Water is H2O. Table salt is NaCl. Carbon dioxide is CO2. These are compounds, substances formed from two or more elements chemically combined in fixed proportions. The elements lose their individual properties when they form a compound. Sodium is a reactive metal that explodes in water. Chlorine is a toxic gas. Together, they form table salt, which is safe to eat. The compound is not a mixture. It is a new substance with its own properties.

Compounds are held together by chemical bonds. Ionic compounds form when metals transfer electrons to nonmetals. Covalent compounds form when nonmetals share electrons. The type of bond affects the properties. Ionic compounds have high melting points and conduct electricity when dissolved. Covalent compounds have lower melting points and do not conduct electricity. Some compounds, like acids and bases, react in characteristic ways.

Compounds are represented by chemical formulas. The formula tells you which elements are present and in what ratio. H2O means two hydrogen atoms for every oxygen atom. The formula is not just a label. It is a precise description of composition. The molecular formula gives the actual number of atoms in a molecule. The empirical formula gives the simplest whole-number ratio.

Compounds can be decomposed into their elements by chemical reactions. Electrolysis of water produces hydrogen and oxygen. Heating calcium carbonate produces calcium oxide and carbon dioxide. These reactions are the reverse of formation. They demonstrate that compounds are combinations, not mixtures.