Most schools cover the transition element topic last in the H2 A Level Chemistry syllabus. This topic is usually consists of very thick notes, and many students find it rather challenging how to learn this topic. In this post here, I’ll discuss how you can break this topic down to learn it.
We can break this topic down into these areas and tackle them:
- Atomic Structure (this is essentially a revision of the atomic structure chapter, where you learn about writing of electronic configurations, and looking at factors affecting effective nuclear charge and relate them to ionisation energy, atomic radii, etc.)
- Physical Properties
- Chemical Properties
- Why transition elements form coloured complexes
Atomic Structure of Transition Elements
Electronic Configuration of transition elements (1st row)
Recall in the atomic structure chapter, we learn that electrons fill the 4s orbitals, before filling up the 3d orbitals? Conversely, when removing electrons to form cations, the electrons are removed from the 4s orbitals first before the 3d orbitals.
Hence, the electronic configuration of first row d-block transition elements are:
Electronic Configuration of Sc: 1s²2s²2p⁶3s²3p⁶3d¹4s²
Electronic Configuration of Ti: 1s²2s²2p⁶3s²3p⁶3d²4s²
Electronic Configuration of V: 1s²2s²2p⁶3s²3p⁶3d³4s²
Electronic Configuration of Cr: 1s²2s²2p⁶3s²3p⁶3d⁵4s¹
Electronic Configuration of Mn: 1s²2s²2p⁶3s²3p⁶3d⁵4s²
Electronic Configuration of Fe: 1s²2s²2p⁶3s²3p⁶3d⁶4s²
Electronic Configuration of Co: 1s²2s²2p⁶3s²3p⁶3d⁷4s²
Electronic Configuration of Ni: 1s²2s²2p⁶3s²3p⁶3d⁸4s²
Electronic Configuration of Cu: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹
Electronic Configuration of Zn: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²
Note that both Cu and Cr are “special”. Instead of filling 2 electrons in the 4s orbital, they only have 1.
Atomic Radii of transition elements
If we were to plot the atomic radii of the first row transition elements (from Scandium to Zinc), this is the trend:
Observe the trend in atomic radii as we go across the first row transition element from Scandium to Zinc. Notice that the general trend is that there is little change in atomic radii from scandium to zinc.
This is because although the number of protons increases from Sc to Zn, the electrons are added to the penultimate 3d shell, which increases the shielding effect on the outermost 4s electrons. Increase in nuclear charge due to addition protons is negated by the increase in shielding effect. Hence, effective nuclear charge remains relatively unchanged. Atomic radius thus remains relatively unchanged from Sc to Zn.
First Ionisation Energy of First Row Transition Elements
The general trend is that the 1st ionisation energy remains relatively the same across the first row transition element.
The reason is similar to why the atomic radii of these elements remain relatively unchanged across the period. This is due to the addition of electrons to the penultimate 3d shell, which increases the shielding effect on the outermost 4s electrons. Increase in nuclear charge due to addition protons is negated by the increase in shielding effect. Hence, effective nuclear charge remains relatively unchanged, and so does the first ionisation energy.
Physical Properties of Transition Elements
Transition elements have a higher melting point than other metals. In the fourth period, the 3d and 4s electrons are involve in the metallic bonding. Hence, there is a stronger electrostatic force of attraction between the metallic ions and the delocalised electrons. The stronger metalllic bonds gives transition elements a higher melting point, density, and electrical conductivity, compared to other metals.
Chemical Properties of Transition Elements
Variable Oxidation State
Transition elements can form ions with different charge: This is due to the relatively close energies of the 4s and 3d electrons, which allow them to lose electrons from these subshells.
Transition elements can also form covalent bonds due to the availability of the partially filled 3d subshell to accept electrons.
Acts as Good Catalysts
Transition elements can act as both heterogeneous catalysts and homogeneous catalysts. A heterogeneous catalyst is one that is of a different state as the reactants. A homogeneous catalysts is one that is of the same state as the reactants.
As a heterogeneous catalyst, the partially filled 3d subshell allows transition elements (or their ions) to form bonds during the adsorption stage of catalysis.
When they act as homogeneous catalyst, transition elements usually change from one oxidation state to another, and then back to their original oxidation state.
Transition element can form complexes due to the availability of the 3d and 4s orbital to accept electron pair from ligands.
Why Transition Elements Form Coloured Compounds
Ligands will split the 3d orbitals into 2 slightly different energy levels. Since the 3d orbitals are partially filled, electrons from the slightly lower energy 3d orbitals can be promoted to the slightly higher energy 3d orbital by absorbing light within the visible light range. Colour observed is complement to the colour absorbed.