Unit 1 · Atomic Structure and Properties

AP Chemistry Ions

What an ion is, how a cation differs from an anion, how to read and write ion symbols, how to write electron configurations for ions, and how ions behave in water, in four worked examples.

Ions show up in nearly every unit of AP Chemistry: in ionic compounds, in solutions, in acid-base reactions, and in electrochemistry. This page covers the vocabulary and the reasoning behind them. How an ion’s charge follows from its position on the periodic table, and how ions combine into compounds, are covered in AP Chemistry ionic bonds.

What an Ion Is

An ion is an atom or group of atoms with a net electric charge, because its number of electrons does not equal its number of protons. In a chemical change, the nucleus is untouched: the number of protons never changes, only the number of electrons. So:

Charge of an ion = number of protons − number of electrons

• Lose electrons: the charge becomes positive.
• Gain electrons: the charge becomes negative.
• Monatomic ions are single charged atoms, such as Na⁺ and O²⁻. Polyatomic ions are charged groups of covalently bonded atoms, such as SO₄²⁻ and NH₄⁺; their names and formulas are in AP Chemistry polyatomic ions.

Counting protons, neutrons, and electrons from a nuclear symbol works the same way for ions as for atoms; the method is in AP Chemistry isotope notation.

How a Cation Differs From an Anion

The size difference comes from Coulomb’s law. A main-group cation usually loses its whole outer shell, so the electrons left behind sit closer to the nucleus. An anion adds electrons that repel each other while the nuclear charge stays the same, so the electron cloud spreads out. Ranking ion sizes, including isoelectronic series, is worked through in AP Chemistry periodic trends. Naming rules for both kinds of ion are in AP Chemistry naming compounds.

How to Read and Write Ion Symbols

• Write the charge as a superscript after the symbol: number first, then the sign, as in Fe³⁺ and O²⁻.
• A charge of one leaves out the 1: Na⁺ and Cl⁻, not Na¹⁺.
• The charge belongs to the whole formula of a polyatomic ion: in CO₃²⁻, the 2− is shared by all four atoms.
• Subscripts count atoms and superscripts show charge. Do not mix them up: O₂ is an oxygen molecule, O²⁻ is an oxide ion.

How to Write Electron Configurations for Ions

Start from the neutral atom’s configuration, written as in how to write electron configurations, then add or remove electrons:

• Anions: add electrons to the next available subshell. Cl is [Ne]3s²3p⁵, so Cl⁻ is [Ne]3s²3p⁶, the same as argon.
• Main-group cations: remove the outermost electrons. Na is [Ne]3s¹, so Na⁺ is [Ne].
• Transition-metal cations: remove electrons from the shell with the highest n first. That means the 4s electrons leave before the 3d electrons, even though 4s filled first.

Ions that end up with the same configuration as a noble gas, such as N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, and Al³⁺ (all like neon), are called isoelectronic. Why sodium stops at +1 while magnesium goes to +2 shows up as a jump in successive ionization energies, explained in AP Chemistry ionization energy.

How Ions Behave in Water

When an ionic compound dissolves, its ions separate and each one is surrounded by water molecules. Water is polar, so it turns its partially negative oxygen end toward a cation and a partially positive hydrogen end toward an anion. This attraction is an ion-dipole force, one of the intermolecular forces.

Free-moving ions are what let a solution conduct electricity. A substance that produces ions in water is an electrolyte: ionic compounds and strong acids break apart completely, weak acids only slightly, and molecular substances such as sugar not at all. The dissolving process from the solvent’s side is covered in AP Chemistry solutions.

Worked Example 1: Identifying an Ion From Its Particle Counts

Identify each species and say whether it is a cation or an anion: (a) 26 protons and 23 electrons, (b) 34 protons and 36 electrons, (c) 7 protons and 10 electrons.

(a) 26 protons is iron. Charge = 26 − 23 = +3, so it is Fe³⁺, a cation named iron(III).
(b) 34 protons is selenium. Charge = 34 − 36 = −2, so it is Se²⁻, an anion named selenide.
(c) 7 protons is nitrogen. Charge = 7 − 10 = −3, so it is N³⁻, an anion named nitride.

The proton count fixes the element; the electron count only sets the charge. Never change the element because the electron count changed.

Worked Example 2: Writing Configurations for Iron and Sulfide Ions

Write the electron configurations of Fe²⁺, Fe³⁺, and S²⁻.

Fe atom: [Ar]4s²3d⁶.
Fe²⁺: remove the two 4s electrons first, giving [Ar]3d⁶.
Fe³⁺: remove one more electron, now from 3d, giving [Ar]3d⁵.
S atom: [Ne]3s²3p⁴. S²⁻: add two electrons to 3p, giving [Ne]3s²3p⁶, isoelectronic with argon.

Writing Fe²⁺ as [Ar]4s²3d⁴ is the classic error. The 4s electrons are in the outer shell, farther from the nucleus, so they are the first to go.

Worked Example 3: Drawing Water Around the Ions of KBr

Potassium bromide dissolves in water. Describe how the water molecules should be drawn around each ion, and say which ion, K⁺ or Mg²⁺, attracts water more strongly.

Around K⁺: the oxygen end of each water molecule (partially negative) points toward the cation, with the hydrogens pointing away.
Around Br⁻: one hydrogen of each water molecule (partially positive) points toward the anion.
K⁺ vs Mg²⁺: Mg²⁺ has twice the charge and a much smaller radius, so by Coulomb’s law it attracts the water dipoles more strongly.

Particle drawings of ions in water appear on the free-response section, and reversing the orientation of water is one of the most common ways to lose the point.

Worked Example 4: Ranking Solutions by Electrical Conductivity

Rank these 0.10 M solutions from highest to lowest electrical conductivity: CaCl₂, NaCl, HC₂H₃O₂ (acetic acid, K_a = 1.8 × 10⁻⁵), and C₆H₁₂O₆ (glucose).

CaCl₂: Ca²⁺ + 2Cl⁻, so 0.30 M of ions.
NaCl: Na⁺ + Cl⁻, so 0.20 M of ions.
Acetic acid: a weak acid. [H⁺] = √(1.8 × 10⁻⁵ × 0.10) = 1.3 × 10⁻³ M, so only about 0.0027 M of ions in total, about 1.3% ionized.
Glucose: a molecular compound that dissolves without forming ions, so it does not conduct.

Ranking: CaCl₂ > NaCl > HC₂H₃O₂ > C₆H₁₂O₆. More ions per liter, and more charge per ion, mean more charge carriers. The acetic acid calculation is explained in AP Chemistry weak acids.

Five Ion Practice Questions With Answers

1. Is K⁺ a cation or an anion, and how many electrons does it have?
   Show answer

   A cation. Potassium has 19 protons, so K⁺ has 19 − 1 = 18 electrons.
2. Write the symbol and name of the ion with 16 protons and 18 electrons.
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   S²⁻, the sulfide ion. 16 protons is sulfur, and 16 − 18 = −2.
3. Which is larger in each pair: Ca or Ca²⁺, and Cl or Cl⁻?
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   Ca is larger than Ca²⁺, because the cation has lost its outer 4s shell. Cl⁻ is larger than Cl, because the added electron increases repulsion with no extra protons.
4. Write the electron configuration of Zn²⁺.
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   Zn is [Ar]4s²3d¹⁰. Remove the two 4s electrons first: Zn²⁺ is [Ar]3d¹⁰.
5. In a particle drawing of NaF(aq), which end of the water molecules should point toward F⁻?
   Show answer

   A hydrogen end. The hydrogens carry a partial positive charge, which is attracted to the negative fluoride ion.

Common Ion Mistakes

• Changing the number of protons to make an ion. Only electrons are gained or lost.
• Getting the sign backward. Losing negative electrons makes an ion positive.
• Writing the charge as +3 or 3 instead of 3+. The number comes first, then the sign.
• Removing 3d electrons before 4s when writing transition-metal ion configurations.
• Pointing oxygen toward anions (or hydrogen toward cations) in particle drawings.
• Assuming everything that dissolves forms ions. Sugar and other molecular compounds dissolve as intact molecules.

Ions underpin questions in nearly every unit of the exam. To see how stronger answers on these questions could move your overall estimate, try the AP Chem Score Calculator.

Related Resources

• AP Chem Score Calculator
• Unit 1 Review: Atomic Structure and Properties
• AP Chemistry Valence Electrons
• How to Write Electron Configurations
• AP Chemistry Ionic Bonds
• AP Chemistry Polyatomic Ions
• AP Chemistry Naming Compounds
• AP Chemistry Periodic Trends
• AP Chemistry Solutions
• AP Chemistry Study Guide

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