Ionisation Enthalpies
Examination of the trends in successive ionisation enthalpies and factors influencing them.
What is Ionisation Enthalpies?
The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
Key formula / rule: First Ionisation Enthalpy (IE1)
Key points
- Define ionisation enthalpy and successive ionisation enthalpies.
- Explain the general trends in ionisation enthalpies across a period and down a group for transition elements.
- Identify and explain the reasons for irregularities in ionisation enthalpy trends, particularly those related to d⁵ and d¹⁰ configurations.
- Relate ionisation enthalpy values to the stability of oxidation states and chemical reactivity of transition elements.
Common exam trap
Assuming a smooth increase in IE across a period for transition elements without considering d-orbital stability.
Definitions
- Term
Ionisation Enthalpy
- Meaning
The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
- Term
Successive Ionisation Enthalpies
- Meaning
The energies required to remove electrons one after another from an atom. IE1 is for the first electron, IE2 for the second, and so on.
- Term
Lanthanoid Contraction
- Meaning
The steady decrease in atomic and ionic radii of lanthanoids with increasing atomic number due to the poor shielding effect of 4f electrons, which significantly affects the properties of subsequent 5d transition elements.
Learning objectives
Define ionisation enthalpy and successive ionisation enthalpies.
Explain the general trends in ionisation enthalpies across a period and down a group for transition elements.
Identify and explain the reasons for irregularities in ionisation enthalpy trends, particularly those related to d⁵ and d¹⁰ configurations.
Relate ionisation enthalpy values to the stability of oxidation states and chemical reactivity of transition elements.
Understand the influence of lanthanoid contraction on the ionisation enthalpies of 3rd transition series elements.
Formulae
- Name
First Ionisation Enthalpy (IE1)
- Note
Energy change (ΔH) for this process. Always positive (endothermic).
- Expression
M(g) → M⁺(g) + e⁻
- Name
Second Ionisation Enthalpy (IE2)
- Note
Energy change (ΔH) for this process. Always positive and greater than IE1.
- Expression
M⁺(g) → M²⁺(g) + e⁻
Prerequisites
Basic understanding of atomic structure (protons, neutrons, electrons).
Knowledge of electronic configuration, especially for d-block elements.
Concepts of effective nuclear charge and shielding effect.
Understanding of orbital stability (half-filled and fully-filled orbitals).
Common mistakes
Assuming a smooth increase in IE across a period for transition elements without considering d-orbital stability.
Confusing ionisation enthalpy with electron gain enthalpy.
Not understanding the impact of lanthanoid contraction on the IE of 3rd series elements.
Incorrectly predicting the relative IE values for elements like Cr, Mn, Cu, and Zn without considering their electronic configurations.
Forgetting that IE values are always positive (endothermic process).
Keywords
Ionisation Enthalpy
Successive Ionisation Enthalpies
Transition Elements
d-block
Electronic Configuration
Half-filled orbitals
Fully-filled orbitals
Lanthanoid Contraction
Oxidation States
Metallic Character
Shielding Effect
Effective Nuclear Charge
Practice preview
Which of the following elements has the highest first ionisation enthalpy?…
easy
The successive ionisation enthalpies of an element X are given as: IE1 = 500 kJ/mol, IE2 = 1200 kJ/mol, IE3 = 2500 kJ/mol, IE4 = 3200 kJ/mol, IE5 = 4000 kJ/mol, IE6 = 4800 kJ/mol. In which group of the periodic table is …
medium
Which of the following factors does NOT significantly affect the ionisation enthalpy of an element?…
easy