Glossary

Chemistry terms,defined plainly.

Short, precise definitions written by the Cheemly Chemistry Team, linked to guides, free tools and topic pages where you can go deeper.

Stoichiometry and solutions18

Atom economy: The molar mass of the desired product divided by the total molar mass of all reactants in the balanced equation, times 100. It shows how many reactant atoms can end up in the product, independent of the yield.Read the guide
Avogadro constant: The number of entities per mole, N_A = 6.02214076 × 10²³ mol⁻¹. Since the 2019 SI revision this value is exact and defines the mole.
Dilution: Lowering a concentration by adding solvent. The amount of solute stays constant, so C₁V₁ = C₂V₂ gives the volume of stock needed.Try the free tool
E-factor: The mass of waste produced per mass of product. Lower is greener; solvents, reagents and by-products all count as waste unless they are recovered.Read the guide
Empirical formula: The simplest whole-number ratio of atoms in a compound, often derived from elemental analysis. Glucose (C6H12O6) and formaldehyde (CH2O) share the empirical formula CH2O.
Equivalence point: The point in a titration where the added titrant exactly matches the analyte by the reaction's stoichiometry. The end point is where you observe it; a good method keeps the two close.
Limiting reagent: The reactant that runs out first and therefore sets the maximum amount of product. It is found by dividing each reactant's moles by its coefficient and taking the smallest value.Open the topic page
Molality: Amount of solute per kilogram of solvent, in mol/kg. It does not depend on temperature, which is why colligative properties use it.
Molar mass: The mass of one mole of a substance, in g/mol. It is calculated from standard atomic weights and links mass to amount: n = m / M.Try the free tool
Molarity: Amount of solute per litre of solution, c = n / V, in mol/L (M). Because volume changes with temperature, molarity does too, slightly.Try the free tool
Mole: The SI unit of amount of substance. One mole contains exactly 6.02214076 × 10²³ specified entities (atoms, molecules, ions or electrons), so chemists can count particles by weighing them.Open the topic page
Molecular formula: The actual number of each kind of atom in one molecule, such as C9H8O4 for aspirin. It is a whole-number multiple of the empirical formula.Try the free tool
Monoisotopic mass: The mass of a molecule built only from the most abundant isotope of each element. It is the value high-resolution mass spectrometry compares with, and it differs slightly from the average molar mass.Try the free tool
Percent yield: Actual yield divided by theoretical yield, times 100. It measures how much of the possible product was isolated, not how pure it is.Open the topic page
Primary standard: A pure, stable, non-hygroscopic substance weighed directly to make a solution of accurately known concentration, such as potassium hydrogen phthalate for standardising bases.
Stoichiometry: The quantitative relationship between reactants and products in a balanced equation. The coefficients give mole ratios, which turn into masses through molar masses.Open the topic page
Theoretical yield: The amount of product expected if the limiting reagent reacts completely by the balanced equation, with no losses or side reactions.Open the topic page
Titration: A method that finds an unknown amount by adding a solution of known concentration until the reaction is just complete, detected with an indicator or an instrument.

Acids, bases and equilibrium15

Acid dissociation constant (Ka): The equilibrium constant for HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA]. It depends on temperature and solvent.Read the guide
Amphoteric: Able to act as either an acid or a base. Water, amino acids and aluminium hydroxide are common examples.
Brønsted acid: A species that donates a proton (H⁺). A Brønsted base accepts one, so every acid–base reaction is a proton transfer between two conjugate pairs.
Buffer: A solution of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists pH change when small amounts of acid or base are added.Try the free tool
Buffer capacity: How much strong acid or base a buffer can absorb per unit pH change. It grows with total buffer concentration and is highest when pH is close to pKa.Read the guide
Conjugate base: What remains after an acid loses a proton: acetate is the conjugate base of acetic acid. The stronger the acid, the weaker its conjugate base.
Equilibrium constant (K): The ratio of product to reactant activities at equilibrium, each raised to its stoichiometric coefficient. It is fixed at a given temperature and tells you how far a reaction goes, not how fast.Read the guide
Henderson–Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]). It estimates buffer pH from the base-to-acid ratio and works best for ratios between 0.1 and 10 at moderate concentrations.Try the free tool
Ionic strength: I = ½ Σ cᵢzᵢ², a measure of the total charge concentration in a solution. Higher ionic strength lowers activity coefficients, so concentration-based equations become less accurate.
Isoelectric point (pI): The pH at which a molecule such as an amino acid or protein carries no net charge. Solubility is often lowest there, which is used in purification.
Le Chatelier's principle: A system at equilibrium that is disturbed shifts in the direction that partly counteracts the change, for example by consuming added reactant. Only a temperature change alters K itself.
Lewis acid: An electron-pair acceptor, such as BF3, AlCl3 or a metal cation. A Lewis base donates the pair. The definition covers reactions with no proton transfer.
pH: The negative base-10 logarithm of the hydrogen-ion activity, approximately −log₁₀[H⁺]. Each unit is a tenfold change; pure water at 25 °C has pH 7.Read the guide
pKa: The negative logarithm of an acid's dissociation constant, pKa = −log₁₀ Ka. Lower pKa means a stronger acid; at pH = pKa the acid is half dissociated.Read the guide
Solubility product (Ksp): The equilibrium constant for dissolving a sparingly soluble salt, such as Ksp = [Ag⁺][Cl⁻] for AgCl. A precipitate forms when the ion product exceeds Ksp.

Thermodynamics and kinetics14

Activation energy (Ea): The minimum energy barrier reactants must cross to form products. It controls how fast a reaction goes, not how far.Read the guide
Arrhenius equation: k = A·e^(−Ea/RT). It relates a rate constant to temperature; plotting ln k against 1/T gives a line with slope −Ea/R.
Catalyst: A substance that speeds up a reaction by offering a lower-energy pathway and is regenerated at the end. It changes the rate, not the position of equilibrium.
Enthalpy (H): A state function whose change, ΔH, equals the heat exchanged at constant pressure. Negative ΔH means the reaction releases heat (exothermic).
Entropy (S): A state function related to the number of microscopic arrangements available to a system. Spontaneous processes increase the total entropy of the system plus surroundings.
Gibbs energy (G): G = H − TS. At constant temperature and pressure a process is spontaneous when ΔG < 0, and ΔG° = −RT ln K links it to the equilibrium constant.
Half-life: The time for a reactant's concentration to fall to half its value. For a first-order reaction it is constant: t½ = ln 2 / k.
Hess's law: The enthalpy change of a reaction is the same whatever route it takes, because enthalpy is a state function. Reaction enthalpies can therefore be added like equations.
Kinetic vs thermodynamic control: Under kinetic control the product that forms fastest dominates; under thermodynamic control, usually at higher temperature or longer times with reversible steps, the most stable product dominates.Read the guide
Rate constant (k): The proportionality constant in a rate law. It depends on temperature and catalyst but not on concentration, and its units depend on the overall order.
Rate law: An experimentally determined expression such as rate = k[A]^m[B]^n. The orders m and n come from measurements, not from the balanced equation.
Reaction intermediate: A species formed in one step and consumed in a later one, sitting in an energy minimum between transition states. Carbocations and enolates are common examples.Open the topic page
Reaction order: The exponent of a concentration in the rate law, or the sum of all exponents for the overall order. Zero, first and second order give different concentration–time curves.
Transition state: The highest-energy arrangement along a reaction step, with bonds partly formed and broken. It cannot be isolated and lasts about one molecular vibration.Open the topic page

Organic chemistry and mechanisms31

Aldol reaction: An enolate (or enol) adds to an aldehyde or ketone to form a β-hydroxy carbonyl compound, building a new C–C bond. Loss of water then gives an α,β-unsaturated carbonyl (aldol condensation).
Aromaticity: Extra stability of a planar, fully conjugated ring with 4n + 2 π electrons (Hückel's rule), as in benzene. Aromatic rings prefer substitution over addition.
Cahn–Ingold–Prelog rules (R/S): Rules that rank substituents by atomic number to assign R or S to a stereocentre. With the lowest priority pointing away, clockwise 1→2→3 is R and anticlockwise is S.
Carbocation: A positively charged carbon with only six valence electrons. Stability increases from methyl to tertiary and with resonance, and carbocations can rearrange by hydride or alkyl shifts.Open the topic page
Chemoselectivity: A reagent's preference for one functional group over another in the same molecule, such as NaBH4 reducing a ketone but leaving an ester untouched.
Chirality: The property of a molecule that is not superimposable on its mirror image, like a left and right hand. A carbon with four different substituents is the most common source.
Curly arrow: Notation for the movement of an electron pair in a mechanism. It starts at the electrons (a lone pair or bond) and points to where they end up; a half-headed arrow moves one electron.Open the topic page
Diastereomer: Stereoisomers that are not mirror images, such as cis/trans alkenes or molecules with several stereocentres differing at only some. They have different physical properties.
Disconnection: An imagined bond break in retrosynthesis, drawn with a double-line arrow (⇒). Each disconnection should correspond to a known forward reaction.Read the guide
E/Z isomerism: Geometric isomerism about a double bond. Rank the two groups on each carbon by CIP priority: higher ones on the same side is Z, on opposite sides is E.
E1 reaction: Unimolecular elimination through a carbocation, followed by loss of a neighbouring proton to form an alkene. It competes with SN1 and generally gives the more substituted alkene.Open the topic page
E2 reaction: Bimolecular elimination in one step: a base removes a proton anti-periplanar to the leaving group as the double bond forms. Strong, bulky bases favour it.Open the topic page
Electrophile: An electron-poor species that accepts an electron pair from a nucleophile. Carbonyl carbons, carbocations and alkyl halides act as electrophiles.Open the topic page
Electrophilic aromatic substitution: An aromatic ring attacks an electrophile, forms a cationic intermediate, then loses a proton to restore aromaticity. Nitration, halogenation and Friedel–Crafts reactions follow this pattern.Open the topic page
Enantiomer: One of two stereoisomers that are non-superimposable mirror images. Enantiomers share most physical properties but rotate plane-polarised light oppositely and can behave very differently in biology.
Functional group: A specific group of atoms, such as –OH, C=O or –COOH, that gives a molecule its characteristic reactions. Molecules are named and classified by their functional groups.
Grignard reagent: An organomagnesium halide, R–MgX, made from an organic halide and magnesium in dry ether. It acts as a carbon nucleophile and strong base, and is destroyed by water.
Inductive effect: Polarisation transmitted through σ bonds by electronegative or electropositive substituents. It fades quickly with distance and helps explain trends in acidity and carbocation stability.
Leaving group: The atom or group that departs with the bonding electron pair in substitution or elimination. Good leaving groups are weak bases, such as iodide, bromide or tosylate.Open the topic page
Nucleophile: An electron-rich species that donates an electron pair to form a new bond with an electron-poor atom. Hydroxide, cyanide, amines and enolates are typical nucleophiles.Open the topic page
Nucleophilic acyl substitution: A nucleophile adds to a carbonyl carbon bearing a leaving group, forming a tetrahedral intermediate that collapses and expels the leaving group. Esters, amides and acid chlorides interconvert this way.Open the topic page
Protecting group: A temporary modification that masks a functional group so it survives a reaction elsewhere in the molecule, then is removed. It adds steps, so routes try to avoid it.
Racemic mixture: A 1:1 mixture of two enantiomers. It shows no net optical rotation, and reactions without a chiral influence typically give it.
Regioselectivity: Preference for bond formation at one position over another when several are possible, as in Markovnikov addition or ortho/para substitution.
Resonance: Describing one molecule with several Lewis structures that differ only in electron placement. The real structure is a single hybrid; delocalisation lowers its energy.
Retrosynthesis: Planning a synthesis backwards from the target molecule to simpler, available starting materials, one disconnection at a time.Read the guide
SN1 reaction: Unimolecular nucleophilic substitution: the leaving group departs first to give a carbocation, which the nucleophile then attacks. Favoured by tertiary substrates and polar protic solvents; stereochemistry is usually scrambled.Open the topic page
SN2 reaction: Bimolecular nucleophilic substitution in one concerted step: the nucleophile attacks from the back as the leaving group departs, inverting the configuration. Fastest for methyl and primary substrates.Open the topic page
Stereoselectivity: Preference for forming one stereoisomer over others. Enantioselective reactions favour one enantiomer; diastereoselective reactions favour one diastereomer.
Synthon: An idealised fragment, often charged, produced by a disconnection. A real reagent that behaves like it is its synthetic equivalent.Read the guide
Tautomer: Constitutional isomers that interconvert rapidly, usually by moving a proton and a double bond. The keto–enol pair is the classic case.

Spectroscopy and analysis18

Absorbance: A = log₁₀(I₀/I), the logarithm of incident over transmitted light. It is unitless; an absorbance of 1 means 10% of the light passes through.Read the guide
Beer–Lambert law: A = εlc: absorbance is proportional to molar absorptivity, path length and concentration. It holds best for dilute solutions within the instrument's linear range.Read the guide
Calibration curve: A plot of instrument response against known standard concentrations, used to read unknown concentrations. Unknowns should fall inside the calibrated range.Read the guide
Chemical shift (δ): The position of an NMR signal relative to a reference such as TMS, in ppm. Electron-withdrawing neighbours deshield a nucleus and move it to higher δ.Read the guide
Degree of unsaturation: The number of rings plus π bonds, from the formula: (2C + 2 + N − H − X) / 2. Benzene, C6H6, has 4: one ring and three double bonds.Open the topic page
Deuterated solvent: A solvent with hydrogen replaced by deuterium, such as CDCl3 or DMSO-d6, so it does not swamp the ¹H spectrum. Its small residual peak is a common reference.
Fingerprint region: The IR region below about 1500 cm⁻¹, full of overlapping bending and skeletal bands. It is characteristic of the whole molecule and is used for matching against reference spectra.
Fragmentation: Breaking of a molecular ion into smaller ions inside the mass spectrometer. Predictable losses, such as 15 for CH3 or 18 for H2O, help identify structure.
Integration (NMR): The area under a ¹H NMR signal, proportional to the number of protons it represents. Integrals give ratios, not absolute counts.Open the topic page
IR spectroscopy: Infrared spectroscopy measures absorption by bond vibrations. It is best for identifying functional groups, such as a strong C=O stretch near 1700 cm⁻¹.Read the guide
Isotope pattern: The cluster of peaks caused by natural isotopes. Chlorine gives an M+2 peak about one third of M, and bromine gives M and M+2 of almost equal height.
Limit of detection (LOD): The lowest concentration that can be reliably distinguished from a blank, often estimated as 3.3 times the standard deviation of the blank divided by the calibration slope.
Mass spectrometry: A technique that ionises molecules and separates the ions by mass-to-charge ratio (m/z). It gives molecular mass, formula (at high resolution) and structural clues from fragments.Open the topic page
Molecular ion: The ion formed from the intact molecule, M⁺• in electron ionisation or [M+H]⁺ in soft ionisation. Its m/z gives the molecular mass.Open the topic page
NMR spectroscopy: Nuclear magnetic resonance measures how nuclei such as ¹H and ¹³C absorb radio frequencies in a strong magnetic field. Shifts, coupling and integrals reveal a molecule's connectivity.Open the topic page
Spin–spin coupling: Splitting of an NMR signal by neighbouring nuclei, measured as the coupling constant J in Hz. In simple first-order ¹H spectra, n equivalent neighbours give n + 1 lines.Open the topic page
UV–Vis spectroscopy: Measures absorption of ultraviolet and visible light by electronic transitions, mainly in conjugated systems and metal complexes. It is widely used for quantification.
Wavenumber: The number of waves per centimetre, in cm⁻¹, used as the IR x-axis. It is proportional to energy: higher wavenumber, higher-energy vibration.

Cheminformatics7

Canonical SMILES: A single standard SMILES string a toolkit generates for a structure, so that different inputs for the same molecule can be compared. Canonical forms differ between toolkits.Try the free tool
CAS Registry Number: A unique numeric identifier assigned by the Chemical Abstracts Service, such as 50-78-2 for aspirin. It identifies a substance but encodes no structural information.
InChI: The IUPAC International Chemical Identifier: a layered, non-proprietary text identifier generated by a standard algorithm, so the same structure always gives the same InChI.
InChIKey: A fixed-length 27-character hash of the InChI, designed for web and database searches. It is not reversible, so it is looked up rather than decoded.
IUPAC nomenclature: The systematic naming rules of the International Union of Pure and Applied Chemistry. A correct systematic name maps to exactly one structure, which is why software can parse it.Read the guide
Skeletal formula: A drawing in which carbon atoms sit at line ends and vertices and hydrogens on carbon are implied. It is the standard way organic chemists draw structures.Try the free tool
SMILES: Simplified Molecular Input Line Entry System: a line of text that encodes a molecule, such as CCO for ethanol. Branches go in parentheses and digits close rings.Try the free tool