CLEP Chemistry key terms

The number of protons in the nucleus of an atom, which uniquely identifies the element and determines its position on the periodic table.

The total count of protons plus neutrons in one atom's nucleus, written as a whole number after the element name or symbol.

One of two or more forms of the same element whose atoms have equal proton counts but different neutron counts, and therefore different masses.

The electrons in the outermost occupied shell of an atom, which are the ones most involved in bonding and in determining chemical reactivity.

The arrangement of an atom's electrons among its orbitals and shells, usually written with shell numbers, subshell letters, and superscript electron counts.

A region of space around a nucleus where an electron is most likely to be found; each orbital holds a maximum of two electrons with opposite spins.

The rule that no two electrons in the same atom can share all four quantum numbers, so an orbital holds at most two electrons of opposite spin.

The guideline that electrons fill degenerate orbitals singly, with parallel spins, before any pairing occurs, minimizing electron repulsion within a subshell.

The minimum energy required to remove one electron from a neutral gaseous atom, giving a positive ion; it generally increases across a period.

A relative measure of how strongly an atom attracts shared electrons in a bond; values rise across a period and fall down a group.

A covalent bond in which the shared electrons are pulled unequally toward the more electronegative atom, creating partial positive and partial negative charges.

An unusually strong intermolecular attraction between a hydrogen atom bonded to N, O, or F and a lone pair on a nearby electronegative atom.

The attractions between separate molecules or ions, such as dispersion, dipole, and hydrogen-bonding forces, which influence boiling point, melting point, and solubility.

The relationship PV = nRT linking pressure, volume, amount in moles, and absolute temperature of a gas, valid when molecules are far apart and interact negligibly.

The pressure a single gas in a mixture would exert if it alone occupied the entire container; the total pressure equals the sum of all partial pressures.

The ratio of the number of moles of one component to the total number of moles in a mixture, a dimensionless quantity that sums to one.

The pressure exerted by vapor in equilibrium with its liquid or solid at a given temperature; it rises with temperature and with weaker intermolecular attractions.

The temperature at which a liquid's vapor pressure equals the surrounding external pressure, so bubbles of vapor can form and rise through the liquid.

A concentration unit giving the number of moles of solute dissolved per liter of solution, commonly abbreviated M.

The process of adding solvent to a solution to lower its concentration, described by the relationship that initial molarity times initial volume equals final molarity times final volume.

The reactant that is completely consumed first in a chemical reaction, thereby fixing the maximum amount of product that can form.

The maximum mass or amount of product predicted from stoichiometry when the limiting reactant is fully converted, assuming a perfectly complete reaction.

The actual experimental mass of product divided by the theoretical yield, multiplied by 100 to express the result as a percentage.

The amount of substance containing 6.022 x 10^23 representative particles, a counting unit that connects masses on the atomic scale to laboratory quantities.

The mass in grams of one mole of a substance, numerically equal to its formula mass in atomic mass units.

The loss of electrons by a species during a redox reaction, accompanied by an increase in its oxidation number.

The gain of electrons by a species during a redox reaction, accompanied by a decrease in its oxidation number.

The reactant that accepts electrons and is itself reduced, causing another substance in the reaction to be oxidized.

The reactant that donates electrons and is itself oxidized, causing another substance in the reaction to be reduced.

A substance that donates a proton to another species, or accepts an electron pair, producing hydronium ions when dissolved in water.

A substance that accepts a proton from another species, or donates an electron pair, producing hydroxide ions when dissolved in water.

The reaction of an acid with a base to form water and a salt, in which hydrogen ions and hydroxide ions combine.

A logarithmic scale of hydrogen ion activity, defined as the negative base-10 logarithm of the hydrogen ion concentration; values below 7 are acidic.

A solution that resists large changes in pH when small amounts of strong acid or base are added, typically containing a weak acid and its conjugate base.

The ratio of product concentrations to reactant concentrations, each raised to its coefficient, for a reaction at equilibrium at a specified temperature.

The rule that a system at equilibrium shifts to partly offset a change in concentration, pressure, or temperature imposed on it.

The same ratio of concentrations used for the equilibrium constant but computed at any moment; comparing it with the equilibrium constant predicts the direction of net change.

The minimum extra energy that colliding particles must possess for a reaction to occur, corresponding to the height of the energy barrier along the reaction path.

A substance that speeds up a reaction by providing an alternative pathway with lower activation energy while being regenerated and not consumed overall.

An equation expressing reaction rate as a constant times reactant concentrations raised to experimentally determined powers called reaction orders.

A thermodynamic quantity representing the heat content of a system at constant pressure; its change equals the heat absorbed or released in a process.

The principle that the enthalpy change of an overall reaction equals the sum of the enthalpy changes of any sequence of steps adding up to that reaction.

The enthalpy change when one mole of a compound forms from its elements in their standard states, measured under defined standard conditions.

A quantitative procedure in which a solution of known concentration is added gradually to a measured volume of unknown concentration until an indicator signals the equivalence point.

The stage in a titration at which the amount of titrant added is exactly stoichiometrically equivalent to the amount of substance being analyzed.

The digits in a measured value that carry meaningful information about its precision, including all certain digits plus one estimated digit.

A defined reference condition, commonly 0 degrees Celsius and 1 atmosphere, used to compare gas volumes and tabulate thermodynamic data.

The simplest whole-number ratio of atoms of each element in a compound, which may be a multiple of the actual molecular formula.