Kaplan MCAT General Chemistry Review

Melting point The temperature at which the solid and liquid phases of a substance coexist in equilibrium.

Metal One of a class of elements located on the left side of the periodic table possessing low ionization energies and electronegativities. Metals readily give up electrons to form cations; they possess relatively high electrical conductivity and are lustrous and malleable.

Metallic bonding The type of bonding in which the valence electrons of metal atoms are delocalized throughout the metallic lattice.

Metalloid An element possessing properties intermediate between those of a metal and those of a nonmetal. Also called a semimetal.

Molality (m) A concentration unit equal to the number of moles of solute per kilogram of solvent.

Molarity (M) A concentration unit equal to the number of moles of solute per liter of solution.

Molar mass The mass in grams of 1 mole of an element or compound.

Mole (mol) One mole of a substance contains Avogadro’s number of molecules or atoms. The mass of 1 mole of substance in grams is the same as the mass of one molecule or atom in atomic mass units.

Mole fraction (X) A unit of concentration equal to the ratio of the number of moles of a particular component to the total number of moles for all species in the system.

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.