Key Terms – Chapter 02

Allotropic modifications (allotropes) Different forms of the same element in the same physical state.

Anhydrous Without water.

Anion An ion with a negative charge.

Atom The smallest particle of an element that maintains its chemical identity through all chemical and physical changes.

Atomic mass unit (amu) One twelfth of the mass of an atom of the carbon-12 isotope; a unit used for stating atomic and formula weights.

Atomic number The number of protons in the nucleus of an atom.

Atomic weight Weighted average of the masses of the constituent isotopes of an element; the relative mass of atoms of different elements.

Avogadro’s number 6.022´1023 units of a specified item. See Mole.

Cation An ion with a positive charge.

Chemical formula Combination of symbols that indicates the chemical composition of a substance.

Composition stoichiometry Describes the quantitative (mass) relationships among elements in compounds. Empirical formula See Simplest formula.

Formula Combination of symbols that indicates the chemical composition of a substance.

Formula unit The smallest repeating unit of a substance—for non-ionic substances, the molecule.

Formula weight The mass, in atomic mass units, of one formula unit of a substance. Numerically equal to the mass, in grams, of one mole of the substance (see Molar mass). This number is obtained by adding the atomic weights of the atoms specified in the formula.

Hydrate A crystalline sample that contains water, H2O, and another compound in a fixed mole ratio. Examples include CuSO4.5H2O and (COOH)2.2H2O.

Ion An atom or group of atoms that carries an electric charge. A positive ion is a cation; a negative ion is an anion.

Ionic compound A compound that is composed of cations and anions. An example is sodium chloride, NaCl.

Law of Constant Composition See Law of Definite Proportions.

Law of Definite Proportions Different samples of a pure compound always contain the same elements in the same proportions by mass; this corresponds to atoms of these elements in fixed numerical ratios. Also known as the Law of Constant Composition.

Law of Multiple Proportions When two elements, A and B, form more than one compound, the ratio of the masses of element B that combine with a given mass of element A in each of the compounds can be expressed by small whole numbers.

Molar mass The mass of substance in one mole of the substance; numerically equal to the formula weight of the substance. See Formula weight; see Molecular weight.

Mole 6.022´1023 (Avogadro’s number of) formula units (or molecules, for a molecular substance) of the substance under discussion. The mass of one mole, in grams, is numerically equal to the formula (molecular) weight of the substance.

Molecular formula A formula that indicates the actual number of atoms present in a molecule of a molecular substance. Compare with Simplest formula.

Molecular weight The mass, in atomic mass units, of one molecule of a nonionic (molecular) substance. Numerically equal to the mass, in grams, of one mole of such a substance. This number is obtained by adding the atomic weights of the atoms specified in the formula.

Molecule The smallest particle of an element or compound that can have a stable independent existence.

Percent composition The mass percentage of each element in a compound.

Percent purity The mass percentage of a specified compound or element in an impure sample.

Polyatomic Consisting of more than one atom. Elements such as Cl2, P4, and S8 exist as polyatomic molecules. Examples of polyatomic ions are the ammonium ion, NH4+, and the sulfate ion, SO42-.

Simplest formula The smallest whole-number ratio of atoms present in a compound; also called empirical formula. Compare with Molecular formula.

Stoichiometry Description of the quantitative relationships among elements in compounds (composition stoichiometry) and among substances as they undergo chemical changes (reaction stoichiometry).

Structural formula A representation that shows how atoms are connected in a compound.