Chemistry Glossary

The lowest possible temperature — 0 K (−273.15°C or −459.67°F) — at which all classical molecular motion ceases and a system has minimum possible energy.

A substance that donates hydrogen ions (H⁺/protons) in solution, has a pH below 7, tastes sour, and reacts with bases to form water and a salt.

The 15 elements from actinium (89) to lawrencium (103) — all radioactive metals, including uranium and plutonium, central to nuclear energy and weapons.

Group 1 elements (Li, Na, K, Rb, Cs, Fr) — the most reactive metals, soft enough to cut with a knife, that react violently with water.

Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra) — reactive metals with two valence electrons, essential for bones, construction, and fireworks.

A mixture of two or more chemical elements, at least one of which is a metal, engineered to have properties superior to those of its individual components.

The smallest unit of a chemical element that retains the element's properties, consisting of a nucleus of protons and neutrons surrounded by an electron cloud.

The weighted average mass of an element's atoms, measured in atomic mass units (amu), accounting for the natural abundance of all its isotopes.

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

A measure of the size of an atom, typically half the distance between two bonded nuclei, measured in picometers and following clear periodic trends.

A substance that accepts hydrogen ions (H⁺/protons) or donates hydroxide ions (OH⁻) in solution, has a pH above 7, feels slippery, and neutralizes acids.

The temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure, causing the liquid to transform into gas throughout its volume.

A substance that increases the rate of a chemical reaction without being permanently consumed or altered in the process.

A lasting attraction between atoms that allows the formation of molecules, crystals, and other chemical structures by sharing or transferring electrons.

A pure substance made entirely of one type of atom, defined by its number of protons, that cannot be broken down into simpler substances by chemical means.

A process in which one or more substances (reactants) are transformed into different substances (products) through the breaking and forming of chemical bonds.

A chemical bond formed when two atoms share one or more pairs of electrons, typically occurring between nonmetal elements.

The highly ordered, repeating three-dimensional arrangement of atoms, ions, or molecules within a crystalline solid.

The mass of a substance per unit volume, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).

A process that uses direct electric current to drive a non-spontaneous chemical reaction, decomposing a compound into its constituent elements or simpler substances.

A negatively charged subatomic particle that orbits the atomic nucleus in an electron cloud, determining an element's chemical behavior and bonding properties.

The energy change when a neutral atom in the gas phase gains an electron to form a negative ion — higher values mean the atom attracts electrons more strongly.

A section of the periodic table (s, p, d, or f block) defined by which type of electron subshell is being filled in the ground-state electron configuration.

The arrangement of electrons in an atom's orbitals, described by quantum numbers and organized into shells and subshells (s, p, d, f) that determine the element's chemical properties.

A measure of an atom's ability to attract shared electrons toward itself in a chemical bond, quantified most commonly using the Pauling scale.

A vertical column in the periodic table, numbered 1 through 18, containing elements with the same number of valence electrons and therefore similar chemical properties.

The time required for exactly half of the atoms in a sample of a radioactive isotope to undergo decay, transforming into a different element or isotope.

Group 17 elements (F, Cl, Br, I, At) — the most reactive nonmetals, known for forming salts and existing in all three states of matter at room temperature.

An atom or molecule that has gained or lost one or more electrons, giving it a net positive or negative electrical charge.

A chemical bond formed by the electrostatic attraction between oppositely charged ions, created when one atom transfers electrons to another.

The minimum energy required to remove the outermost electron from a neutral atom in its gas phase — a key indicator of how tightly an element holds its electrons.

Atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses.

The SI base unit of temperature, starting at absolute zero (0 K = −273.15°C) — used universally in science because it has no negative values.

The 15 elements from lanthanum (57) to lutetium (71) — rare earth metals with similar chemical properties, vital for magnets, lasers, and green technology.

The temperature at which a solid substance transitions to a liquid state at standard atmospheric pressure, with both phases coexisting in equilibrium.

A class of chemical elements characterized by high electrical and thermal conductivity, metallic luster, malleability, and a tendency to lose electrons and form positive ions.

A type of chemical bond in which a lattice of positive metal ions is held together by a shared 'sea' of delocalized electrons that move freely throughout the structure.

An element with properties intermediate between metals and nonmetals, typically a semiconductor — including silicon, germanium, boron, arsenic, antimony, and tellurium.

The mass of one mole (6.022 x 10²³ particles) of a substance, expressed in grams per mole (g/mol), serving as the bridge between atomic-scale and laboratory-scale measurements.

A group of two or more atoms bonded together, representing the smallest unit of a chemical compound that retains its chemical properties.

An electrically neutral subatomic particle found in the nucleus of atoms, with a mass slightly greater than a proton's, whose number determines the isotope of an element.

Group 18 elements (He, Ne, Ar, Kr, Xe, Rn) — extremely unreactive gases with completely filled electron shells, used in lighting, cryogenics, and diving.

A class of chemical elements that lack metallic properties — they are typically poor conductors, brittle in solid form, and tend to gain or share electrons in chemical reactions.

A nuclear reaction in which two light atomic nuclei merge to form a heavier nucleus, releasing enormous amounts of energy — the process that powers stars.

A mathematical function describing a region of space around an atom's nucleus where an electron has the highest probability of being found.

A hypothetical charge an atom would have if all its bonds were fully ionic, used as a bookkeeping tool to track electron transfer in chemical reactions.

A horizontal row in the periodic table (numbered 1 through 7) in which all elements share the same number of electron shells.

The principle stating that when elements are arranged by increasing atomic number, their physical and chemical properties recur in a regular, periodic pattern.

A logarithmic scale from 0 to 14 that measures the concentration of hydrogen ions (H⁺) in a solution, indicating how acidic (below 7), neutral (7), or basic (above 7) it is.

Metallic elements (Al, Ga, In, Sn, Tl, Pb, Bi) located between transition metals and metalloids — softer, with lower melting points and weaker metallic character.

A positively charged subatomic particle found in the nucleus of every atom, whose number defines the identity of the chemical element.

The spontaneous emission of particles or energy from unstable atomic nuclei as they transform into more stable configurations.

The amount of energy required to raise the temperature of one gram of a substance by one degree Celsius — a measure of how well a material stores thermal energy.

The physical form of a substance — solid, liquid, or gas — determined by temperature, pressure, and the strength of intermolecular forces.

A measure of how efficiently a material transfers heat, expressed in watts per meter-kelvin (W/m·K) — metals and diamond are the best conductors, while gases and aerogels are the worst.

Elements in Groups 3–12 with partially filled d-orbitals — they form colorful compounds, exhibit variable oxidation states, and include the most widely used metals.

The combining capacity of an atom, determined by the number of electrons it can lose, gain, or share to form chemical bonds with other atoms.