphate.
How transition elements form ions
When forming ions, atoms of transition elements lose electrons from the 4s subshell first, followed by 3d electrons to form ions that have partially filled 3d subshells.
For example:
- V atom = 1s2 2s2 2p6 3s2 3p6 3d3 4s2
- V3+ ion = 1s2 2s2 2p6 3s2 3p6 3d2 4s0
- Fe atom = 1s2 2s2 2p6 3s2 3p6 3d6 4s2
- Fe3+ ion = 1s2 2s2 2p6 3s2 3p6 3d5 4s0
- Cu atom = 1s2 2s2 2p6 3s2 3p6 3d10 4s1
- Cu2+ ion = 1s2 2s2 2p6 3s2 3p6 3d9 4s0
The most common oxidation state among transition elements is +2, which is usually formed when the two 4s electrons are lost from the atoms. From titanium to manganese, the maximum oxidation number involves the loss of all the 4s and 3d electrons in their atoms. For example:
- titanium’s maximum oxidation state is +4, when it loses its two 4s electrons and its two 3d electrons.
- vanadium’s maximum oxidation state is +5, when it loses its two 4s electrons and its three 3d electrons.
However, from iron onwards, the +2 oxidation state is common as the 3d electrons become increasingly harder to remove due to the increase in the nuclear charge across the period.