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 (l ) The second quantum number, denoting the sublevel or subshell in which an electron can be found. Reveals the shape of the orbital. This quantum number represents the orbital angular momentum of the motion of the electron about a point in space.
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 n > 2 to n = 2. These are four lines in the visible region of the spectrum.
Base A species that donates hydroxide ions or electrons or that accepts protons. See arrhenius base; basic solution; brønsted-lowry base; lewis base.
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 colligative property.
Bond energy The energy (enthalpy change) required to break a particular bond under given conditions.
Bonding orbital A molecular orbital formed by the overlap of two or more atomic orbitals whose energy is less than that of the original orbital. Compare antibonding orbital.
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.
Chemical bond The interaction between two atoms resulting from the overlap of electron orbitals, holding the two atoms together at a specific average distance from each other.
Chemical equation An expression used to describe the quantity and identity of the reactants and products of a reaction.
Chemical properties Those properties of a substance related to the chemical changes that it undergoes, such as ionization energy and electronegativity.
Closed system A system that can exchange energy but not matter with its surroundings.
Colligative properties Those properties of solutions that depend only on the number of solute particles present but not on the nature of those particles. See boiling point elevation; freezing point depression; vapor pressure depression.
Common ion effect A shift in the equilibrium of a solution due to the addition of ions of a species already present in the reaction mixture.
Compound A pure substance that can be decomposed to produce elements, other compounds, or both.
Concentration The amount of solute per unit of solvent (denoted by square brackets) or the relative amount of one component in a mixture.
Conjugate acid-base pair Brønsted-Lowry acid and base reacted by the transfer of a proton (e.g., H2CO3 and HCO3-).
Coordinate covalent bond A covalent bond in which both electrons of the bonding pair are donated by only one of the bonded atoms. Also called a dative bond.
Covalent bond A chemical bond formed by the sharing of an electron pair between two atoms. See coordinate covalent bond; nonpolar covalent bond; polar covalent bond.
Critical pressure The vapor pressure at the critical temperature of a given substance.
Critical temperature The highest temperature at which the liquid and vapor phases of a substance can coexist; above this temperature, the substance does not liquefy at any pressure.