1. a compound which releases hydrogen ions (H+) in solution (Arrhenius). 2. a compound which donates hydrogen ions in a chemical reaction (Bronsted-Lowry). 3. a compound that can accept a pair of electrons from a base (Lewis)..

addition compound
Two or more compounds can be packed in a definite ratio into a crystal. A dot is used to separate the compounds in the formula. For example, ZnSO4•7 H2O is an addition compound of zinc sulfate and water. This represents a compound, and not a mixture, because there is a definite 1:7 ratio of zinc sulfate to water in the compound. Hydrates are a common type of addition compound.

A series of organic compounds* with general formula CnH2n+2. Examples are propane* (with n=3) and octane* (with n=8).

base (also: alkali)
1. a compound that reacts with an acid* to form a salt*. 2. a compound that produces hydroxide ions in aqueous solution (Arrhenius). 3. a molecule or ion that captures hydrogen ions.(Bronsted-Lowry). 4. a molecule or ion that donates an electron pair to form a chemical bond.(Lewis).

chemical bond
A chemical bond is a strong attraction between two or more atoms. Bonds hold atoms in molecules* and crystals together. There are many types of chemical bonds, but all involve electrons which are either shared or transferred between the bonded atoms.
binary compound
A compound that contains two different elements.

covalent bond
A covalent bond is a very strong attraction between two or more atoms that are sharing their electrons. In structural formulas*, covalent bonds are represented by a line drawn between the symbols of the bonded atoms.

covalent compound
A compound made of molecules*- not ions. The atoms in the compound are bound together by shared electrons. Also called a molecular compound.

diatomic molecule
A molecule that contains only two atoms. All of the noninert gases occur as diatomic molecules; e. g. hydrogen, oxygen, nitrogen, fluorine, and chlorine are H2, O2, N2, F2, and Cl2, respectively.

empirical formula
Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts. For example, the empirical formula of glucose is CH2O, which means that for every mole of carbon in the compound, there are 2 moles of hydrogen and one mole of oxygen. Contrast with molecular formula*.

formula weight
The formula weight is the sum of the atomic weights* of the atoms in an empirical formula*. Formula weights are usually written in atomic mass units* (u). Compare with molecular weight*.

A hydrate is an addition compound* that contains water in weak chemical combination with another compound. For example, crystals of CuSO4•5 H2O (copper sulfate pentahydrate) are made of regularly repeating units, each containing 5 molecules of water weakly bound to a copper(II) ion and a sulfate ion.

Hydrocarbons are organic compounds that contain only hydrogen and carbon. The simplest hydrocarbons are the alkanes*.

inorganic compound
A compound that does not contain carbon chemically bound to hydrogen. Carbonates, bicarbonates, carbides, and carbon oxides are considered inorganic compounds, even though they contain carbon.

ionic compound
A compound made of distinguishable cations* and anions*, held together by electrostatic forces.

law of conservation of mass
There is no change in total mass during a chemical change. The demonstration of conservation of mass by Antoine Lavoisier in the late 18th century is was a milestone in the development of modern chemistry.

law of definite proportions
When two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen, the 'definite proportion' is 1 g of H for every 8 g of O.

law of multiple proportions
When one element can combine with another to form more than one compound, the mass ratios of the elements in the compounds are simple whole-number ratios of each other. For example, in CO and in CO2, the oxygen-to-carbon mass ratios are 16:12 and 32:12, respectively. The the second ratio is exactly twice the first, because there are exactly twice as many oxygens in CO2 per carbon as there are in CO.

molecular compound
A compound made of molecules*- not ions. The atoms in the compound are bound together by shared electrons. Also called a covalent compound.

molecular formula
A notation that indicates the type and number of atoms in a molecule. The molecular formula of glucose is C6H12O6, which indicates that a molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. Compare with empirical formula*.

molecular weight
The average mass of a molecule, calculated by summing the atomic weights* of atoms in the molecular formula*. Compare with formula weight*.

A molecule is the smallest particle of a substance that retains the chemical properties of the substance. Making or breaking chemical bonds* in a molecule changes it into a new molecule.

Flammable liquid compounds found in petroleum and natural gas. There are 18 different octanes- they have different structural formulas but share the molecular formula C8H18. Octane is used as a fuel and as a raw material for building more complex organic molecules. It is the eighth member of the alkane series.

organic compound
Compounds that contain carbon and hydrogen. They often contain other elements (particularly O, N, halogens, or S). Organic compounds were once thought to be produced only by living things. We now know that any organic compound can be synthesized in the laboratory (although this can be extremely difficult in practice!)
Organic chemistry is concerned with the compounds of carbon. Biochemistry is concerned with compounds of carbon that crawl. -Mike Adams

polar bond
A bond* involving electrons that are unequally shared. Polar bonds can be thought of as intermediate between the extremes represented by covalent bonds* and ionic bonds*.

polar molecule
An asymmetric molecule with polar bonds*. H2O, NH3, and HCl are examples of polar molecules. Non-examples are CO2, CCl4, and BCl3 which contain polar bonds but are nonpolar because they have symmetric shapes. Alkanes are usually asymmetric but are nonpolar because they contain no polar bonds.

polyatomic ion
A polyatomic ion is a charged particle that contains more than two covalently bound* atoms.

polyatomic molecules
A polyatomic molecule is a particle that contains more than two atoms.

A colorless, odorless, flammable gas, found in petroleum and natural gas. It is used as a fuel and as a raw material for building more complex organic molecules. Propane is the third member of the alkane series.

proton donor
Because a free H+ ion is technically a bare proton, acids* are sometimes referred to as "proton donors" because they release hydrogen ions in solution. The term "proton donor" is misleading, since in aqueous solution, the hydrogen ion is never a bare proton- it's covalently bound* to a water molecule as an H3O+ ion.

Stoichiometry is a branch of chemistry that quantitatively relates amounts of elements and compounds involved in chemical reactions.

structural formula
A structural formula is a diagram that shows how the atoms in a molecule are bonded together. Atoms are represented by their element symbols and covalent bonds* are represented by lines. The symbol for carbon is often not drawn. Most structural formulas don't show the actual shape of the molecule (just as a floor plan shows the actual shape of a house).