Mixtures are made up of unlike parts with a variable composition. Puresubstances are the same throughout and have a definite composition. Mixtures can be separated into their components by physical changes, changes that do not alter the identity of matter. Some pure substances can be broken down into simpler substances by a chemical change, a change that alters the identity of matter as it produces new substanceswith different properties. A pure substance that can be decomposed by chemical change into simpler substances with a definite composition is a compound. A pure substance that cannot be broken down into anything simpler is an element.

A chemical change produces new substances by making or breaking chemical bonds. The process of chemical change is called a chemicalreaction. During a chemical reaction different chemical substances with greater or lesser amounts of internal potential energy are produced. Chemical energy is the change of internal potential energy during a chemical reaction. A chemical equation is a shorthand way of describing a chemical reaction. An equation shows the substances that are changed, the reactants, on the left side, and the new substances produced, the products, on the right side.

Chemical reactions involve valence electrons, the electrons in the outermost energy level of an atom. Atoms tend to lose or acquire electrons to achieve the configuration of the noble gases with stable, filled outer orbitals. This tendency is generalized as the octet rule, that atoms lose or gain electrons to acquire the noble gas structure of eight electrons in the outer orbital. Atoms form negative or positive ions in the process.

A chemical bond is an attractive force that holds atoms together in a compound. Chemical bonds formed when atoms transfer electrons to become ions are ionic bonds.

An ionic bond is an electrostatic attraction between oppositely charged ions. Chemical bonds formed when ions share electrons are covalent bonds. Ionic bonds result in ionic compounds with a crystalline structure. The energy released when an ionic compound is formed is called the heat of formation. It is the same amount of energy that is required to decompose the compound into its elements.

A formula of a compound uses symbols to tell what elements are in a compound and in what proportions. Ions of representative elements have a single, fixed charge, but many transition elements have variable charges. Electrons are conserved when ionic compounds are formed, and the ionic compound is electrically neutral. A formula shows the overall balance of charges.

Covalent compounds are molecular, composed of electrically neutral groups of atoms bound together by covalent bonds. The sharing of a pair of electrons, with each atom contributing a single electron to the shared pair, forms a single covalent bond. Covalent bonds formed when two pairs of electrons are shared are called double bonds and a triplebond is the sharing of three pairs of electrons.

Compounds are named with different rules for ionic and covalent compounds. Both ionic and covalent compounds that are made up of only two different elements always end with an -ide suffix, but there are a few exceptions.

The rule for naming variable-charge ions states the English name and gives the charge with Roman numerals in parentheses. Ionic compounds are electrically neutral, and formulas must show a balance of charge. The crossover technique is an easy way to write formulas that show a balance of charge.

Covalent compounds are molecules of two or more nonmetal atoms held together by a covalent bond. The system for naming covalent compounds uses Greek prefixes to identify the numbers of atoms, since more than one compound can form from the same two elements (CO and CO₂, for example).

A concise way to describe a chemical reaction is to use formulas in a chemical equation. A chemical equation with the same number of each kind of atom on both sides is called a balanced equation. A balanced equation is in accord with the law of conservation of mass, which states that atoms are neither created nor destroyed in a chemical reaction. To balance a chemical equation, coefficients are placed in front of chemical formulas. Subscripts of formulas may not be changed since this would change the formula, meaning a different compound.

One important group of chemical reactions is called oxidationreductionreactions, or redox reactions for short. Redox reactions are reactions where shifts of electrons occur. The process of losing electrons is called oxidation, and the substance doing the losing is said to be oxidized. The process of gaining electrons is called reduction, and the substance doing the gaining is said to be reduced. Substances that take electrons from other substances are called oxidizing agents. Substances that supply electrons are called reducing agents.

Chemical reactions can also be classified as (1) combination, (2) decomposition, (3) replacement, or (4) ion exchange. The first three of these are redox reactions, but ion exchange is not.