alexkich/iStock/GettyImages
Ionic compounds are built from ions, not molecules. Electrons are transferred from one atom to another, creating positively and negatively charged ions that attract each other through electrostatic forces. This electron transfer yields stable, charged particles that form the lattice of an ionic compound.
Atoms that possess loosely held outer electrons—such as hydrogen, sodium, and potassium—can donate these electrons to atoms that need them to complete their outer shells. Typical electron acceptors include halogens (chlorine, bromine) with seven electrons in their outermost shell, and oxygen or sulfur, each needing two electrons to achieve a stable configuration.
For instance, a sodium atom (Na) with one valence electron reacts with a chlorine atom (Cl) that requires one more electron. Sodium donates its electron, becoming a positively charged Na⁺ ion, while chlorine gains the electron, becoming a negatively charged Cl⁻ ion. The resulting electrostatic attraction between Na⁺ and Cl⁻ forms the ionic bond that constitutes sodium chloride (NaCl).
Similarly, two potassium atoms can donate a total of two electrons to a sulfur atom, yielding potassium sulfide (K₂S). In each case, the transfer of electrons ensures that both species achieve full outer shells, thereby stabilizing the compound.
Some molecules themselves become charged ions—known as polyatomic ions—and can participate in ionic bonding. A common example is the ammonium ion (NH₄⁺), which forms when nitrogen bonds covalently with four hydrogen atoms but carries an extra positive charge. When NH₄⁺ encounters the sulfide ion (S²⁻), they combine to form ammonium sulfide ((NH₄)₂S). The bond between NH₄⁺ and S²⁻ is ionic, while the bonds within the NH₄⁺ unit remain covalent.
Ionic compounds exhibit distinctive traits due to their charged constituents:
Understanding these characteristics aids chemists in predicting reactivity, solubility, and physical behavior of ionic materials.
