Absolute Entropy of a Substance
The absolute entropy of a substance is the total entropy measured from absolute zero (0 K), where a perfect crystal has zero entropy according to the Third Law of Thermodynamics. I…
Chemical Glossary — Letter A
66 chemicals found starting with "A"
Absolute Zero
Absolute zero is the lowest theoretically possible temperature, equivalent to 0 Kelvin (−273.15°C). At this point, all classical molecular motion ceases and a system possesses only…
Absorption Spectrum
An absorption spectrum is the pattern of wavelengths (or frequencies) of electromagnetic radiation that are absorbed by a substance as radiation passes through it. Electrons in ato…
Accuracy
Accuracy is the degree of closeness of a measured or calculated value to the true or accepted value of the quantity being measured. It is distinct from precision, which refers to r…
Acetic Acid
Acetic acid is a weak organic acid with a distinctive pungent smell. It is the main component of vinegar (4–8% solution) and is one of the simplest carboxylic acids. It is widely u…
Acid
An acid is a substance that donates protons (H⁺ ions) to a base (Brønsted–Lowry definition), accepts electron pairs (Lewis definition), or produces hydrogen ions in aqueous solutio…
Acid Anhydride
An acid anhydride is a compound formed by the condensation of two carboxylic acid molecules with the loss of one water molecule. The resulting compound contains two acyl groups joi…
Acidic Salt
An acidic salt is a salt formed by partial neutralisation of a polyprotic acid (an acid with more than one ionisable hydrogen) with a base. The salt still contains one or more repl…
Actinides
The actinides are the series of 15 metallic chemical elements with atomic numbers 89 to 103 (actinium through lawrencium). They all have electrons filling the 5f subshell. All acti…
Actinium
Actinium is a radioactive silvery-white metal and the first member of the actinide series. It glows blue in the dark due to its radioactivity and was the first non-primordial radio…
Activation Energy
Activation energy (Ea) is the minimum amount of energy that reacting molecules must possess in order for a chemical reaction to occur. It represents the energy barrier that must be…
Active Metal
An active metal is a metal that reacts readily with water, dilute acids, or air under ordinary conditions. Active metals are strong reducing agents and appear high in the activity …
Activity of a Component of Ideal Mixture
The activity of a component in an ideal mixture equals its mole fraction (a_i = x_i). Activity is a dimensionless quantity that expresses the effective concentration of a species i…
Activity Series
The activity series (also called the reactivity series) is a list of metals arranged in decreasing order of their chemical reactivity. A more active metal can displace a less activ…
Actual Yield
The actual yield is the amount of product that is experimentally obtained from a chemical reaction. It is always less than or equal to the theoretical yield (the maximum amount cal…
Acyl Group
An acyl group is a functional group derived from a carboxylic acid by the removal of a hydroxyl (–OH) group, leaving a carbonyl group (C=O) bonded to one organic substituent (R). T…
Addition Reaction
An addition reaction is a chemical reaction in which two or more molecules combine to form a single larger molecule, with no atoms lost as byproducts. Addition reactions typically …
Adhesive Forces
Adhesive forces are attractive forces between molecules of different substances. They are responsible for the adhesion of a liquid to a solid surface and for phenomena such as capi…
Adsorption
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid onto a solid or liquid surface. The adsorbed material is called the adsorbate and the…
Alcohol
Alcohols are organic compounds containing one or more hydroxyl (–OH) groups attached to a saturated carbon atom. They are classified as primary, secondary, or tertiary based on the…
Aldehyde
Aldehydes are organic compounds containing a carbonyl group (C=O) bonded to at least one hydrogen atom, giving the characteristic –CHO functional group at the end of a carbon chain…
Alkali Metals
The alkali metals are the six elements in Group 1 (IA) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). They all hav…
Alkaline Battery
An alkaline battery is a type of primary (non-rechargeable) electrochemical cell that uses an alkaline electrolyte (usually potassium hydroxide, KOH) and zinc metal as the anode wi…
Alkaline Earth Metals
The alkaline earth metals are the six elements in Group 2 (IIA) of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). T…
Alkenes (Olefins)
Alkenes (also called olefins) are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). The general formula for acyclic alkenes is CₙH₂ₙ. The simplest a…
Alkyl Group
An alkyl group is a substituent derived from an alkane by removing one hydrogen atom. The general formula is CₙH₂ₙ₊₁– (or R–). Common alkyl groups include methyl (–CH₃), ethyl (–C₂…
Alkylbenzene
An alkylbenzene is an aromatic compound consisting of a benzene ring with one or more alkyl group substituents. The simplest is toluene (methylbenzene, C₆H₅CH₃). Alkylbenzenes are …
Alkynes
Alkynes are unsaturated hydrocarbons containing at least one carbon–carbon triple bond (C≡C). The general formula for acyclic alkynes is CₙH₂ₙ₋₂. The simplest alkyne is ethyne (ace…
Allotropes
Allotropes are different structural modifications of the same element in the same physical state, having different physical properties. Allotropy is a property of elements (not com…
Allotropic Modifications (Allotropes)
Allotropic modifications refer to the different physical forms in which an element can exist in the same state of matter, each having distinct physical properties but the same chem…
Alloying
Alloying is the process of combining a metal with one or more other metals or non-metals to produce an alloy with improved or modified properties compared to the pure metals. The p…
Alpha (a) Particle
An alpha particle is a helium-4 nucleus consisting of two protons and two neutrons, emitted during alpha decay of radioactive nuclei. With a charge of +2 and relatively large mass,…
Alpha Particles
Alpha particles are positively charged particles identical to helium-4 nuclei, consisting of two protons and two neutrons bound together. They are emitted by heavy radioactive nucl…
Aluminium
Aluminium is a silvery-white lightweight metal, atomic number 13, the most abundant metal in Earth crust. Forms thin protective oxide layer preventing corrosion. Highly malleable, …
Alums
Alums are a class of double sulfate salts with the general formula XM(SO₄)₂·12H₂O, where X is a monovalent cation and M is a trivalent metal ion. The most common is potassium alum …
Americium
Americium is a synthetic radioactive actinide. Americium-241 is widely used in ionisation-type smoke detectors, making it the most common artificial element encountered in everyday…
Amide
An amide is an organic functional group or compound containing a carbonyl group (C=O) bonded to a nitrogen atom. Amides are formed by the reaction of carboxylic acids with amines, …
Amine
Amines are organic compounds derived from ammonia by replacing one or more hydrogen atoms with organic substituents. Classified as primary, secondary, or tertiary based on the numb…
Amine Complexes
Amine complexes are coordination compounds in which amine ligands (NH₃ or organic amines) are bonded to a central metal ion. Also known as ammine complexes when ammonia is the liga…
Amino Acid
Amino acids are organic molecules containing both an amino group (–NH₂) and a carboxyl group (–COOH) attached to the same carbon atom (the alpha carbon). They are the fundamental b…
Amorphous Solid
An amorphous solid is a solid material whose atoms, ions, or molecules are not arranged in a definite crystalline pattern. Unlike crystalline solids, amorphous solids have no long-…
Ampere
The ampere (A) is the SI base unit of electric current, defined as the flow of one coulomb of charge per second. In electrochemistry, the ampere is fundamental to calculations of c…
Amphiprotism
Amphiprotism describes the ability of a substance to act as either a proton donor (acid) or a proton acceptor (base) depending on the reaction conditions. Water is the most common …
Amphoterism
Amphoterism is the property of a substance to react both as an acid and as a base. Amphoteric substances can neutralise both acids and bases. Examples include aluminium hydroxide A…
Anion
An anion is a negatively charged ion formed when an atom or molecule gains one or more electrons. Anions are attracted to the positively charged anode during electrolysis. Common a…
Anode
The anode is the electrode at which oxidation occurs in an electrochemical cell. In an electrolytic cell, the anode is connected to the positive terminal of the power source; in a …
Antibonding Orbital
An antibonding orbital (denoted with an asterisk, e.g., σ* or π*) is a molecular orbital whose electron density weakens the bond between atoms. Electrons in antibonding orbitals de…
Antimony
Antimony is a lustrous, silvery metalloid, atomic number 51. Known since antiquity (kohl eye makeup was Sb2S3). Brittle and flaky. Used in alloys for hardening lead. Antimony triox…
Argon
Argon is a colourless, odourless noble gas, atomic number 18, the most abundant noble gas in Earth atmosphere (0.934%). Chemically inert with no known stable compounds. Produced by…
Aromatic Hydrocarbons
Aromatic hydrocarbons are cyclic organic compounds containing conjugated pi electrons that satisfy Hückel's rule (4n+2 π electrons). Benzene (C₆H₆) is the prototype aromatic compou…
Arsenic
Arsenic is a metalloid, atomic number 33. A notorious poison since antiquity. Occurs naturally and from industrial contamination of groundwater. Also has industrial uses in semicon…
Artificial Transmutation
Artificial transmutation is the conversion of one element into another by bombarding atomic nuclei with high-energy particles such as protons, neutrons, or alpha particles, using p…
Aryl Group
An aryl group is a substituent derived from an aromatic ring (typically benzene) by the removal of one hydrogen atom. The simplest aryl group is the phenyl group (C₆H₅–). Aryl grou…
Associated Ions
Associated ions are oppositely charged ions that are close enough in solution to exert mutual electrostatic attraction without forming a covalent bond. Ion association is particula…
Astatine
Astatine is a radioactive halogen, atomic number 85. The rarest naturally occurring element in Earth crust (total amount ~31 grams at any time). All isotopes are short-lived. At-21…
Atmosphere
In chemistry, the atmosphere (atm) is a unit of pressure equal to 101,325 pascals (101.325 kPa), representing the average air pressure at sea level. The term also refers to the lay…
Atom
An atom is the smallest unit of a chemical element that retains the chemical properties of that element. Atoms consist of a nucleus containing protons and neutrons, surrounded by e…
Atomic Mass Unit (amu)
The atomic mass unit (amu, also written as u or Da for Dalton) is the standard unit of mass used to express the mass of atoms and molecules. It is defined as exactly 1/12 of the ma…
Atomic Number
The atomic number (symbol: Z) is the number of protons found in the nucleus of an atom of a given element. It uniquely identifies each chemical element — no two elements have the s…
Atomic Orbital
An atomic orbital is a mathematical function (wavefunction, ψ) that describes the region of space around an atom's nucleus where an electron is most likely to be found. Each orbita…
Atomic Radius
The atomic radius is a measure of the size of an atom, defined as the distance from the nucleus to the outermost electron shell. Because electrons form a probability cloud, atomic …
Atomic Weight
Atomic weight (also called relative atomic mass, Ar) is the weighted average mass of all naturally occurring isotopes of an element, relative to 1/12 the mass of carbon-12. It take…
Aufbau (Building-Up) Principle
The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy, from the lowest available orbital upward. The filling sequence follows the n + l rule…
Autoionization
Autoionization (also called autoprotolysis) is the spontaneous transfer of a proton from one molecule to another of the same substance. The most important example is the autoioniza…
Avogadro's Law
Avogadro's law states that equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules. Equivalently, the volume of a gas is directly propo…
Avogadro's Number
Avogadro's number (Nₐ, also called the Avogadro constant) is the number of particles (atoms, molecules, or ions) in one mole of a substance. Its exact value is 6.02214076 × 10²³ mo…
About Chemicals Starting With A
This page lists all chemicals in our database beginning with the letter A. Each entry provides the chemical formula, CAS registry number, physical and chemical properties, common uses, and safety information. Use the alphabetical navigation above to browse other letters, or use the search function to find a specific chemical quickly.
Our chemical glossary covers acids, bases, salts, organic compounds, inorganic compounds, solvents, and many more categories. Click on any chemical name to view its full detailed profile.