Question

Discuss the variation of ionisation potential across a transition series.

Short answer

Generally, ionisation potential increases across a transition series due to increased nuclear charge. However, exceptions exist in the case of chromium and copper where the ionisation potential is lower due to entering of the last electron into the inner d-orbital resulting in increased stability and hence, lower ionisation potential.

Step-by-step solution

Understanding the Trend

In general, across a transition series, the ionisation potential increases. This is due to the increase in nuclear charge as you move across the series. This increased nuclear charge results in stronger attraction between the nucleus and the outermost electron. This means that more energy (higher ionisation potential) is required to remove this electron.

Identifying the Exceptions

There are exceptions to this general trend. The ionisation potential of chromium and copper is less than that of their preceding elements (iron, cobalt and nickel). This is due to the fact that in these elements (chromium and copper), the last electron enters the inner d-orbital instead of the outer s-orbital, resulting in increased stability and hence, lower ionisation potential.

Understanding the Cause of Exceptions

The lowered ionisation potential in the case of chromium and copper is due to the symmetry and increased stability provided by a completely filled or half-filled d-subshell. These configurations result in lower energy and hence, lower ionisation potential.

Key concepts

Transition Series

The transition series are a set of metallic elements in the periodic table, primarily found in the d-block. These elements share a common trait: they have d-orbital electrons in addition to s-orbital electrons. This unique electronic configuration lends various physical and chemical properties to the transition metals.
The transition series is divided into three distinct series based on the principal energy level that the elements occupy. These are:

• 3d series: Starting from Scandium (Sc) to Zinc (Zn)
• 4d series: Starting from Yttrium (Y) to Cadmium (Cd)
• 5d series: Starting from Lanthanum (La) or Hafnium (Hf) to Mercury (Hg)

As you move across each series from left to right, an electron is added to the d-orbitals. Understanding this arrangement helps explain the trends in properties such as ionization potential among these elements.

Nuclear Charge

Nuclear charge refers to the total charge of the nucleus, which is determined by the number of protons present. As you proceed across a transition series, the nuclear charge increases because protons are added to the nucleus. This increase in nuclear charge directly affects the ionisation potential.
What happens here is that the added protons lead to a stronger pull on the electrons, including those in the outermost shell. This stronger attraction typically requires more energy to remove an electron, upgrading the ionisation potential. However, due to the screening effect of the inner electrons and the specific arrangement of d-electrons, the increase in ionisation potential is gradual rather than abrupt.
Thus, an increase in nuclear charge is a fundamental factor influencing not just ionisation potential but also various other properties of transition metals.

Electron Configuration

Electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals. For transition elements, this typically involves the filling of d-orbitals and s-orbitals. This filling impacts the chemical behavior and reactivity of the elements.
In each transition series, electrons are being added to d-orbitals (except for a few exceptions). This d-orbital filling is significant as it affects the properties of the atom, including its ionisation potential. For instance, a filled or half-filled d-subshell tends to provide additional stability to the atom, influencing the energy required to remove an electron.
Therefore, understanding electron configurations helps explain why certain transition elements demonstrate exceptions in trends like ionisation potential.

D-Orbitals

D-orbitals are a set of five orbitals in the second highest energy level of an atom within the d-block of the periodic table. They have distinct shapes and orientations that allow them to uniquely interact with other atoms and molecules, making the study of transition metals and their properties quite fascinating.
The presence of d-orbitals in transition elements leads to complex behaviors. For example:

• They can hold up to ten electrons, affecting how these atoms bond and interact.
• The partially filled d-orbitals allow elements to exhibit multiple oxidation states.

This d-orbital behavior also informs why exceptions in ionisation potential are present across the transition series. In some cases, like chromium and copper, electron configurations with half-filled or fully filled d-orbitals offer increased stability and result in unique shifts in ionisation potential.

Exception Trends

Exception trends are often seen in the properties of transition metals, including their ionisation potentials. The exception trends occur due to the unique electron configurations that offer specific stability advantages.
For instance, in chromium and copper, the last electron enters a d-orbital instead of an s-orbital, resulting in configurations that are either half-filled or fully filled. This configuration provides a peculiar stability because these configurations are particularly low in energy. As a result, despite an increasing nuclear charge, these elements show a decrease in ionisation potential.
Thus, examining these exceptions provides insight not only into the transition series itself but also the broader periodic trends governing chemical behavior.