Ionic Compounds
Definition and meaning of Ionic Compounds in chemistry.
Ionic compounds are substances made of positively charged cations and negatively charged anions. Strong electrostatic attractions hold these ions together in a repeating three-dimensional pattern called a crystal lattice. Table salt is the most familiar example.
In more detail
Ionic compounds typically form when a metal atom transfers one or more electrons to a nonmetal atom. This creates a stable cation and a stable anion, each with a full outer electron shell. Polyatomic ions, like ammonium (NH4+) or sulfate (SO4 2-), can also pair with an oppositely charged ion to form ionic compounds.
In the solid state, ions stay locked in fixed positions within the lattice. Breaking apart this lattice takes a lot of energy, since many strong ionic bonds must be overcome at once. This is why ionic compounds tend to be hard and brittle, with high melting and boiling points.
Solid ionic compounds do not conduct electricity, because the charged ions cannot move freely. Once melted or dissolved in water, though, the ions separate and become mobile, letting the resulting liquid or solution carry an electric current. The strength of an ionic bond depends on the charges of the ions and the distance between them, a relationship described by Coulomb's law.
Ions with higher charges, or ions that pack closer together, form stronger bonds and generally higher melting points. Ionic compounds also tend to dissolve well in polar solvents like water. Polar water molecules can surround and separate individual ions from the crystal lattice.
Key facts
| Field | General Chemistry |
|---|---|
| Common example formula | NaCl (sodium chloride) |
| Bond type | Ionic (electrostatic attraction) |
| Typical formation | Metal + nonmetal, via electron transfer |
| Conducts electricity | Only when molten or dissolved |
Sodium chloride (NaCl) forms when a sodium atom transfers its single valence electron to a chlorine atom. This creates a Na+ ion and a Cl- ion. These oppositely charged ions pack into a cubic lattice, the crystal structure of ordinary table salt.
Frequently asked questions
Why do ionic compounds have such high melting points?
Their ions are held in a rigid lattice by strong electrostatic attractions in all directions. Melting requires enough energy to overcome many of these bonds at the same time.
Can ionic compounds conduct electricity?
Not as solids, since the ions are fixed in place. Once molten or dissolved in water, the ions can move freely, allowing the liquid or solution to conduct electric current.
Are ionic compounds always made of a metal and a nonmetal?
Usually, but not always. Most ionic compounds form between a metal and a nonmetal, but compounds containing polyatomic ions, such as ammonium nitrate, can be made entirely of nonmetal elements and still be ionic.