CSET Science: 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 and used to distinguish isotopes of an element.

A form of an element whose atoms have the same number of protons but different numbers of neutrons, giving different masses and sometimes different stability.

The weighted average mass of an element's naturally occurring atoms, expressed in atomic mass units and dominated by the most abundant isotopes.

The amount of substance containing 6.022 x 10^23 representative particles, used to connect counted particles with measurable masses in the laboratory.

The value 6.022 x 10^23, the count of particles in one mole of any substance, allowing conversion between particle counts and moles.

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

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

The actual number of atoms of each element in one molecule of a compound, always a whole-number multiple of the empirical formula.

The mass percentage of each element in a compound, calculated by dividing that element's total mass in the formula by the molar mass.

The arrangement of electrons among an atom's orbitals, typically written using shell and subshell notation that reflects increasing orbital energy.

An electron in the outermost shell of an atom that participates in bonding and largely determines chemical properties and reactivity.

A region of space around a nucleus where an electron is most likely found, described by a wave function and holding at most two electrons.

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

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

The energy required to remove the most loosely held electron from a neutral gaseous atom, generally increasing across a period and decreasing down a group.

A measure of how strongly an atom attracts shared electrons in a bond, increasing across a period and decreasing down a group.

A measure of atomic size, usually half the distance between nuclei of bonded identical atoms; it decreases across a period and increases down a group.

The principle that element properties recur periodically when elements are arranged by increasing atomic number, underlying the layout of the periodic table.

The electrostatic attraction between oppositely charged ions formed when one atom transfers electrons to another, typically between metals and nonmetals.

A bond formed when two atoms share one or more pairs of electrons, typically between nonmetals and often directional in character.

A covalent bond in which electrons are shared unequally because the bonded atoms differ in electronegativity, producing partial charges.

A separation of positive and negative charge within a molecule or bond, often represented by an arrow pointing toward the more electronegative end.

A model predicting molecular shape by assuming electron pairs around a central atom repel one another and spread as far apart as possible.

The mixing of atomic orbitals on a central atom to form equivalent new orbitals that explain observed bond angles and molecular geometry.

A bond formed by head-on overlap of orbitals along the line between nuclei, allowing free rotation and providing the strongest part of a covalent connection.

A bond formed by side-by-side overlap of parallel p orbitals, creating electron density above and below the bond axis and preventing free rotation.

The description of a molecule or ion as a weighted blend of several valid Lewis structures, used when one structure cannot account for observed bonding.

A bookkeeping value comparing an atom's valence electrons with those it owns in a Lewis structure, used to judge the plausibility of structures.

The tendency of main-group atoms to gain, lose, or share electrons until surrounded by eight valence electrons, with many recognized exceptions.

An attractive force between separate molecules, such as hydrogen bonding or dispersion forces, that influences boiling point, solubility, and phase behavior.

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

The quantitative relationship among amounts of reactants and products in a balanced chemical equation, used to predict masses, moles, or volumes.

The reactant that is fully consumed first in a reaction and therefore determines the maximum amount of product that can form.

The actual product mass obtained divided by the theoretical maximum mass predicted from stoichiometry, multiplied by one hundred.

A concentration unit giving moles of solute per liter of solution, widely used to relate solution volume to the amount of dissolved substance.

The process of adding solvent to a solution to lower its concentration, described by the relationship that moles of solute stay constant.

A volumetric procedure in which a solution of known concentration is added until a reaction reaches its equivalence point, allowing unknown concentration to be calculated.

The point in a titration where the amount of titrant added exactly matches the stoichiometric amount needed to react with the analyte.

A logarithmic scale expressing hydrogen ion concentration as the negative base-ten logarithm, with values below seven indicating acidic solutions.

A solution containing a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists large pH changes on adding acid or base.

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

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

The minimum energy colliding particles need to convert reactants into products, which a catalyst lowers without changing the overall energy change.

A substance that speeds a reaction by providing a lower-energy pathway while being regenerated and not consumed in the overall process.

The heat content of a system at constant pressure; its change during a reaction determines whether the process is exothermic or endothermic.

The principle that the total enthalpy change of a reaction is the same regardless of the pathway, allowing sums of known steps to be used.

The loss of electrons by a species, accompanied by an increase in oxidation state and occurring together with reduction in a redox reaction.

A number assigned to an atom reflecting its electron ownership under agreed rules, used to track electron transfer in redox reactions.

The gain of electrons by a species, accompanied by a decrease in oxidation state and paired with oxidation in a redox reaction.

The separate oxidation or reduction portion of a redox reaction, written with electrons shown explicitly and used to balance complex equations.

A device that converts chemical energy into electrical energy, or the reverse, through redox reactions occurring at separate electrodes.

The spontaneous emission of particles or radiation from an unstable nucleus, changing the nucleus into a different nuclide over time.

The time required for half the nuclei in a radioactive sample to decay, a constant characteristic of each isotope.

The relationship PV = nRT linking pressure, volume, moles, and absolute temperature for a gas modeled as noninteracting particles.

A model describing gases as many small particles in constant random motion whose collisions with container walls produce pressure.

The substance dissolved in a solution, present in the lesser amount or the one whose phase change is described as dissolving.

The dissolving medium in a solution, usually the component present in the greatest amount and the one that determines the solution's phase.