R-group
In organic chemistry, R-group refers to a variable substituent attached to the main chain or functional group of a molecule. In amino acids, the R-group (side chain) distinguishes …
Chemical Glossary — Letter R
48 chemicals found starting with "R"
Radiation
The emission and propagation of energy through space as electromagnetic waves or subatomic particles. Types include alpha (α), beta (β), and gamma (γ) radiation from radioactive de…
Radical
A radical (free radical) is a highly reactive chemical species with one or more unpaired electrons, generated by the homolytic cleavage of a covalent bond. Radicals play key roles …
Radioactive Dating
Radioactive dating (radiometric dating) is a technique for determining the age of materials by measuring the ratio of a radioactive parent isotope to its stable daughter product(s)…
Radioactive Decay
The spontaneous transformation of an unstable atomic nucleus into a more stable configuration by emitting particles or radiation. Types include alpha decay (α particle emission), b…
Radioactive Tracer
A radioactive tracer is a radioisotope attached to or incorporated into a substance to allow monitoring of its distribution, movement, or chemical transformation within a system. B…
Radioactivity
Radioactivity is the spontaneous emission of radiation (alpha particles, beta particles, or gamma rays) from unstable atomic nuclei as they decay to more stable configurations. Dis…
Radioactivity
The property of certain isotopes (radioisotopes) to spontaneously emit radiation from their nuclei. Discovered by Henri Becquerel in 1896. Radioactive materials emit alpha, beta, o…
Radium
Radium is a radioactive alkaline earth metal discovered by Marie and Pierre Curie in 1898. It was historically used in luminous paints and medical treatments but was phased out onc…
Radon
Radon is a colourless, odourless, radioactive noble gas, atomic number 86. The heaviest noble gas. Produced by radioactive decay of radium. Second leading cause of lung cancer (aft…
Raoult's Law
Raoult's Law states that the partial vapor pressure of a component in an ideal solution is equal to the mole fraction of that component multiplied by its pure vapor pressure at the…
Raoult's Law
Raoult's law states that the partial vapor pressure of a component in an ideal solution is equal to the product of the vapor pressure of the pure component and its mole fraction in…
Rate Constant
The proportionality constant (k) in a rate law relating reaction rate to reactant concentrations. It depends on temperature (Arrhenius equation: k = Ae^(-Ea/RT)) but not on concent…
Rate Determining Step
The slowest elementary step in a reaction mechanism, which controls the overall rate of the reaction. The rate law for the overall reaction is determined by the rate of this step. …
Rate Law
A mathematical expression relating the rate of a chemical reaction to the concentrations of reactants raised to experimentally determined powers: Rate = k[A]ᵐ[B]ⁿ. The exponents (m…
Rate Law Expression
The rate law expression (rate equation) for a chemical reaction relates the rate of reaction to the concentrations of reactants raised to experimentally determined powers: rate = k…
Rate Of Reaction
The rate of a chemical reaction is the change in the concentration of a reactant or product per unit time, typically expressed in units of mol L⁻¹ s⁻¹. It depends on factors such a…
Rate-Determining Step
The rate-determining step is the slowest elementary step in a multi-step reaction mechanism, which limits the overall rate of the reaction. It acts as a bottleneck — even if other …
Reactants
Reactants (or reagents) are the starting materials in a chemical reaction that are consumed as the reaction proceeds to form products. In a balanced chemical equation, reactants ap…
Reactants
The starting materials consumed in a chemical reaction, appearing on the left side of a chemical equation. Reactants are transformed into products through bond breaking and forming…
Reaction Mechanism
A reaction mechanism is a step-by-step description of the sequence of elementary reactions (steps) by which overall chemical change occurs, including bond-breaking, bond-forming ev…
Reaction Mechanism
The step-by-step sequence of elementary reactions by which a reaction proceeds from reactants to products. Each step involves bond breaking or formation. The sum of all elementary …
Reaction Quotient
The reaction quotient (Q) is the ratio of the concentrations (or partial pressures) of products to reactants at any point during a reaction, using the same expression as the equili…
Reaction Rate
The change in concentration of a reactant or product per unit time, typically expressed in mol/L/s. Rates depend on concentration, temperature, catalysts, and surface area. The rat…
Reaction Ratio
Reaction ratio refers to the molar ratios in which reactants combine and products are formed in a balanced chemical equation, derived directly from the stoichiometric coefficients.…
Reaction Stoichiometry
Reaction stoichiometry is the quantitative relationship between the amounts of reactants and products in a balanced chemical equation, based on the law of conservation of mass and …
Reactivity
Reactivity in chemistry refers to the tendency of a substance to undergo chemical reaction, either by itself or with other materials, and to release energy during that reaction. Re…
Reactivity Series
A list of metals arranged in order of decreasing reactivity, from most reactive (potassium, sodium) to least reactive (gold, platinum). A more reactive metal can displace a less re…
Recrystallization
Recrystallization is a purification technique used to remove impurities from solid compounds by dissolving them in a hot solvent (in which the compound has greater solubility at hi…
Redox Reaction
A chemical reaction involving the simultaneous transfer of electrons, in which one species is oxidized (loses electrons) and another is reduced (gains electrons). Redox reactions a…
Reducing Agent
A reducing agent (reductant) is a substance that donates electrons to another substance in a redox reaction, causing that substance to be reduced. In the process, the reducing agen…
Reducing Agent
A substance that donates electrons to another species in a redox reaction, causing that species to be reduced. The reducing agent itself is oxidized in the process. Examples includ…
Reduction
A process involving the gain of electrons, decrease in oxidation state, or loss of oxygen. In redox reactions, reduction always accompanies oxidation. The species gaining electrons…
Reduction Potential
Reduction potential (standard electrode potential, E°) is the measure of the tendency of a chemical species to be reduced — to gain electrons — measured under standard conditions (…
Relative Atomic Mass
The relative atomic mass (Ar, also called atomic weight) is the ratio of the average mass of atoms of an element (taking into account the natural abundances of isotopes) to one-twe…
Relative Molecular Mass
The relative molecular mass (Mr, also called molecular weight) is the ratio of the average mass of a molecule to one-twelfth of the mass of a carbon-12 atom. It is calculated by su…
Resonance
Resonance is a concept in chemistry used when a single Lewis structure cannot adequately represent the bonding in a molecule or ion. Instead, two or more resonance structures (diff…
Resonance
The description of a molecule by two or more Lewis structures (resonance structures) where electrons are delocalized over the molecule. The actual structure is a hybrid (average) o…
Resonance Hybrid
A resonance hybrid is the actual structure of a molecule or polyatomic ion that cannot be represented adequately by a single Lewis structure. It is the weighted average of all cont…
Reverse Osmosis
Reverse osmosis is a water purification process in which pressure exceeding the osmotic pressure is applied to a solution to force solvent (water) through a semipermeable membrane …
Reversible Reaction
A chemical reaction that can proceed in both forward and reverse directions and eventually reaches an equilibrium state. Represented with double arrows (⇌). At equilibrium, both re…
Reversible Reaction
A reaction in which the products can react to reform the starting materials under the same conditions. The forward and reverse reactions reach a dynamic equilibrium with constant c…
Rhenium
Rhenium is a dense, silvery-grey transition metal, atomic number 75. One of the rarest elements in Earth crust. Has the second highest melting point after tungsten. Critical compon…
Rhodium
Rhodium is a hard, silvery-white platinum group metal, atomic number 45. One of the rarest and most expensive metals. Key component of catalytic converters in cars. Very high refle…
Roentgenium
Roentgenium is a synthetic radioactive transactinide element named after Wilhelm Röntgen. Expected to behave somewhat similarly to gold. First synthesised in 1994 at GSI Darmstadt.
Rubidium
Rubidium is a soft, silvery-white alkali metal, atomic number 37. Highly reactive - ignites spontaneously in air and reacts explosively with water. Used in atomic clocks and as get…
Ruthenium
Ruthenium is a hard, lustrous, white platinum group metal, atomic number 44. Relatively rare. Used as a hardener in platinum and palladium alloys. Important catalyst in organic che…
Rutherfordium
Rutherfordium is a synthetic radioactive transactinide element named after Ernest Rutherford. It is the first element beyond the actinides and is predicted to behave similarly to h…
About Chemicals Starting With R
This page lists all chemicals in our database beginning with the letter R. 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.