In this topic you will learn about how atoms link together to form new substances.

Chemical changes produce substances that have different properties. The rusting of iron is a chemical change. When iron rusts, iron atoms link to oxygen atoms. Rust is an example of a compound, a chemical combination of two or more elements. A compound has its own properties, different from the substances it is made of.

When chemical changes occur, atoms link together in new ways. A link that atoms or atomic-sized particles can form with each other is called a chemical bond. Chemical bonds result from electrical attraction between atoms.

Sometimes atoms link together in a certain ratio. The ratio in which atoms are bonded together in a compound is shown by a chemical formula, a way of using letters and numbers to show how much of each element is in a substance.

Atoms of nonmetals attract extra electrons. Most metals have little attraction for extra electrons. When these two types of atoms come in contact, the nonmetallic atoms may be able to "take" electrons from the metallic atoms, resulting in the formation of ions. An ion is an electrically charged particle with unequal numbers of protons and electrons. Ions with opposite charges may bond together.

A group of bonded atoms that act like a single particle is called a molecule. The molecules of any given substance are always alike. Compounds made of ions are not made of molecules. Instead they are just collections of ions that are held together by opposite charges. Ammonia is made of molecules; salt is made of ions.

A way of describing how a substance changes chemically with other substances is called a chemical property. An element's chemical properties can be predicted from its position in the periodic table.

Chemical reactions give off or absorb heat. Chemical reactions that give off heat are said to be exothermic. The burning of acetylene gas is an example of an exothermic reaction. In this reaction, energy is released. Chemical reactions that absorb heat are said to be endothermic. Endothermic reactions require that a supply of energy be applied.