Transition Elements
- Transition metals are elements with an incomplete d-subshell or that can form at least one stable cation with an incomplete d-subshell
- This definition distinguishes them from d-block elements, because scandium and zinc do not fit the definition
- Scandium only forms the ion Sc3+, configuration [Ar] 3d0
- Zinc only forms the ion Zn2+, configuration [Ar] 3d10
- The elements of the first transition series are therefore titanium to copper
The transition elements and the d-block elements
Electron Configuration
- The full electronic configuration of the first d-series transition metals is shown in the table below
- Following the Aufbau Principle electrons occupy the lowest energy subshells first
- The 4s overlaps with the 3d subshell so the 4s is filled first
- Remember that you can abbreviate the first five subshells, 1s-3p, as [Ar] representing the configuration of argon (known as the argon core)
Table showing the electronic configuration of the first d-series transition elements
- From your previous studies in chemistry, you should recall there are two exceptions to the Aufbau Principle in the first row of d-block, chromium and copper
- In both cases an electron is promoted from the 4s to the 3d to achieve a half full and full d-subshell, respectively
- Chromium and copper have the following electron configurations, which are different to what you may expect:
- Cr is [Ar] 3d5 4s1 not [Ar] 3d4 4s2
- Cu is [Ar] 3d10 4s1 not [Ar] 3d9 4s2
- This is because the [Ar] 3d5 4s1 and [Ar] 3d10 4s1 configurations are energetically more stable and are preferred configurations
- When forming cations, remove the 4s electrons first
Worked example
Writing electronic configuration of transition element ions
State the full electronic configuration of the manganese(III) ion
Answer
Step 1: Write out the electron configuration of the atom first:
Mn atomic number = 25
1s22s22p63s23p64s23d5
2 + 2 + 6 + 2 + 6 + 2 + 5 = 25 electrons
Step 2: Subtract the appropriate number of electrons starting from the 4s subshell
Mn(III) = 22 electrons
1s22s22p63s23p63d4
General properties
- Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
- Variable oxidation states
- Form complex ions
- Form coloured compounds
- Behave as catalysts
- Have magnetic properties
Variable Oxidation States
- 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
Forming 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
- A molecule or ion surrounding the central metal atom or ion is called a ligand
- 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
Forming coloured compounds
- 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
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 from or donating electrons to reagents within the reaction
- Substances can also be adsorbed onto their surface and activated in the process
Magnetic Properties
- Materials are classified as diamagnetic, paramagnetic or ferromagnetic according to their behaviour when placed in an external magnetic field
- Transition metals exhibit these properties depending on their electronic configurations