4-Aminobenzoic acid
4-aminobenzoic acid is an aminobenzoic acid in which the amino group is para to the carboxy group. It has a role as an allergen, a plant metabolite and an Escherichia coli metaboli…
Chemical Glossary — Letter A
122 chemicals found starting with "A"
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…
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…
Acenaphthene
Acenaphthene is a polycyclic aromatic hydrocarbon derived from naphthalene by the addition of an ethylene bridge connecting C-1 and C-8.
Acetaldehyde
Acetaldehyde can cause cancer according to an independent committee of scientific and health experts.
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…
Acetic anhydride
Acetic anhydride is an acyclic carboxylic anhydride derived from acetic acid. It has a role as a metabolite and a reagent.
Acetone
Acetone is a methyl ketone that consists of propane bearing an oxo group at C2. It has a role as a polar aprotic solvent, a human metabolite and an EC 3.5.1.4 (amidase) inhibitor. …
Acetonitrile
Acetonitrile is a nitrile that is hydrogen cyanide in which the hydrogen has been replaced by a methyl group. It has a role as a polar aprotic solvent and an EC 3.5.1.4 (amidase) i…
Acetophenone
Acetophenone is a methyl ketone that is acetone in which one of the methyl groups has been replaced by a phenyl group. It has a role as a xenobiotic, a photosensitizing agent and a…
Acetyl chloride
Acetyl chloride is an acyl chloride. It is functionally related to an acetic acid.
Acetylene
Acetylene is a gas molecular entity, an alkyne and a terminal acetylenic compound.
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…
Acridine
Acridine is a polycyclic heteroarene that is anthracene in which one of the central CH groups is replaced by a nitrogen atom. It has a role as a genotoxin. It is a member of acridi…
Acrylamide
Acrylamide can cause cancer according to The World Health Organization's International Agency for Research on Cancer (IARC) and The Environmental Protection Agency (EPA). It can ca…
Acrylic acid
Acrylic acid is a alpha,beta-unsaturated monocarboxylic acid that is ethene substituted by a carboxy group. It has a role as a metabolite. It is a conjugate acid of an acrylate.
Acrylonitrile
Acrylonitrile can cause cancer according to state or federal government labeling requirements.
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 …
Adenine
Adenine is the parent compound of the 6-aminopurines, composed of a purine having an amino group at C-6. It has a role as a mouse metabolite, a Saccharomyces cerevisiae metabolite,…
Adenosine
Adenosine is a ribonucleoside composed of a molecule of adenine attached to a ribofuranose moiety via a betaN9-glycosidic bond. It has a role as an analgesic, a vasodilator agent, …
Adenosine triphosphate
ATP is an adenosine 5'-phosphate in which the 5'-phosphate is a triphosphate group. It is involved in the transportation of chemical energy during metabolic pathways. It has a role…
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…
Adipic acid
Adipic acid is an alpha,-dicarboxylic acid that is the 1,4-dicarboxy derivative of butane. It has a role as a food acidity regulator and a human xenobiotic metabolite. It is a dica…
Adiponitrile
Adiponitrile (C6H8N2) is an organic compound that appears as Adiponitrile appears as a colorless to light yellow liquid which is fairly soluble and is less dense than water. Contac…
Adrenaline
(R)-adrenaline is the R-enantiomer of adrenaline. It is a hormone secreted by the adrenal glands resulting in the 'fight-or-flight' response. It has a role as a hormone, a beta-adr…
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…
Agar
Agar (C14H24O9) is a polymer or plastic that appears as Agar is odourless or has a slight characteristic odour. Unground agar usually occurs in bundles consisting of thin, membrano…
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…
Aldosterone
Aldosterone is a pregnane-based steroidal hormone produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland, and acts on the distal tubules and co…
Alizarin
Alizarin is a dihydroxyanthraquinone that is anthracene-9,10-dione in which the two hydroxy groups are located at positions 1 and 2. It has a role as a chromophore, a dye and a pla…
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…
Allyl alcohol
Allyl alcohol is a propenol in which the C=C bond connects C-2 and C-3. It is has been found in garlic (Allium sativum). Formerly used as a herbicide for the control of various gra…
Allyl bromide
Allyl bromide (C3H5Br) is a laboratory reagent that appears as Allyl bromide appears as a clear colorless to light yellow liquid with an irritating unpleasant odor. Flash point 30 …
Allyl chloride
Allyl chloride (C3H5Cl) is an organic compound that appears as Allyl chloride appears as a clear colorless liquid with an unpleasant pungent odor. Flash point -20 °F. Boiling point…
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…
Alpha-chloroacetophenone
Alpha-chloroacetophenone (C8H7ClO) is an organic compound that appears as 2-chloroacetophenone appears as a riot control agent and chemical warfare tear gas agent. A white crystall…
alpha-Tocopherol
(R,R,R)-alpha-tocopherol is an alpha-tocopherol that has R,R,R configuration. The naturally occurring stereoisomer of alpha-tocopherol, it is found particularly in sunflower and ol…
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, …
Aluminium chloride
Aluminium trichloride is an aluminium coordination entity. It has a role as a Lewis acid.
Aluminium hydroxide
Aluminium hydroxide (Al(OH)3) is an inorganic compound that appears as White solid in various forms; [ICSC] White odorless granules; Slightly soluble in water; [MSDSonline]. It has…
Aluminium nitrate
Aluminium trinitrate is an inorganic nitrate salt having Al(3+) as the counterion. It has a role as a NMR chemical shift reference compound. It is an aluminium coordination entity …
Aluminium oxide
Aluminium oxide (Al2O3) is an inorganic compound that appears as Aluminum oxide is a white odorless crystalline powder. Water insoluble. Properties (both physical and chemical) var…
Aluminum sulfate
Aluminium sulfate (anhydrous) is an aluminium sulfate that contains no water of crystallisation.
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…
Ammonia
Ammonia is an azane that consists of a single nitrogen atom covelently bonded to three hydrogen atoms. It has a role as a NMR chemical shift reference compound, a metabolite, a neu…
Ammonium acetate
Ammonium acetate is an ammonium salt obtained by reaction of ammonia with acetic acid. A deliquescent white crystalline solid, it has a relatively low melting point (114C) for a sa…
Ammonium carbonate
Ammonium carbonate is an ammonium salt that is the diammonium salt of carbonic acid. It has a role as a food acidity regulator and a raising agent. It is a carbonate salt and an am…
Ammonium chloride
Ammonium chloride is an inorganic chloride having ammonium as the counterion. It has a role as a ferroptosis inhibitor. It is an inorganic chloride and an ammonium salt.
Ammonium hydroxide
Ammonium hydroxide is a solution of ammonia in water. It has a role as a food acidity regulator.
Ammonium molybdate
Ammonium molybdate (H32Mo7N6O28-42) is a salt that appears as Ammonium molybdate is a white to greenish-yellow solid. Sinks and mixes with water. (USCG, 1999). It has a molecular w…
Ammonium nitrate
Ammonium nitrate is the ammonium salt of nitric acid. It has a role as a fertilizer, an oxidising agent and an explosive. It is an inorganic molecular entity, an ammonium salt and …
Ammonium oxalate
Ammonium oxalate (C2H10N2O5) is a salt that appears as Odorless solid; [Merck Index] White granules; [Alfa Aesar MSDS]. It has a molecular weight of 142.11 g/mol. Its IUPAC name is…
Ammonium sulfate
Ammonium sulfate is an inorganic sulfate salt obtained by reaction of sulfuric acid with two equivalents of ammonia. A high-melting (decomposes above 280C) white solid which is ver…
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-…
Amoxicillin
Amoxicillin is a penicillin in which the substituent at position 6 of the penam ring is a 2-amino-2-(4-hydroxyphenyl)acetamido group. It has a role as an antibacterial drug. It is …
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…
Ampicillin
Ampicillin is a penicillin in which the substituent at position 6 of the penam ring is a 2-amino-2-phenylacetamido group. It has a role as an antibacterial drug. It is a penicillin…
Aniline
Aniline is a primary arylamine in which an amino functional group is substituted for one of the benzene hydrogens. It is a member of anilines and a primary arylamine.
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…
Anisole
Anisole is a monomethoxybenzene that is benzene substituted by a methoxy group. It has a role as a plant metabolite.
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 …
Anthracene
Anthracene can cause cancer according to California Labor Code and the World Health Organization's International Agency for Research on Cancer (IARC).
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…
Antimony trioxide
Antimony Oxide (Antimony trioxide) can cause cancer according to The World Health Organization's International Agency for Research on Cancer (IARC).
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…
Arsenic trioxide
Arsenic trioxide (As2O3) is an inorganic compound that appears as Arsenic trioxide appears as white or transparent, glassy amorphous lumps or crystalline powder. Slightly soluble i…
Arsine
Arsane is an arsine, a member of arsanes and a mononuclear parent hydride. It is a conjugate base of an arsonium. It is a conjugate acid of an arsanide.
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…
Ascorbic acid
L-ascorbic acid is the L-enantiomer of ascorbic acid and conjugate acid of L-ascorbate. It has a role as a geroprotector, a coenzyme, a cofactor, a flour treatment agent, a plant m…
Aspirin
Aspirin can cause developmental toxicity and female reproductive toxicity according to an independent committee of scientific and health experts.
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…
Astaxanthin
Astaxanthin is a carotenone that consists of beta,beta-carotene-4,4'-dione bearing two hydroxy substituents at positions 3 and 3' (the 3S,3'S diastereomer). A carotenoid pigment fo…
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…
Atorvastatin
Atorvastatin is a dihydroxy monocarboxylic acid that is a member of the drug class known as statins, used primarily for lowering blood cholesterol and for preventing cardiovascular…
Atropine
Atropine is a racemate composed of equimolar concentrations of (S)- and (R)-atropine. It is obtained from deadly nightshade (Atropa belladonna) and other plants of the family Solan…
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…
Azobisisobutyronitrile
Azobisisobutyronitrile (C8H12N4) is a laboratory reagent that appears as Insoluble in water and denser than water. Moderately toxic by ingestion. Readily ignited by sparks or flame…
Azulene
Azulene is a mancude carbobicyclic parent consisting of a cycloheptatriene and cyclopentadiene rings. It has a role as a volatile oil component and a plant metabolite. It is an ort…
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.