Successive Ionisation Energies

Introduction

Magnesium commonly forms Mg²⁺, but a third electron removal is far harder than the first two. Once its two outer 3s electrons are gone, the next electron would come from the filled inner [Ne] core. A sequence of ionisation energies can therefore reveal a valence-electron pattern.

Core explanation

Successive ionisation steps each begin with a gaseous species one charge more positive than the last. For magnesium, the first is Mg(g) → Mg⁺(g) + e⁻, the second Mg⁺(g) → Mg²⁺(g) + e⁻, and the third Mg²⁺(g) → Mg³⁺(g) + e⁻. Each step requires energy. The corresponding IE₁, IE₂ and IE₃ refer to these distinct state changes, not to three names for one total energy.

For a given element, later removals generally require more energy. After an electron is removed, the remaining ion is more positively charged, so the remaining electrons experience stronger net attraction. Electron-electron repulsion is reduced, and the ion's distribution can contract. Thus IE₂ is greater than IE₁, and IE₃ greater than IE₂ in the ordinary successive sequence. Exact values depend on configuration and cannot be obtained by simply multiplying IE₁.

A particularly large increase occurs when removal switches from outer valence electrons to an inner core. Magnesium's neutral configuration is [Ne]3s². The first two removals can take its 3s electrons, leaving Mg²⁺ with [Ne]. The third would remove a core n = 2 electron held much closer to the twelve-proton nucleus. The large IE₂-to-IE₃ jump supports two outer electrons for neutral magnesium. Sodium, [Ne]3s¹, shows a large jump between IE₁ and IE₂ because the second removal attacks the core.

To infer a group from tabulated energies, find the first dramatic jump, not simply the largest absolute number. If IE₁ and IE₂ are moderate relative to IE₃, the atom likely has two relatively accessible valence electrons in a simple main-group case. If the large jump follows IE₃, a three-valence-electron pattern may be plausible. The inference needs context: transition metals can have more complex valence structures, and “dramatic” should be assessed on an appropriate scale, often by ratios or a graph.

Successive energies are often listed per mole of ions for each step. Their sum gives the energy to remove multiple electrons from one mole of initial gaseous atoms in the sequential process, provided all steps are represented in a consistent molar convention. For example, energy to make X²⁺(g) and two electrons from X(g) equals IE₁ + IE₂. It is not IE₂ alone, because the first electron also had to be removed.

The existence of a large IE₃ does not mean Mg³⁺ can never appear under any high-energy condition. It means the third removal is very costly compared with formation of Mg²⁺. Ordinary compound chemistry often favours +2 magnesium because the surrounding energetics compensate more plausibly for two removals than for stripping a core electron. Do not turn an energy trend into an absolute logical impossibility.

An ionisation-energy table is evidence about electron arrangement, not a direct image of shells. The interpretation relies on a model connecting core proximity and attraction with removal energy. If observed data conflict with a naive shell diagram, revisit the configuration or the context rather than forcing the numbers into a memorised family rule.

Step-by-step reasoning

1. Write each Xⁿ⁺(g) → X⁽ⁿ⁺¹⁾⁺(g) + e⁻ equation correctly. 2. Plot or compare the successive positive energy values. 3. Locate the first unusually large increase after the outer electrons are gone. 4. Infer a plausible valence count and check it against the element's configuration.

Visual explanation

Draw three bars for magnesium IE₁, IE₂ and IE₃, with the third much taller. Beside them draw [Ne]3s² losing two 3s arrows and then a third arrow from an inner n = 2 box. The diagram links the jump to a change in the electron being removed.

Real-world analogy

Removing two books from an open desk is easier than removing the next one locked in a cabinet. The jump in effort resembles a valence-to-core change. Actual electrons are not stored in physical compartments, and ionisation energies arise from quantum states.

Real-world example

Chemical analysis of ionisation-energy sequences can support where an element belongs in a main group. A large jump after one removal is consistent with an alkali-metal outer ns¹ pattern, while a jump after two fits a group-two pattern.

Why?

Why is magnesium's third ionisation much harder than its second? After two 3s electrons are removed, the next electron comes from a more tightly held inner [Ne] core.

Common misconception

“The second ionisation energy means the total energy to remove two electrons.” IE₂ is only the second step, Mg⁺(g) → Mg²⁺(g) + e⁻. The two-step total is IE₁ + IE₂.

Worked example

An unknown main-group atom has successive energies in arbitrary equal units: 5, 11, 95 and 130. The first especially large jump is from 11 to 95, after two removals. Infer two relatively accessible outer electrons and a likely group-two-like ns² pattern, subject to the atom's identity and measurement context. The energies do not reveal its isotope neutron count.

Quick check

1. What energy sum is needed to form X²⁺(g) from X(g) by removing two electrons? Answer: The first and second ionisation energies added together, because both removal steps occur.

Exam focus

Write the initial charge for each step and avoid calling IE₂ the two-electron total. Locate the first major jump, connect it to core-electron removal and qualify group inference for complex elements.

Advanced insight

Successive energies encode electron relaxation as well as orbital occupancy. After one electron leaves, the remaining distribution reorganises, so the next energy is not just the previous orbital energy with one charge unit added.

Summary

Each successive ionisation removes one electron from a progressively more positive gaseous ion. Values rise, with a large jump often marking the switch from valence to core removal. The jump can reveal a simple main-group outer-electron count.

Practice questions

1. What species starts magnesium's third ionisation? Answer: Mg²⁺(g). 2. Where is sodium's major early ionisation jump expected? Answer: Between IE₁ and IE₂, after its single 3s valence electron is removed. 3. If a major jump follows IE₃, what simple valence count is suggested? Answer: Three relatively accessible valence electrons. 4. Is IE₁ + IE₂ equal to IE₂ alone? Answer: No; the sum includes both sequential removal energies.