A to Z chemistry dictionary

A to Z chemistry  Dictionary 

This alphabetical chemistry dictionary offers definitions and examples of important chemistry and chemical engineering terms. For each term, a brief definition is given. Each link leads to a more comprehensive discussion of the word.

A- Absolute Alcohol to Azimuthal Quantum Number

absolute alcohol - common name for high purity ethanol or ethyl alcohol.

absolute error - expression of the uncertainty or inaccuracy of a measurement.

absolute temperature - temperature measured using the Kelvin scale.

absolute uncertainty - the uncertainty of a scientific measurement, given in the same units as the measurement.

absolute zero - the lowest possible state at which matter can exist, 0 K or -273.15°C.

absorbance - measure of the amount of light absorbed by a sample.

absorption - process by which atoms, ions, or molecules enter a bulk phase.

absorption spectroscopy - technique used to determine concentration and structure of a sample based on which wavelengths of liquid are absorbed.

absorption spectrum - graph of amount of absorption as a function of wavelength.

absorptivity - absorption cross section of extinction coefficient, which is the absorbance of a solution per unit path length and concentration.

accuracy - the closeness of a measurement to a true or accepted value.

acid - a chemical species that accepts electrons or donate protons or hydrogen ions.

acid anhydride - a nonmetal oxide that reacts with water to form an acidic solution.

acid-base indicator - a weak acid or weak base that changes color when the concentration of hydrogen or hydroxide ions changes in an aqueous solution.

acid-base titration - a procedure to find the concentration of an acid or base by reacting a known concentration with the unknown until the equivalence point is reached.

acid dissociation constant - Ka - a quantitative measure of how strong an acid is.

acidic solution - an aqueous solution with a pH less than 7.0.

actinides - Usually, the actinides are considered to be elements 90 (thorium) through 103 (lawrencium). Otherwise, the actinides are defined according to their common properties.

actinium - the name for the element with atomic number 89 and is represented by the symbol Ac. It is a member of the actinide group.

activated complex - an intermediate state at the maximum energy point on the reaction path that occurs as reactants are being converted into product in a chemical reaction.

activation energy - Ea - the minimum amount of energy needed for a chemical reaction to occur.

active transport - the movement of molecules or ions from a region of lower concentration to higher concentration; requires energy

activity series - list of metals ranked in order of decreasing activity, used to predict which metals displace others in aqueous solutions.

actual yield - the quantity of product experimentally obtained from a chemical reaction.

acute health effect - the effect caused by initial exposure to a chemical.

acyl group - a functional group with the formula RCO- where R is bound to carbon via a single bond.

adsorption - the adhesion of a chemical species onto a surface

adulterant - a chemical that acts as a contaminant in the context of another substance's purity.

aether - a medium believed to carry light waves in the 18th and 19th century.

air - the mixture of gases that make up the Earth's atmosphere, consisting mainly of nitrogen, with oxygen, water vapor, argon, and carbon dioxide.

alchemy -Several definitions of alchemy exist. Originally, alchemy was an ancient tradition of sacred chemistry used to discern the spiritual and temporal nature of reality, its structure, laws, and functions.

alcohol - a substance that contains an -OH group attached to a hydrocarbon.

aliphatic amino acid - amino acid that has an aliphatic side chain.

aliphatic compound - an organic compound containing carbon and hydrogen joined into straight chains, branches chains, or non-aromatic rings.

aliphatic hydrocarbon - a hydrocarbon containing carbon and hydrogen joined into straight chains, branches chains, or non-aromatic rings.

alkali metal - any element found in group IA (first column) of the periodic table.

alkaline - an aqueous solution with a pH greater than 7.

alkalinity - a quantitative measure of a solution's ability to neutralize an acid.

alkene - a hydrocarbon containing a double carbon-carbon bond.

alkenyl group - the hydrocarbon group formed when a hydrogen atom is removed from an alkene group.

alkoxide - an organic functional group formed when a hydrogen atom is removed from the hydroxyl group of an alcohol when it is reacted with a metal.

alkoxy group - functional group containing an alkyl group bonded to oxygen.

allotrope - a form of an elemental substance.

alloy - substance made by melting together two or more elements, at least one of which must be a metal.

alpha decay - spontaneous radioactive decay which produces an alpha particle or helium nucleus.

alpha radiation - the ionizing radiation released from radioactive decay emitting an alpha particle.

aluminum or aluminium - the name for the element with atomic number 13 and is represented by the symbol Al. It is a member of the metal group.

amalgam - any alloy of mercury and one or more other metals.

americium - radioactive metal with element symbol Am and atomic number 95.

amide - functional group containing a carbonyl group linked to a nitrogen atom.

amine - compound in which one or more hydrogen atom in ammonia is replaced by an organic functional group.

amino acid - an organic acid containing a carboxyl (-COOH) and amine (-NH₂) functional group along with a side chain.

amorphous - term describing a solid that does not have crystalline structure.

amphiprotic - species that can both accept and donate a proton or hydrogen ion.

amphoteric - substance capable of acting as either an acid or a base.

amphoteric oxide - oxide that can act as either an acid or a base in a reaction to produce a salt and water.

amu - atomic mass unit or 1/12th the mass of an unbound atom of carbon-12.

analytical chemistry - chemistry discipline that studies the chemical composition of materials and tools used to examine them.

angstrom - unit of length equal to 10^-10meters.

angular momentum quantum number - ℓ, the quantum number associated with the angular momentum of an electron.

anhydrous - describes a substance that does not contain water or else is as concentrated as it can get.

anion - an ion with a negative electrical charge.

anode - electron where oxidation occurs; positive charged anode

antibonding orbital - molecular orbital with an electron outside the region between the two nuclei.

anti-Markovnikov addition - an addition reaction between an electrophilic compound HX and either an alkene or alkyne in which the hydrogen atom bonds to the carbon with the least number of hydrogen atoms and X bonds to the other carbon.

antimony - Antimony is the name for the element with atomic number 36 and is represented by the symbol Kr. It is a member of the metalloid group.

anti-periplanar - periplanar conformation where the dihedral atom between atoms is between 150° and 180°.

aqueous - describes a system containing water.

aqueous solution - a solution in which water is the solvent.

aqua regia - mixture of hydrochloric and nitric acids, capable of dissolving gold, platinum, and palladium.

argon - Argon is the name for the element with atomic number 18 and is represented by the symbol Ar. It is a member of the noble gases group.

aromatic compound - an organic molecule that contains a benzene ring.

Arrhenius acid - species that dissociates in water to form protons or hydrogen ions.

Arrhenius base - species that increases the number of hydroxide ions when added to water.

arsenic - metalloid with element symbol As and atomic number 33.

aryl - a functional group derived from a simple aromatic ring when one hydrogen is removed from the ring.

astatine - Astatine is the name for the element with atomic number 85 and is represented by the symbol At. It is a member of the halogen group.

atom - the defining unit of an element, which cannot be subdivided using chemical means.

atomic mass - average mass of atoms of an element.

atomic mass unit (amu) - 1/12th the mass of an unbound atom of carbon-12, used to represent atomic and molecular masses.

atomic number - the number of protons in the nucleus of an atom of an element.

atomic radius - value used to describe the size of an atom, usually half the distance between two atoms just touching each other.

atomic solid - solid in which atoms are bonded to other atoms of the same type.

atomic volume - volume occupied by one mole of an element at room temperature.

atomic weight - average mass of atoms of an element.

atmosphere - surrounding gases, such as the gases surrounding a planet that are held in place by gravity.

ATP - ATP is the acronym for the molecule adenosine triphosphate. 

Aufbau principle - idea that electrons are added to orbitals as protons are added to an atom.

austenite - the face-centered cubic crystalline form of iron.

Avogadro's Law - relation that states equal volumes of all gases contain the same number of molecules at the same pressure and temperature.

Avogadro's number - the number of particles in one mole of a substance; 6.0221 x 10²³

azeotrope - a solution that retains its chemical composition when distilled.

azimuthal quantum number - the quantum number associated with the angular momentum of an electron, determining the shape of its orbital.

B definitions - Background Radiation to Buffer

background radiation - radiation from external sources, typically from cosmic radiation and radioisotope decay.

back titration - titration in which the analyte concentration is determined by reacting it with a known quantity of excess reagent.

balanced equation - chemical equation in which the number and type of atoms and the electric charge is the same on both the reactant and product sides of the equation.

Balmer series - the part of the hydrogen emission spectrum for electron transitions n=2 and n>2, There are four lines in the visible spectrum.

barium - alkaline earth metal with element symbol Ba and atomic number 56.

barometer - instrument used to measure atmospheric pressure.

base - chemical species that either accepts protons or else donates electrons or hydroxide ions.

base anhydride (basic anhydride) - a metal oxide formed from the reaction between water and a basic solution.

base metal - any metal besides a precious or noble metal used for jewelry or in industry.

basic - alkaline or having a pH > 7.

basic solution - aqueous solution containing more hydroxide ions than hydrogen ions; solution with pH > 7.

Beer's law (Beer-Lambert Law) - law that states the concentration of a solution is directly proportional to its light absorbance.

berkelium - radioactive metal with element symbol Bk and atomic number 97.

beryllium - alkaline earth metal with element symbol Be and atomic number 4.

beta decay - type of radioactive decay that results in spontaneous emission of a beta particle.

beta particle - an electron or positron emitted during beta decay.

beta radiation - ionizing radiation from beta decay in the form of an energetic electron or positron.

binary acid - an acidic binary compound in which one element is hydrogen and the other element is another nonmetal.

binary compound - a compound made up of two elements (e.g., HF).

binding energy - energy needed to remove an electron from an atom or to separate a proton or neutron from the atomic nucleus.

biochemistry - Biochemistry is the chemistry of living things.

bismuth - Bismuth is the name for the element with atomic number 83 and is represented by the symbol Bi. It is a member of the metal group.

bitumen - natural mixture of polycyclic aromatic hydrocarbons (PAHs).

black light - a lamp that emits ultraviolet radiation or the invisible radiation emitted by it.

block copolymer - copolymer formed by repeating monomer subunits.

bohrium - transition metal with element symbol Bh and atomic number 107.

boiling - phase transition from the liquid to gas state.

boiling point - temperature at which a liquid's vapor pressure is equal to the external gas pressure.

boiling point elevation - the increase in a liquid boiling point caused by adding another compound to it.

bond - a chemical link formed between atoms in molecules and molecules and ions in crystals.

bond angle - the angle formed between two adjacent chemical bonds within the same atom.

bond-dissociation energy - energy required to homolytically break a chemical bond.

bond energy - quantity of energy needed to break one mole of molecules into component atoms.

bond enthalpy - enthalpy change resulting when one mole of bonds in a species are broken at 298 K.

bond length - the equilibrium distance between atomic nuclei or groups of nuclei that share a chemical bond.

bond order - a measure of the number of electrons involved in chemical bonds between two atoms in a molecule; usually equal to the number of bonds between the atoms.

boron - Boron is the name for the element with atomic number 5 and is represented by the symbol B. It is a member of the semimetal group.

Boyle's law - ideal gas law that states the volume of a gas is inversely proportional to its absolute pressure, assuming constant temperature.

branched chain alkane - an alkane with alkyl groups bonded to the central carbon chain. The molecules are branched, but all C-C bonds are single bonds.

brass - Brass is defined as an alloy of copper and zinc.

bromine - Bromine is the name for the element with atomic number 35 and is represented by the symbol Br. It is a member of the halogen group.

Bronsted-Lowry acid - species that yields hydrogen ions.

Bronsted-Lowry base - species that accepts hydrogen ions in a reaction.

bronze - Bronze is an alloy of copper, usually containing tin as its main addition.

buffer - either a weak acid and its salt or else a weak base and its salt that form an aqueous solution that resists pH changes.

C - Cadmium to Current

n alloy of copper and zinc.

bromine - Bromine is the name for the element with atomic number 35 and is represented by the symbol Br. It is a member of the halogen group.

Bronsted-Lowry acid - species that yields hydrogen ions.

Bronsted-Lowry base - species that accepts hydrogen ions in a reaction.

bronze - Bronze is an alloy of copper, usually containing tin as its main addition.

buffer - either a weak acid and its salt or else a weak base and its salt that form an aqueous solution that resists pH changes.

Continue Reading Below

03

of 26

C - Cadmium to Current

 The Celsius scale is a common temperature scale in chemistry. Indeed / Getty Images

cadmium - Cadmium is the name for the element with atomic number 48 and is represented by the symbol Cd. It is a member of the transition metals group.

caffeine - Caffeine is a chemical substance naturally found in tea and coffee and added to colas.

calcium - Calcium is the name for the element with atomic number 20 and is represented by the symbol Ca. It is a member of the alkaline earth metal group.

calorie - unit of thermal energy; the amount of energy required to raise the temperature of 1 gram of water 1 degree C or K at standard pressure.

calorimeter - instrument designed to measure heat flow of a chemical reaction or physical change.

capillary action - the spontaneous floe of liquid into a narrow tube or porous material.

carbon - Carbon is the name for the element with atomic number 6 and is represented by the symbol C. It is a member of the nonmetal group.

carbonate - an ion consisting of one carbon bonded to three oxygen atoms (CO₃^2-) or a compound containing this ion.

carbonyl - functional group consisting of a carbon atom double bonded to oxygen, C=O.

carboxyl group - functional group consisting of a carbon double bonded to oxygen and single bonded to a hydroxyl (-COOH).

catalyst - substance that increases the chemical reaction rate by decreasing its activation energy.

catenation - binding of an element to itself via covalent bonds, forming a chain or ring

cathode - electrode where reduction occurs; usually the negative electrode.

cathode ray tube - a vacuum tube with a source of electrons, a fluorescent screen, and means of accelerating and deflecting the electron beam.

cation - ion with a positive electrical charge.

Celsius temperature scale - temperature scale where 0°C and 100°C are defined as the freezing and boiling points of water, respectively.

cerium - rare earth metal with element symbol Ce and atomic number 58.

cesium - Cesium is the name for the element with atomic number 55 and is represented by the symbol Cs. It is a member of the alkali metal group.

cetane number (CN) - value that describes the combustion quality of diesel fuel, based on the delay between injection and ignition.

chain reaction - set of chemical reactions in which products become reactants of another reaction.

charge - an electrical charge, a conserved property of subatomic particles determining their electromagnetic interaction.

Charles's law - ideal gas law that states the volume of an ideal gas is directly proportional to absolute temperature, assuming constant pressure.

chelate - organic compound formed by bonding a polydentate ligand to a central metal atom, or the act of forming such a compound.

chemical - any matter or substance that has mass.

chemical change - process by which one or more substances are altered to form new substances.

chemical energy - energy contained in the internal structure of an atom or molecule.

chemical equation - description of a chemical reaction, including the reactants, products, and direction of the reaction.

chemical equilibrium - state of a chemical reaction where the concentration of the reactants and products remains stable over time.

chemical formula - expression which states the number and type of atoms in a molecule.

chemical kinetics - the study of chemical processes and rates of reactions.

chemical property - characteristic which may be observed when matter undergoes a chemical change.

chemical reaction - a chemical change in which reactants form one or more new products.

chemical symbol - one- or two-letter representation of a chemical element (e.g., H, Al).

chemiluminescence - light emitted as a result of a chemical reaction

chemistry - study of matter and energy and the interactions between them

Cherenkov radiation - Cherenkov radiation is the electromagnetic radiation emitted when a charged particle moves through a dielectric medium faster than the velocity of light in the medium.

chiral center - the atom in a molecule bonded to four chemical species, allowing optical isomerism.

chirality - Chirality or chiral describes a nonsuperimposable mirror image, like left and right hands. Usually in chemistry the term is used to describe a pair of molecules that have the same formulas, but form a pair of structures.

chlorine - halogen with atomic number 17 and element symbol Cl.

chlorofluorocarbon - A chlorofluorocarbon or CFC is a compound that contains atoms of chlorine, fluorine, and carbon.

chromatography - group of techniques used to separate mixture components by passing the mixture through a stationary phase.

chromium - Chromium is the name for the element with atomic number 24 and is represented by the symbol Cr. It is a member of the transition metals group.

closed system - thermodynamic system in which mass is conserved within the system, but energy can freely enter or exit.

coagulation - the gelling or clumping of particles, usually in a colloid.

cobalt - transition metal that is atomic number 27 with element symbol Co.

coenzyme - substance that works with an enzyme to aid its function or initiate its action.

cohesion - measure of how well molecules stick to each other or group together.

collagen - an important family of proteins found in humans and other animals, found in skin, cartilage, blood vessels, and tendons.

colligative properties - properties of a solution that depend on the number of particles in a volume of solvent.

colloid - a homogenous mixture in which dispersed particles do not settle out.

combined gas law - law which states the ratio of the product of pressure and volume, divided by the absolute temperature, is a constant value.

combination reaction - reaction in which two reactants combine to form a single product.

combustion - chemical reaction between a fuel and oxidizer that yields energy (usually heat and light).

common-ion effect - suppressing effect an electrolyte has on the ionization of another electrolyte that shares a common ion.

compound - chemical species formed when two or more atoms form a chemical bond.

complex ion - ion in which a central metal ion is bonded to one or more ions or molecules.

concentrated - having a large ratio of solute to solvent.

concentration - an expression of the quantity of a substance in a defined volume.

condensation - state of matter change from vapor phase to liquid phase.

condensation reaction - chemical reaction in which one of the products is water or ammonia, also known as a dehydration reaction.

condensed formula - chemical formula in which atom symbols are listed in the order they appear in the molecular structure, with limited bond dashes.

conductor - material which permits the flow of energy (e.g., electrical conductor, thermal conductor).

conformer - an isomer that differs from another isomer by rotation around a single bond.

congener - member of the same group of elements of the periodic table (e.g., iodine and chlorine).

conjugate - multiple chemistry definitions, referring to Bronsted acids and bases, a compound formed by combining other compounds, or the overlap of p-orbitals across a sigma bond.

conjugate acid - HX, a compound differing from a base X by a proton.

conjugate base - the species that gains a proton in an acid-base reaction.

conservation of energy - law which states energy can change forms but may not be created or destroyed.

conservation of mass - law that states, in a closed system, matter can change forms but not be created or destroyed.

controlled variable - variable that a scientist holds constant in an experiment; the control or constant variable

conversion factor - numerical ratio that converts a measurement from one unit into another.

coordinate bond - covalent bond between two atoms in which one atom supplies both electrons for the bond.

coordination compound - compound containing one or more coordinate bonds.

coordination number - number of atoms bonded to a central atom.

copernicium - radioactive element with symbol Cn and atomic number 112.

copper - Copper is the name for the element with atomic number 29 and is represented by the symbol Cu. It is a member of the transition metals group.

corrosion - irreversible damage to a material or tissue due to a chemical reaction.

corrosive - having the power to cause irreversible chemical damage upon contact.

Coulomb's law - law which states the force between two charges is proportional to the quantity of both charges and inversely proportional to the square of the distance between them.

covalent bond - chemical link between atoms or ions in which the electron pairs are more or less evenly shared between them.

covalent compound - molecule that contains covalent chemical bonds.

covalent radius - half the diameter of the part of an atom that participates in a covalent bond.

crenation - forming a scalloped shape upon exposure to a hypertonic solution.

critical point - critical state; point at which two phases of matter become indistinguishable from one another.

cyrogenics - study of matter at extremely low temperatures

crystal - matter in which atoms, ions, or molecules are packed into an ordered, repeating three-dimensional pattern.

crystal field splitting - the difference in energy between the d orbitals of ligands.

crystallize - solidification of matter into the highly ordered form of a crystal.

curium - radioactive metal with element symbol Cm and atomic number 96.

current - rate of flow of electricity.