Atomic number The number of protons in a given element. See
Atomic orbital The square of the wavefunction of an electron. It describes the region of space where there is a high probability of finding the electron.
Atomic radius The radius of an atom. The average distance between a nucleus and the outermost electron. Usually measured as one-half the distance between two nuclei of an element in its elemental form.
Aufbau principle The principle that electrons fill energy levels in a given atom in order of increasing energy, completely filling one sublevel before beginning to fill the next.
Avogadro’s number The number of atoms of molecules in one mole of a substance: 6.022 × 1023.
Avogadro’s principle The law stating that under the same conditions of temperature and pressure, equal volumes of different gases will have the same number of molecules.
Azimuthal quantum number (
Balanced equation An equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge are the same for the reactants and the products.
Balmer series Part of the emission spectrum for hydrogen, representing transitions of an electron between energy levels
Base A species that donates hydroxide ions or electrons or that accepts protons. See
Base dissociation constant (Kb) The equilibrium constant that measures the degree of dissociation for a base under specific conditions. For a base BOH,
Basic solution An aqueous solution that contains more OH- ions than H+ ions. The pH of a basic solution is greater than 7.
Body-centered cubic A crystalline unit cell described as a cube with one atom at each corner and an additional atom in the center (or body) of the cube.
Bohr model The model of the hydrogen atom postulating that atoms are composed of electrons that assume certain circular orbits about a positive nucleus.
Boiling point The temperature at which the vapor pressure of a liquid is equal to the surrounding pressure. The normal boiling point of any liquid is defined as its boiling point at a pressure of 1 atmosphere.
Boiling point elevation The amount by which a given quantity of solute raises the boiling point of a liquid; a
Bond energy The energy (enthalpy change) required to break a particular bond under given conditions.
Bonding orbital A
Boyle’s law The law stating that at constant temperature, the volume of a gaseous sample is inversely proportional to its pressure.
Brønsted-Lowry acid Proton donor (e.g., H3PO4).
Brønsted-Lowry base Proton acceptor (e.g., OH-).
Buffer A solution containing a weak acid and its salt (or a weak base and its salt) that tends to resist changes in pH.
Buffer region The region of a titration curve in which the concentration of a conjugate acid is approximately equal to that of the corresponding base. The pH remains relatively constant when small amounts of H+ or OH- are added because of the combination of these ions with the buffer species already in solution.
Calorie (cal) A unit of thermal energy (1 cal = 4.184 J).
Calorimeter An apparatus used to measure the heat absorbed or released by a reaction.
Catalysis Increasing a reaction rate by adding a substance (the catalyst) not permanently changed by the reaction. The catalyst lowers the activation energy.
Catalyst A substance that increases the rates of the forward and reverse directions of a specific reaction but is itself left unchanged.
Cathode The electrode at which reduction takes place.
Cation An ionic species with a positive charge.
Celsius (°C) A temperature scale defined by having 0°C equal to the freezing point of water and 100°C equal to the boiling point of water; also the units of that scale. Otherwise known as the centigrade temperature scale. 0 × C = 273.15K.
Charles’ law The law stating that the volume of a gaseous sample at constant pressure is directly proportional to its absolute (Kelvin) temperature.
