Formulae and Equations

General Concepts

Chemical formulas are a shorthand way of describing a chemical compound using atomic symbols and numerical subscripts.
The molecular formula gives the exact number of atoms of each element in a molecule, while the empirical formula gives the simplest ratio of atoms in a compound.
Structural formulas show the arrangement of atoms in a molecule.

Balancing Equations

Chemical equations represent the changes that occur during a chemical reaction.
A balanced chemical equation obeys the law of conservation of mass, showing that the number of atoms of each element are the same on both sides of the equation.
Balancing equations involves adjusting the coefficients of the reactants and products until the number of atoms of each element is balanced.
Reactants are the substances that react in a chemical reaction and appear on the left side of the equation, while products are the substances that are formed and appear on the right side.

State Symbols in Equations

State symbols are used in chemical equations to indicate the physical state of each reactant and product: (s) for solid, (l) for liquid, (g) for gas, and (aq) for substances dissolved in water (aqueous).

Ions and Ionic Equations

An ion is an atom or group of atoms that has lost or gained electrons and so has an electric charge.
The charge on an ion is shown after the atomic symbol: e.g., Na⁺, Cl^-, OH^-.
Ionic equations show only the ions that change in a reaction. Substances not changed by the reaction, known as spectator ions, are not included.

Moles and Stoichiometry

The mole is a unit for amount of substance. One mole of a substance contains the same number of entities (such as atoms, molecules, or ions) as there are in 12 grams of carbon-12.
Stoichiometry is the study of the quantitative relationships in a chemical reaction, determining the proportions in which elements or compounds react.

Chemical Reactions

Chemical reactions may be categorized as combination, decomposition, displacement, or exchange reactions.
In combination reactions, two or more substances react to form one product.
In decomposition reactions, one substance breaks down into two or more substances.
In displacement reactions, one element is replaced by another in an compound.
In exchange reactions, the positive ions in the reacting compounds switch places.