GACE Chemistry (712) key terms

A systematic approach to investigation in which observations generate questions, hypotheses are proposed and tested through controlled experiments, and data are analyzed to support, refine, or reject explanations. Conclusions are provisional and open to revision when new evidence appears.

A testable, falsifiable proposed explanation for an observed phenomenon, stated specifically enough that predictions can be derived and checked against measurements. It differs from a guess because evidence could show it to be wrong.

An investigation in which all variables except the one being tested are held constant, so any change in the outcome can be attributed to the manipulated variable. A parallel control group receives no treatment or a standard treatment for comparison.

The factor deliberately changed or selected by the experimenter in an investigation. Its effect on the measured outcome is what the study is designed to reveal, and it is plotted on the horizontal axis of a graph.

The measured response in an experiment; its value depends on the independent variable. It is recorded as data and plotted on the vertical axis, and it must be defined operationally so results can be compared across trials.

The digits in a measured value that carry meaningful information about precision, including all certain digits plus one estimated digit. Results of calculations are rounded to match the least precise measurement used, preventing false claims of accuracy.

Precision describes how closely repeated measurements agree with one another, while accuracy describes how close a measurement is to the accepted true value. A set of readings can be tightly clustered yet consistently off target, indicating systematic error rather than random scatter.

A comparison of an experimental result with an accepted value, calculated as the absolute difference divided by the accepted value, multiplied by one hundred. It quantifies accuracy and is reported alongside units and appropriate significant figures.

The process by which independent experts evaluate a study's methods, data, and reasoning before publication. It screens for flaws and unsupported claims but does not guarantee correctness, so published findings remain subject to replication and challenge.

One of two or more forms of the same element whose atoms have identical proton counts but different neutron counts, giving different mass numbers. Isotopes share chemical behavior but differ in mass and sometimes in nuclear stability.

The number of protons in an atom's nucleus, which uniquely identifies the element and determines the number of electrons in a neutral atom. It fixes the element's position on the periodic table and its typical chemical behavior.

The total number of protons and neutrons in a specific nucleus, written as a superscript before the element symbol. It distinguishes isotopes of an element and is always a whole number, unlike average atomic mass.

The weighted mean mass of an element's naturally occurring isotopes, expressed in atomic mass units and weighted by relative abundance. It is the value shown on the periodic table and is usually not a whole number.

A description of how an atom's electrons occupy orbitals, following the Aufbau order, the Pauli exclusion principle, and Hund's rule. The arrangement, especially of valence electrons, explains chemical properties and periodic trends.

A region of space around a nucleus where an electron is most likely to be found, characterized by a shape (s, p, d, f) and an energy level. Each orbital holds at most two electrons with opposite spins.

The electrons in an atom's outermost occupied shell, which are chiefly responsible for bonding and chemical reactivity. Main-group elements tend to gain, lose, or share electrons to reach a stable noble-gas count.

The principle that when elements are arranged by increasing atomic number, their chemical and physical properties repeat periodically. This repetition reflects recurring valence electron configurations and underlies the structure of the periodic table.

A measure of how strongly an atom attracts shared electrons within a bond. Differences in electronegativity between bonded atoms predict bond polarity, and the general increase across a period and decrease down a group explains many trends.

The energy required to remove an electron from a gaseous atom or ion, always an endothermic quantity. It generally increases across a period and decreases down a group, and successive removals require progressively more energy.

The electrostatic attraction between oppositely charged ions formed when one atom transfers electrons to another, typically between a metal and a nonmetal. The resulting lattice gives high melting points and conductivity when molten or dissolved.

A bond formed when two atoms share one or more pairs of electrons, usually between nonmetals. The shared pair is attracted to both nuclei, and bond strength and length depend on how many pairs are shared.

A covalent bond in which shared electrons are pulled unequally toward the more electronegative atom, producing partial charges. The resulting dipole affects solubility, boiling point, and intermolecular forces.

The tendency of main-group atoms to gain, lose, or share electrons until surrounded by eight valence electrons, matching a noble-gas configuration. It predicts many formulas but has notable exceptions such as hydrogen and expanded octets.

A diagram showing valence electrons as dots and bonded pairs as lines to represent connectivity in a molecule or polyatomic ion. Formal charge and the octet rule guide which arrangement is most plausible.

A model predicting molecular shape by assuming electron domains around a central atom repel one another and spread as far apart as possible. Lone pairs occupy more space than bonding pairs, compressing bond angles.

A way of describing molecules whose bonding cannot be captured by a single Lewis structure, treating the true structure as a hybrid of several valid contributors. Delocalized electrons spread bond order evenly, as in carbonate and benzene.

An unusually strong intermolecular attraction between a hydrogen atom bonded to nitrogen, oxygen, or fluorine and a lone pair on a nearby electronegative atom. It raises boiling points and shapes proteins and nucleic acids.

Attractions between separate molecules, including London dispersion, dipole-dipole, and hydrogen bonding, that are weaker than covalent bonds. Their strength determines boiling point, viscosity, surface tension, and vapor pressure.

The amount of substance containing Avogadro's number (6.022 × 10²³) of representative particles. It bridges microscopic particle counts and macroscopic masses, allowing conversion among mass, volume of gas, and number of particles.

The mass in grams of one mole of a substance, numerically equal to its formula mass in atomic mass units. It is the conversion factor between mass and moles in stoichiometric calculations.

The quantitative relationship among reactants and products in a balanced chemical equation, based on mole ratios. It allows prediction of product amounts, reactant requirements, and percent yield.

The reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product that can form. Any other reactant is in excess and remains after the reaction stops.

The actual mass of product obtained divided by the theoretical maximum predicted from stoichiometry, multiplied by one hundred. Values below one hundred percent reflect side reactions, incomplete reaction, or loss during handling.

The heat content of a system at constant pressure; the change in enthalpy equals the heat absorbed or released during a reaction. Negative values indicate exothermic processes and positive values endothermic ones.

The principle that the overall enthalpy change of a reaction is the same regardless of the route taken, so it can be computed by summing enthalpy changes of steps. It allows indirect determination of values that are hard to measure directly.

The minimum energy colliding particles must possess for a reaction to occur, corresponding to the energy barrier between reactants and the transition state. Catalysts lower it without altering the overall enthalpy change.

An equation expressing reaction rate as a function of reactant concentrations raised to experimentally determined orders. Orders are not taken from the balanced equation but must be found by measurement.

A substance that increases reaction rate by providing an alternative pathway with lower activation energy, without being consumed overall. It shifts neither the equilibrium position nor the equilibrium constant.

When a stress such as a concentration change, pressure change, or temperature change is applied to a system at equilibrium, the system shifts in the direction that partially offsets the stress. Temperature changes also alter the equilibrium constant itself.

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient. Its magnitude indicates whether products or reactants are favored, and it varies with temperature.

The equilibrium constant for dissolution of a sparingly soluble ionic solid, equal to the product of the dissolved ion concentrations raised to their coefficients. Comparing the ion product with this value predicts whether a precipitate forms.

A logarithmic measure of hydrogen ion concentration, defined as the negative base-ten logarithm of that concentration. Values below seven are acidic, seven is neutral in water, and above seven is basic.

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 strong acid or base are added. Capacity is greatest near the acid's pKa.

A specific grouping of atoms within an organic molecule that determines its characteristic reactions and physical properties. Common examples include hydroxyl, carbonyl, carboxyl, amino, and ester groups.

One of two or more compounds with the same molecular formula but different arrangements of atoms, and therefore different properties. Structural isomers differ in connectivity, while stereoisomers differ only in spatial arrangement.

The property of a molecule that cannot be superimposed on its mirror image, usually due to a carbon bearing four different groups. Chiral molecules exist as enantiomers that rotate polarized light in opposite directions.

A large molecule built from many repeating small units called monomers joined by covalent bonds. Addition polymers form by opening double bonds, while condensation polymers form with the loss of a small molecule such as water.

A protein, or occasionally RNA, that catalyzes a biochemical reaction by binding substrates at an active site and lowering activation energy. Activity depends on temperature and pH and can be regulated by inhibitors.

A polymer of amino acids linked by peptide bonds that folds into a specific three-dimensional shape determining its function. Structure is described at primary, secondary, tertiary, and quaternary levels.

Warming of Earth's surface as atmospheric gases absorb and re-emit infrared radiation escaping from the planet. Human emissions of carbon dioxide and methane intensify this natural process and drive climate change.