OAE Chemistry key terms

A cyclical process of observation, hypothesis formation, controlled experimentation, and revision of explanations based on evidence; conclusions are provisional and open to further testing.

A testable, falsifiable proposed explanation for an observation, stated precisely enough that experiments could show it to be wrong.

A factor deliberately held constant across experimental trials so that any measured change can be attributed to the manipulated variable rather than to incidental differences.

The digits in a measured value that carry meaningful information about precision, including all certain digits plus one estimated digit; rules for rounding and for multiplication versus addition differ.

A problem-solving technique in which unit labels are treated algebraically so unwanted units cancel, converting between quantities and checking that a setup is plausible.

The absolute difference between an experimental result and an accepted value, divided by the accepted value and multiplied by 100, expressing accuracy as a percentage.

Evaluation of a study's methods and claims by independent experts before publication, intended to catch flaws in design, analysis, or interpretation.

In a closed system, matter is neither created nor destroyed during a chemical change; total mass of reactants equals total mass of products, which justifies balancing equations.

The smallest unit of an element that retains that element's chemical identity, composed of a dense nucleus of protons and neutrons surrounded by electrons.

The number of protons in an atom's nucleus, which uniquely identifies the element and equals the number of electrons in a neutral atom.

The sum of protons and neutrons in one atom's nucleus; it distinguishes isotopes of the same element and is written as a superscript beside the symbol.

A form of an element with the same proton count but a different neutron count, so atoms differ in mass while sharing nearly identical chemical behavior.

The weighted mean mass of an element's naturally occurring isotopes, each weighted by its fractional abundance; this is the value shown on the periodic table.

An atom or group of atoms that has gained or lost electrons, giving it a net electrical charge; cations are positive and anions are negative.

An electron in the outermost occupied shell of an atom; these electrons are chiefly responsible for bonding and for periodic trends in reactivity.

The modern description of electrons in atoms as probability distributions called orbitals, defined by quantum numbers rather than fixed circular paths.

A region around the nucleus where an electron is most likely to be found, holding at most two electrons of opposite spin; s, p, d, and f types differ in shape.

The arrangement of an atom's electrons among orbitals, written in order of increasing energy and governed by the Aufbau principle, Pauli exclusion principle, and Hund's rule.

A regular pattern in properties such as atomic radius, ionization energy, and electronegativity that repeats across periods and varies predictably down groups.

Electrostatic attraction between oppositely charged ions, typically formed when a metal transfers electrons to a nonmetal; the resulting solid forms a crystal lattice.

A bond formed when two atoms share one or more pairs of electrons, usually between nonmetals; shared pairs may be polar or nonpolar depending on electronegativity difference.

A measure of how strongly an atom attracts shared electrons in a bond; larger differences between bonded atoms produce more polar bonds and favor ionic character.

Unequal sharing of bonding electrons caused by differing electronegativities, creating partial positive and partial negative ends on the bond.

A diagram showing atoms, bonding electron pairs as lines, and lone pairs as dots, used to predict molecular shape and bond order from valence electrons.

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

The three-dimensional arrangement of atoms in a molecule, named by shapes such as linear, trigonal planar, tetrahedral, and bent, and determined largely by electron-domain repulsion.

A way of describing delocalized bonding in which several valid Lewis structures contribute to one real structure whose bond lengths lie between those drawn.

A quantitative measure of molecular polarity arising from unequal charge distribution; it depends on both bond polarities and molecular geometry.

An attraction between separate molecules, such as hydrogen bonding, dipole-dipole interaction, or London dispersion forces; these forces govern boiling point and solubility.

A relatively strong intermolecular attraction between a hydrogen bonded to N, O, or F and a lone pair on a nearby electronegative atom; it raises boiling points notably.

The amount of substance containing Avogadro's number (6.022 x 10^23) of representative particles; it links mass in grams to particle count and to gas volume.

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

Calculation of quantitative relationships among reactants and products using mole ratios from a balanced equation, often to find limiting reagent or expected product amount.

A chemical equation in which coefficients give equal numbers of each kind of atom on both sides, satisfying conservation of mass and providing mole ratios.

The reactant that is completely consumed first and therefore determines the maximum amount of product that can form; other reactants are in excess.

Actual product mass divided by theoretical product mass, multiplied by 100; it measures how completely a reaction proceeded relative to prediction.

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

The overall enthalpy change of a reaction is the sum of enthalpy changes for any sequence of steps into which the reaction can be divided, regardless of pathway.

The minimum energy colliding particles must possess for a reaction to occur; it corresponds to the energy barrier between reactants and the transition state.

A substance that increases reaction rate by providing a lower-energy pathway, without being consumed or changing the overall enthalpy change.

The change in concentration of a reactant or product per unit time; it depends on concentration, temperature, surface area, and the presence of a catalyst.

An equation relating reaction rate to reactant concentrations raised to experimentally determined orders, which are not necessarily the equation coefficients.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance.

The ratio of product concentrations to reactant concentrations, each raised to its balanced-equation coefficient, at equilibrium; its value depends on temperature.

Concentration expressed as moles of solute per liter of solution; because volume changes with temperature, molarity is temperature dependent.

Adding solvent to a solution to lower its concentration; the moles of solute stay constant, so initial molarity times initial volume equals final molarity times final volume.

The negative base-10 logarithm of hydrogen ion concentration; values below 7 are acidic, 7 is neutral, and above 7 is basic at standard conditions.

The negative base-10 logarithm of hydroxide ion concentration; at 25 degrees Celsius, pH plus pOH equals 14 in aqueous solution.

An acid that ionizes essentially completely in water, producing hydronium ions and a very low pH; examples include hydrochloric and nitric acids.

An acid that ionizes only partially in water, establishing an equilibrium described by an acid dissociation constant; its solutions have higher pH than equal concentrations of a strong acid.

A procedure that adds a solution of known concentration from a buret until a reaction with the analyte is just complete, allowing calculation of the unknown concentration.

A solution of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists large pH changes when small amounts of acid or base are added.