Configurations of Simple Cations

Introduction

Neutral sodium is [Ne]3s¹, but Na⁺ is [Ne]. The sodium atom forms a positive ion by losing its outer electron, not by gaining a proton. The same counting method works for Mg²⁺, Al³⁺, K⁺ and Ca²⁺. Writing the starting and ending configurations makes the charge change and retained element identity clear.

Core explanation

Start with the neutral ground-state configuration. Sodium has eleven protons and eleven electrons: 1s² 2s² 2p⁶ 3s¹. Remove one electron from its outer occupied 3s state to obtain Na⁺ with ten electrons: 1s² 2s² 2p⁶, or [Ne]. Its nucleus still has eleven protons, so it remains sodium. The net charge is +11 − 10 = +1 in elementary-charge units.

Magnesium has [Ne]3s² and twelve electrons when neutral. Mg²⁺ has lost the two outer 3s electrons and has ten electrons, [Ne]. Aluminium, [Ne]3s² 3p¹, can form Al³⁺ by losing the 3p electron and two 3s electrons, also yielding ten electrons. These ions share a neon-like electron configuration but are not neon; they have 11, 12 and 13 protons, respectively. Their nuclear charges and sizes differ.

Potassium is [Ar]4s¹ and calcium is [Ar]4s² in neutral ground states. K⁺ loses one 4s electron to become [Ar], while Ca²⁺ loses two to become [Ar]. Again, identical electron configurations do not erase the different proton numbers. An ion and a noble-gas atom with the same number of electrons are isoelectronic, not identical in every chemical or physical property.

The charge superscript records how many electrons have been lost relative to a neutral atom when ordinary cations form. For Fe²⁺ and Fe³⁺, the counts are 24 and 23 electrons because iron has Z = 26, but transition-metal configuration details require care. In many transition-metal cations, 4s electrons are removed before 3d despite the simple neutral filling order. This page concentrates on main-group examples where the outer-electron removal pattern is straightforward. Do not automatically subtract from the last written term in a complex configuration without checking the atom and ion.

Formation of a cation costs energy to remove electron(s) from an isolated gaseous atom. A stable ionic compound forms only when the overall process, including lattice formation or solvation and other steps, is favourable. The attractive “noble-gas configuration” pattern is a helpful electron-count result, but it is not a complete energetic explanation. A student should not claim a free Na atom releases energy merely because one electron vanishes from it.

Mass number and neutron count remain unchanged in ordinary ion formation. If a sodium-23 atom forms Na⁺, its nucleus still has eleven protons and twelve neutrons; only electron count changes from eleven to ten. The actual ion's mass changes slightly because an electron is removed, but the integer A remains 23. This distinction is vital in mixed isotope-and-charge questions.

To check a proposed cation configuration, first sum its electrons. If someone writes Mg²⁺ as [Ne]3s², that still has twelve electrons and is neutral magnesium's configuration, not the 2+ ion's. If someone writes Na⁺ with eleven electrons but a positive sign, proton minus electron arithmetic reveals the inconsistency. Count before memorising.

Step-by-step reasoning

1. Write the neutral atom's configuration and proton number Z. 2. Subtract the positive charge magnitude from Z to get the ion's electron count. 3. Remove the relevant outer electrons for the stated simple main-group ion. 4. Verify the remaining configuration sums to the ion electron count and leaves Z unchanged.

Visual explanation

Draw Na [Ne]3s¹ → Na⁺ [Ne] + e⁻, with the 3s arrow disappearing from the orbital box while the eleven-proton nucleus remains. Beneath it draw Mg [Ne]3s² → Mg²⁺ [Ne] + 2e⁻.

Real-world analogy

A person can lose a coat without changing their identity. A sodium atom can lose an outer electron without changing its eleven-proton nucleus. The analogy is limited because electron loss has energetic and electrical consequences far beyond clothing.

Real-world example

In sodium chloride, sodium is present as Na⁺, not as neutral metallic Na atoms. Its ten-electron arrangement helps describe the ion, while the ionic lattice's attraction helps explain why the compound forms and has different properties from sodium metal.

Why?

Why are Na⁺, Mg²⁺ and Al³⁺ all written [Ne]? Each has ten electrons after the stated losses. Their nuclei have different proton counts, so equal electron configuration does not make them the same element or identical-sized ions.

Common misconception

“A cation becomes the noble gas whose electron arrangement it matches.” Na⁺ has a neon-like arrangement but remains sodium because its nucleus still has eleven protons, not neon's ten.

Worked example

Write the configuration of Ca²⁺. Neutral calcium has 20 electrons and [Ar]4s². Remove two outer 4s electrons to give 18 electrons, [Ar]. Check charge: 20 protons minus 18 electrons gives +2. If the isotope is calcium-40, its 20 neutrons and mass number 40 stay unchanged.

Quick check

1. What configuration results when neutral magnesium [Ne]3s² forms Mg²⁺? Answer: [Ne], because two outer 3s electrons are removed.

Exam focus

Show the neutral starting configuration, electron loss and final count. Keep protons and mass number fixed. Treat transition-metal cations separately rather than extending the simple 4s/3d filling mnemonic without care.

Advanced insight

Isoelectronic cations differ in ionic radius because greater nuclear charge attracts the same number of electrons more strongly, subject to electron interactions. Energetic stability of a salt also depends on environment, so electron configuration is only one part of an ion-formation explanation.

Summary

Simple main-group cations form by losing outer electrons. Na⁺, Mg²⁺ and Al³⁺ can all have ten-electron [Ne] configurations; K⁺ and Ca²⁺ can have eighteen-electron [Ar] configurations. Their different proton counts preserve element identity.

Practice questions

1. Write Na⁺ configuration from neutral [Ne]3s¹. Answer: [Ne], with ten electrons. 2. Write Al³⁺ configuration from neutral [Ne]3s² 3p¹. Answer: [Ne], after losing three outer electrons. 3. How many electrons and protons does Ca²⁺ have? Answer: Eighteen electrons and twenty protons. 4. Does forming K⁺ alter potassium's mass number? Answer: No. Its proton and neutron counts remain unchanged in ordinary ion formation.