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Writing Chemical Formulas for Ionic Compounds

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ionic formulas charge balance cations anions

Core Idea

Ionic formulas are determined by balancing positive (cation) and negative (anion) charges. The formula shows the simplest whole-number ratio of ions, with no net charge. Polyatomic ions are written as units. The cation is named first, followed by the anion. Understanding charge and valence is essential for writing correct formulas.

Explainer

You already understand that ionic bonds form when a metal transfers electrons to a nonmetal, producing a positively charged cation and a negatively charged anion. Writing the formula for the resulting compound is essentially a charge-balancing exercise: the total positive charge must exactly cancel the total negative charge so that the compound is electrically neutral.

The procedure is straightforward. First, identify the charges on each ion. Sodium is Na⁺, chloride is Cl⁻ — one of each gives NaCl with zero net charge. But consider calcium (Ca²⁺) and chloride (Cl⁻): one calcium ion carries +2, so you need two chloride ions at −1 each to balance. The formula is CaCl₂. A useful shortcut is the criss-cross method: take the magnitude of each ion's charge and use it as the subscript for the other ion. For Al³⁺ and O²⁻, the 3 becomes oxygen's subscript and the 2 becomes aluminum's, giving Al₂O₃. Always reduce to the simplest whole-number ratio — Mg²⁺ and O²⁻ would criss-cross to Mg₂O₂, but that simplifies to MgO.

Polyatomic ions — groups like sulfate (SO₄²⁻), nitrate (NO₃⁻), or ammonium (NH₄⁺) — are treated as single units. When you need more than one of a polyatomic ion, enclose it in parentheses before adding the subscript. Calcium nitrate is Ca(NO₃)₂, not CaNO₃₂, because the subscript 2 applies to the entire NO₃⁻ unit. Forgetting the parentheses changes the meaning of the formula entirely.

Two conventions complete the picture. The cation is always written first in the formula, regardless of how the compound is named verbally — so it is Na₂SO₄, never SO₄Na₂. And ionic formulas represent the simplest ratio of ions in the crystal lattice, not a discrete molecule. NaCl does not mean one sodium atom is bonded to one chlorine atom in isolation; it means the ratio of sodium to chloride in the extended crystal is 1:1. This distinction between formula units and molecules matters when you move into stoichiometry and solution chemistry.

Practice Questions 5 questions

Prerequisite Chain

Counting to 10 → Counting to 20 → Understanding Zero → The Number Zero → Counting to Five → One-to-One Correspondence → Combining Small Groups Within 5 → Addition Within 10 → Addition Within 20 → Two-Digit Addition Without Regrouping → Two-Digit Addition with Regrouping → Addition Within 100 → Repeated Addition as Multiplication → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Two-Digit by One-Digit Division → Division with Remainders → Remainders and Quotients in Division → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Reading and Writing Decimals → Comparing and Ordering Decimals → Adding and Subtracting Decimals → Multiplying Decimals → Dividing Decimals → Dividing Fractions → Mixed Number Arithmetic → Order of Operations → Integer Order of Operations → Variable Expressions → Combining Like Terms → One-Step Equations → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals over Rectangular Regions → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Green's Theorem → Surface Integrals and Flux of Vector Fields → Surface Integrals and Flux of Vector Fields → Divergence Theorem: Flux and Outflow → Divergence Theorem → Electric Flux → Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → The Electromagnetic Spectrum → Blackbody Radiation and Planck's Law → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → Heisenberg Uncertainty Principle → Wavefunction and the Born Rule → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Scattering Theory → Introduction to Scattering Theory → Partial Wave Analysis in Scattering → Spin Angular Momentum → Electron Spin and Intrinsic Magnetic Moment → Stern-Gerlach Experiment: Spin Quantization and Measurement → Electron Diffraction and Matter Wave Properties → Davisson-Germer Experiment: Crystal Diffraction of Electrons → Electron Diffraction and Matter Wave Interference → Wavefunctions and Probability Density Interpretation → Quantum Superposition and Linear Combinations of States → Quantum Operators and Observables → Canonical Commutation Relations and Uncertainty → Heisenberg Uncertainty Principle and Measurement Limits → Time-Independent Schrödinger Equation and Eigenvalues → Hydrogen Atom in Quantum Mechanics → Spectral Lines and Energy Transitions → Selection Rules for Atomic Transitions → LS and jj Coupling Schemes in Multi-Electron Atoms → Pauli Exclusion Principle and Antisymmetric Wavefunctions → Electron Configuration and the Aufbau Principle → The Periodic Table and Atomic Electronic Structure → The Periodic Table → Electron Configuration → Periodic Trends → Electron Affinity → Ionic Bonding: Electron Transfer and Electrostatic Forces → Writing Chemical Formulas for Ionic Compounds

Longest path: 152 steps · 709 total prerequisite topics

Prerequisites (1)

Ionic Bonding: Electron Transfer and Electrostatic Forceshard

Leads To (1)

Chemical Equations: Writing and Balancing Reactionssoft