Molecular formula A formula showing the actual number and identity of all atoms in each molecule of a compound.
Molecular orbital The region of electron density in chemical bonding that results from the overlap of two or more atomic orbitals. See Antibonding orbital; Bonding orbital.
Molecular weight The sum of the atomic weights of all the atoms in a molecule.
Molecule The smallest polyatomic unit of an element or compound that exists with distinct chemical and physical properties.
Monoprotic acid An acid that can donate only one proton (e.g., HNO3). The molarity of a monoprotic acid solution is equal to its normality.
Nernst equation An equation that relates the voltage of an electrochemical cell to the concentrations of the reactant and products within that cell.
Net ionic equation A reaction equation showing only the species actually participating in the reaction.
Neutralization reaction A reaction between an acid and base in which H+ ions and OH- ions combine to produce water and a salt solution.
Neutral solution An aqueous solution in which the concentration of H+ and OH- ions are equal (pH = 7).
Neutron A subatomic particle contained within the nucleus of an atom. It carries no charge and has a mass very slightly larger than that of a proton.
Noble gases See inert gases.
Nonelectrolyte A compound that does not ionize in water.
Nonmetal One of a class of elements with high ionization potentials and very negative electron affinities that generally gain electrons to form anions. Nonmetals are located on the upper right side of the periodic table.
Nonpolar covalent bond A covalent bond between elements of the same electronegativity. There is no charge separation, and the atoms do not carry any partially positive or partially negative charge. Compare polar covalent bond.
Nonpolar molecule A molecule that exhibits no net separation of charge and, therefore, no net dipole moment.
Normality (N) A concentration unit equal to the number of gram equivalent weights of solute per liter of solution.
Nucleus The small central region of an atom; a dense, positively charged area containing protons and neutrons.
Octet Eight valence electrons in a subshell around a nucleus.
Octet rule A rule stating that bonded atoms tend to undergo reactions that will produce a complete octet of valence electrons. Applies without exception only to C, N, O, and F with zero or negative formal charges.
Open system A system that can exchange both energy and matter with its surroundings.
Orbital A region of electron density around an atom or molecule containing no more than two electrons of opposite spin. See atomic orbital; molecular orbital; paired electrons.
Order of reaction In a calculation of the rate law for a reaction, the sum of the exponents to which the concentrations of reactants must be raised.
Osmosis The movement of a solvent or solute through a semipermeable membrane across its concentration gradient (i.e., from a container in which the concentration is high to a container in which the concentration is low).
Osmotic pressure The pressure that must be applied to a solution to prevent the passage of a pure solvent through a semipermeable membrane across its concentration gradient.
Oxidation A reaction involving the net loss of electrons or, equivalently, an increase in oxidation number.
Oxidation number The number assigned to an atom in an ion or molecule that denotes its real or hypothetical charge. Atoms, alone or in molecules, of standard state elements have oxidation numbers of zero. Also called the oxidation state.
Oxidizing agent In a redox reaction, a species that gains electrons and is thereby reduced.
p subshell The subshells corresponding to the angular momentum quantum number l = 1, found in the second and higher principal energy levels. Each subshell contains three dumbbell-shaped p-orbitals oriented perpendicular to each other and referred to as the px, py, and pz orbitals.
Paired electrons Two electrons in the same orbital with assigned spins of +
Paramagnetism A property of a substance that contains unpaired electrons; it is attracted by a magnetic field.
Partial pressure The pressure that one component of a gaseous mixture would exert if it were alone in the container.
Pathway The specific sequence of events bringing a system from one state to another.
Pauli exclusion principle The principle stating that no two electrons within an atom may have an identical set of all four quantum numbers.
Percent composition The percentage of the total formula weight of a compound attributed to a given element.
Percent yield The percentage of the theoretical product yield that is actually recovered when a chemical reaction occurs.
Period A horizontal row of the periodic table containing elements with the same number of electron shells. Compare group.
Periodic law The law stating that the chemical properties of elements depend on the atomic number of the elements and change in a periodic fashion.
Periodic table The table displaying all known chemical elements arranged in rows (periods) and columns (groups) according to their electronic structure.
pH A measure of the hydrogen ion content of an aqueous solution, defined to be equal to the negative log of the H+ concentration.
Phase One of the three states of matter: solid, liquid, or gas. Compare State.
Phase diagram A plot, usually of pressure versus temperature, showing which phases a compound will exhibit under any set of conditions.
Phase equilibrium For a particular substance, any temperature and pressure at which two or three phases coexist in equilibrium. See triple point.
Photon A quantum of energy in the form of light with a value of Planck’s constant multiplied by the frequency of the light.