In chemistry, “ionic” describes a type of chemical bond formed when one atom transfers electrons to another, creating two oppositely charged particles (ions) that are held together by electrical attraction. This is one of the two main ways atoms bond to each other, the other being covalent bonding, where atoms share electrons instead. Ionic bonding typically occurs between metals and nonmetals, and it produces many of the crystalline solids you encounter every day, from table salt to baking soda.
How Ionic Bonds Form
Every atom has electrons orbiting in layers around its nucleus. The outermost layer, called the valence shell, is what determines how an atom bonds with others. Atoms are most stable when this outer shell is full, which for most elements means holding eight electrons.
Some atoms, particularly metals like sodium or calcium, have only one or two electrons in their outer shell. Rather than finding six or seven more to complete the set, it’s energetically easier for them to give those electrons away. Other atoms, particularly nonmetals like chlorine or oxygen, are just one or two electrons short of a full shell and readily accept them.
When a metal atom gives up electrons, it becomes positively charged (a cation). When a nonmetal accepts those electrons, it becomes negatively charged (an anion). These opposite charges attract each other strongly, and that electrostatic pull is the ionic bond. In sodium chloride (table salt), sodium gives one electron to chlorine. Sodium becomes Na⁺, chlorine becomes Cl⁻, and the two snap together.
What Makes a Bond Ionic vs. Covalent
The key factor is how unevenly the two atoms attract electrons, a property called electronegativity. When two atoms have very different electronegativities, the atom with stronger pull essentially takes electrons from the other, forming an ionic bond. When two atoms have similar electronegativities, neither can overpower the other, so they share electrons in a covalent bond.
There’s no sharp cutoff between the two types. Chemists sometimes use an electronegativity difference of about 1.7 or higher as a rough guideline for calling a bond ionic, but this is a simplification. Sodium chloride has an electronegativity difference of 2.2 and is clearly ionic. But some compounds with smaller differences, like manganese iodide (difference of 1.1), still behave as ionic compounds. In practice, most bonds fall on a spectrum between purely ionic and purely covalent, with many being “polar covalent,” a middle ground where electrons are shared unevenly but not fully transferred.
The Crystal Lattice Structure
Unlike covalent compounds, which exist as individual molecules, ionic compounds don’t pair off into neat two-atom units. Instead, each cation attracts every nearby anion, and each anion attracts every nearby cation, forming a vast three-dimensional grid called a crystal lattice. In this lattice, positive and negative ions alternate in a repeating pattern, with each ion surrounded by several ions of the opposite charge.
Because the anions are typically much larger than the cations, the anions form the main framework of the crystal, and the smaller cations fit into the gaps between them. The smallest repeating unit of this pattern is called a unit cell, and it tiles in every direction to build the visible crystal. This is why ionic compounds like salt form rigid, geometric shapes rather than soft or flexible structures.
Physical Properties of Ionic Compounds
The crystal lattice gives ionic compounds a distinctive set of physical properties that set them apart from covalent compounds.
- High melting and boiling points. Breaking apart a crystal lattice requires overcoming enormous numbers of simultaneous attractions between ions. Sodium chloride, for example, melts at about 800°C. Covalent compounds, by contrast, typically melt and boil at much lower temperatures.
- Solid at room temperature. Because of those strong attractions, ionic compounds are almost always hard, rigid solids. Covalent compounds are more often liquids or gases at room temperature.
- Brittle, not flexible. If you strike an ionic crystal, the layers of ions can shift so that like charges line up next to each other, causing the crystal to shatter rather than bend.
- Conduct electricity only when dissolved or melted. In solid form, the ions are locked in place and can’t move, so the compound doesn’t conduct a current. Dissolve it in water or melt it, however, and the ions are free to flow, carrying electrical charge.
What Determines Bond Strength
Not all ionic bonds are equally strong. Two factors matter most: the charge on the ions and their size. Ions with higher charges attract each other more powerfully. A bond between a 2+ and a 2- ion is significantly stronger than one between a 1+ and a 1- ion. Size matters too: smaller ions pack more closely together, which intensifies the electrostatic attraction. This is why magnesium oxide (small, doubly charged ions) has a melting point above 2,800°C, while sodium chloride (larger, singly charged ions) melts at a comparatively modest 800°C.
How Ionic Compounds Dissolve in Water
Water molecules are polar, meaning one end carries a slight positive charge and the other a slight negative charge. When you drop an ionic compound into water, the water molecules surround the ions in the crystal. The positive ends of water molecules cluster around the anions, and the negative ends cluster around the cations, gradually pulling them away from the lattice and into solution. Once dissolved, the ions float freely, which is why saltwater conducts electricity.
Not all ionic compounds dissolve easily, though. Chemists use a set of solubility rules to predict which ones will. Compounds containing sodium, potassium, or ammonium ions are nearly always soluble. Nitrate salts dissolve readily, as do most chloride, bromide, and iodide salts (with notable exceptions like silver chloride). On the other hand, most sulfides, oxides, and hydroxides are insoluble in water unless they contain one of those highly soluble metal ions.
How Ionic Compounds Are Named
Naming ionic compounds follows a straightforward system. The metal (cation) comes first, the nonmetal (anion) comes second, and the nonmetal’s name gets an “-ide” ending. Sodium and chlorine become sodium chloride. Calcium and oxygen become calcium oxide.
Some metals, especially transition metals like iron or copper, can form ions with different charges. In those cases, a Roman numeral in parentheses indicates the charge: iron(II) chloride means the iron has a 2+ charge, while iron(III) chloride means 3+. This distinction matters because the two compounds have completely different properties.
Many ionic compounds contain polyatomic ions, which are groups of atoms that carry a collective charge. Calcium carbonate, for instance, pairs a calcium ion (Ca²⁺) with a carbonate group (CO₃²⁻). When a formula needs more than one of the same polyatomic ion, parentheses are used: calcium nitrate is Ca(NO₃)₂, showing that two nitrate groups pair with one calcium ion. No Greek prefixes like “di-” or “tri-” are used in ionic naming, unlike the system for covalent compounds.
Ionic Compounds in Everyday Life
Ionic compounds are everywhere. Table salt (sodium chloride) is the most familiar, but calcium carbonate shows up in chalk, antacid tablets, and limestone buildings. Sodium bicarbonate is baking soda. Potassium chloride is used as a salt substitute for people watching their sodium intake. Calcium fluoride occurs naturally in minerals and is the source of fluoride added to drinking water. Even the plaster in your walls is likely made from calcium sulfate. These compounds’ high stability and predictable solubility make them essential in cooking, medicine, construction, and industry.