Ch 4 · Ionisation energy · AS & A Level Chemistry 9701
Topic 4 of 4 in Atomic structure — AS & A Level Chemistry 9701 — 59 lessons.
Introduction
Ionisation energy measures how tightly an atom holds its electrons, and its trends and successive values reveal the shell structure of the atom directly. This lesson defines ionisation energy, explains the periodic trends and the pattern of successive values, and shows how ionisation energy data can be used to deduce electronic configuration and periodic position.
Key ideas — Defining first ionisation energy
The first ionisation energy of an element is the energy required to remove one electron from each atom in one mole of gaseous atoms of that element, to form one mole of gaseous 1+ ions. It is always endothermic (positive), since energy must be supplied to overcome the electrostatic attraction between the nucleus and the electron being removed.
🧑🏫 Teacher's explanation
Imagine you are trying to pull a magnet away from a metal surface. The closer the magnet is to the surface, and the stronger the magnet is, the more force you need to pull it away. In an atom, the nucleus is the magnet and the electron is the metal surface. Ionisation energy is simply the 'effort' or energy required to pull that electron away.
📖 Definitions
🔍 Worked example — Easy
Calculate the first ionisation energy of Magnesium (Mg) if 1.20 imes 10^3 J is required to remove one mole of electrons from one mole of gaseous Mg atoms.
🔍 Worked example — Standard
Write the balanced equation for the first ionisation energy of Aluminium (Al) in the gaseous state.
🔍 Worked example — Exam-style
An element X has the following successive ionisation energies (in kJ mol^-1): I_1 = 500, I_2 = 700, I_3 = 4500, I_4 = 6000. Identify the group of element X.
⚠️ Common errors
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🎯 Exam question
Key ideas — Equations for successive ionisation energies
Each ionisation energy is represented by an equation showing the removal of one electron from a species already in the gas phase. For an element X:
🧑🏫 Teacher's explanation
Imagine you are trying to pull a series of heavy magnets away from a metal surface. The first magnet is easy to slide off, but once you remove it, the next magnet is much closer to the surface and held much more tightly. This is exactly how ionisation energy works. Each time you remove an electron, the remaining electrons feel a stronger pull from the nucleus because there is less 'shielding' and the same amount of positive charge is pulling on fewer negative charges.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the second ionisation energy of Magnesium (Mg).
🔍 Worked example — Standard
An element X has the following successive first ionisation energies (in kJ mol^-1): I_1 = 500, I_2 = 4500, I_3 = 6100, I_4 = 25000. Identify the group of element X.
🔍 Worked example — Exam-style
Calculate the total energy required to convert 1 mole of gaseous Al atoms into Al^3+(g) ions.
⚠️ Common errors
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🎯 Exam question
I_1 = 738 I_2 = 1450 I_3 = 7733 I_4 = 10500
Key ideas — Trends across a period and down a group
Across a period, first ionisation energy generally increases from left to right (with small dips, e.g. at Group 13 and Group 16 due to sub-shell and spin-pairing effects). Nuclear charge increases while electrons are added to the same outer shell, so shielding stays roughly constant; the increased nuclear charge attracts the outer electron more strongly, and atomic radius decreases, so more energy is needed to remove an electron.
🧑🏫 Teacher's explanation
Think of an ionisation energy as the 'cost' of stealing an electron from an atom. Imagine you are trying to pull a magnet away from a metal surface. If the magnet is very strong (high nuclear charge) and the surface is very close (small radius), it is incredibly hard to pull it away. If you put a thick layer of rubber between the magnet and the surface (shielding), it becomes much easier to pull.
📖 Definitions
🔍 Worked example — Easy
Calculate the first ionisation energy of Magnesium (Mg) if the enthalpy change for the reaction Mg(g) arrow Mg^+(g) + e^- is 738 kJ mol^-1.
🔍 Worked example — Standard
An element X has the following successive first ionisation energies (in kJ mol^-1): I_1 = 502, I_2 = 7455, I_3 = 10010, I_4 = 13500. Identify the group of element X.
🔍 Worked example — Exam-style
Explain why the first ionisation energy of Nitrogen (N) is higher than that of Oxygen (O).
⚠️ Common errors
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🎯 Exam question
Key ideas — Successive ionisation energies of one element
Successive ionisation energies of a given element always increase, since each electron is removed from an ion of increasing positive charge, so the remaining electrons are held more tightly by a fixed nuclear charge acting on fewer electrons (reduced electron–electron repulsion, and for a given shell, decreasing radius). There are large jumps in ionisation energy whenever an electron is removed from a shell closer to the nucleus than the previous electron, because that shell experiences less shielding and a smaller radius, and hence much stronger nuclear attraction.
Key ideas — Factors determining the size of an ionisation energy
Ionisation energy arises fundamentally from the electrostatic attraction between the positive nucleus and the electron being removed; the size of an ionisation energy is governed by four factors:
🧑🏫 Teacher's explanation
Think of an ionisation energy as the 'cost' of stealing an electron from an atom. The more the nucleus 'wants' to keep that electron, the higher the cost. It is like a magnet pulling on a metal bead; if the magnet is stronger, or the bead is closer, or there is less stuff in the way, it is much harder to pull that bead away.
📖 Definitions
🔍 Worked example — Easy
Calculate the first ionisation energy of Magnesium (M_r = 24) if 1.20 ext g of Magnesium reacts with 0.50 ext mol of HCl to produce H_2 gas and MgCl_2 (assume 100\% yield). How many moles of electrons were removed in total for the 1.20 g sample?
🔍 Worked example — Standard
An element X has the following successive first ionisation energies (in kJ mol^-1): I_1 = 502, I_2 = 7456, I_3 = 10300. Identify the group of element X in the Periodic Table.
🔍 Worked example — Exam-style
Explain why the first ionisation energy of Nitrogen (N) is higher than that of Phosphorus (P).
⚠️ Common errors
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🎯 Exam question
Key ideas — Deducing configuration and periodic position from successive ionisation energies
A graph of log_10(ionisation energy) against the number of electrons removed shows a series of steadily rising values within each shell, separated by sharp jumps between shells. Counting the number of electrons removed before each jump reveals the number of electrons in each shell, and hence the electronic configuration; the total number of electrons removed before the final, largest jump (removing an electron from the innermost, n=1 shell) also gives the total electron count. The number of electrons in the outermost group (before the first large jump) corresponds directly to the Group number of the element in the Periodic Table, since Group number equals the number of outer-shell electrons.
🧑🏫 Teacher's explanation
Imagine you are pulling a group of energetic toddlers away from a parent in a crowded playground. The first few toddlers are easy to pull away because they are only loosely holding hands. However, once you have removed all the 'outer' toddlers, the next one you try to pull away is being held much more tightly by the parent (the nucleus) because they are in a much closer, more stable group.
📖 Definitions
🔍 Worked example — Easy
An element X has the following first four ionisation energies (in kJ\ mol^-1): 502, 746, 5775, 9530. Determine the group of element X.
🔍 Worked example — Standard
Calculate the mass of magnesium atoms that would require a total of 2500\ kJ\ mol^-1 to remove their first two electrons, given the first ionisation energy is 738\ kJ\ mol^-1 and the second is 1450\ kJ\ mol^-1.
🔍 Worked example — Exam-style
An element has the following successive ionisation energies (kJ\ mol^-1): IE_1=1050, IE_2=2020, IE_3=15300, IE_4=18200. Deduce the electronic configuration of the ground state atom.
⚠️ Common errors
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🎯 Exam question
IE_1 = 738\ kJ\ mol^-1 IE_2 = 1450\ kJ\ mol^-1 IE_3 = 7733\ kJ\ mol^-1 IE_4 = 10540\ kJ\ mol^-1
Worked example
Question. An element Y has successive ionisation energies (in kJ mol⁻¹) of: 590, 1145, 4900, 6500, 8100. There is a large jump between the second and third ionisation energies. (a) Deduce the number of electrons in the outer shell of Y, and hence its Group in the Periodic Table. [2] (b) Explain, in terms of shielding and nuclear attraction, why there is a large jump between the second and third ionisation energies. [2]
🔍 Worked example — Easy
Calculate the first ionisation energy of Magnesium (Mg) in kJ\ mol^-1 if the energy required to remove one electron from one atom of Mg is 739\ kJ/mol.
🔍 Worked example — Standard
An element X has the following successive ionisation energies (in kJ\ mol^-1): 801, 1519, 7923, 10500. Determine the group of element X.
🔍 Worked example — Exam-style
An element Z has successive ionisation energies (kJ\ mol^-1): 702, 1410, 2300, 12000, 15000. Deduce the electronic configuration of the element Z (assume it is in the second period).
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Practice
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