Chemistry

Organic compound An organic compound is a molecule built around carbon atoms and covalent frameworks; it includes hydrocarbons, functionalized derivatives, and the macromolecules central to life an…

Organic matter: definition, roles in soil, ecology and climate Organic matter Organic matter is material derived from living organisms that decomposes into soil, humus and organic compounds; it influences soil fertility, water retention, carbon cycling and p…

Organic reactions: types, mechanisms, and applications Organic reaction Overview of organic reactions, their main classes and mechanisms, historical development, and practical importance in synthesis, industry, and research.

Organic synthesis: principles, methods, history and applications Organic synthesis Study and practice of constructing organic molecules through designed reactions and strategies; includes planning, methods, notable challenges, and applications in medicines, mater…

Organolithium reagent Organolithium reagents are organometallic compounds featuring a highly polar carbon–lithium bond. They serve as very strong bases and nucleophiles and are widely used in synthesis…

Organometallic chemistry: metal–carbon compounds, reactions and applications Organometallic chemistry Study of compounds with direct metal–carbon bonds, their properties, reactions and uses in catalysis, materials and biology, including notable examples and historical development.

Osmium (Os) — the densest naturally occurring element and a rare platinum-group metal Osmium Osmium (Os, atomic number 76) is a very dense, rare noble metal of the platinum group. It is notable for its hardness, low reactivity, toxic volatile oxide (OsO4), and specialized…

Ostwald process — industrial production of nitric acid Ostwald process A chemical method for converting ammonia into nitric acid using platinum catalysts. Central to fertilizer manufacture and linked to the Haber process; involves high-temperature oxi…

Oxidation state (oxidation number) Oxidation state The oxidation state (oxidation number) is a bookkeeping value assigned to atoms to track electron transfer in compounds and redox reactions; it helps predict reactivity, balance eq…

Oxide (chemical compounds of oxygen and other elements) Oxide Oxides are compounds containing oxygen and another element; they range from gases to minerals, play roles in geology, industry and the environment, and include acidic, basic and am…

Oxidizing agent (oxidant): definition, properties, examples and uses Oxidizing agent An oxidizing agent accepts electrons or transfers oxygen to another substance. This article explains the two senses of the term, common examples, mechanisms, applications, and safe…

Oxygen (element O) Oxygen Oxygen (O, atomic number 8) is a reactive nonmetal essential for respiration, a major part of Earth's crust and atmosphere, and widely used in industry, medicine, and propulsion.

Oxygen difluoride (OF2) Oxygen difluoride Oxygen difluoride, OF2, is a reactive, toxic, oxygen–fluorine compound with a bent molecular shape. It is a strong oxidizer with limited practical uses and notable chemical behavio…

Oxygen saturation: dissolved oxygen in water and oxygenation in blood Oxygen saturation Oxygen saturation measures the proportion of oxygen present—either dissolved in water or bound in blood—affecting aquatic ecosystems, industrial processes, and human respiration an…

Ozone (O3): properties, atmospheric role, health effects and uses Ozone Ozone is a reactive allotrope of oxygen found in the stratosphere as a protective layer and in the troposphere as a pollutant. This article explains its chemistry, formation, effec…

Palladium: properties, occurrence, history and uses Palladium Palladium (Pd, atomic number 46) is a silver-white noble metal of the platinum group used widely as a catalyst, in electronics and jewellery and recovered from automotive catalysts…

Paraffin wax Paraffin wax is a low-reactivity, white to off-white solid of long-chain alkanes (typically C20–C40), produced mainly from petroleum refining and used in candles, coatings, cosmeti…

Paris green (copper(II) acetoarsenite) Paris green Paris green is an arsenic-containing copper pigment (copper(II) acetoarsenite) once used in paints and insecticides; vivid but highly toxic and largely abandoned for safety reasons…

Patina: natural and applied surface coatings on metal and other materials Patina Patina is a surface layer—often oxide, carbonate or sulfide—formed on metals and other materials by aging, environment, or deliberate treatment; valued for appearance and protectio…

Pentaerythritol tetranitrate (PETN) Pentaerythritol tetranitrate PETN is a highly energetic nitrate ester used both as a powerful explosive and, in medicine, as a nitrate vasodilator for angina; it is crystalline, energetic, and used in detonato…

Pentane: properties, isomers, uses and safety Pentane Pentane is a five‑carbon alkane (C5H12) with three structural isomers. This article covers its structure, physical and chemical properties, production, industrial uses, nomenclatur…

Pentene (C5H10): isomers, properties and industrial uses Pentene Pentene is a class of five-carbon alkenes (C5H10). This article explains its isomers, physical and chemical traits, production from petroleum processing, and principal applications…

Pepper spray (OC) — uses, effects, history and legal considerations Pepper spray Personal and crowd-control inflammatory spray made from oleoresin capsicum (capsaicin). Covers composition, effects, delivery forms, history, first aid, differences from tear gas,…

Peptide bond (amide linkage between amino acids) Peptide bond A peptide bond is the amide linkage that connects amino acids into peptides and proteins. It forms by condensation, has partial double-bond character that constrains geometry, and…

Perbromate (BrO4−): properties, preparation and significance Perbromate Perbromate (BrO4−) is the bromine +7 oxoanion. It is a powerful but unstable oxidizer, known mainly from laboratory synthesis by radiochemical decay, electrolysis, or strong fluori…

Perbromic acid (HBrO4): properties, preparation and safety Perbromic acid Perbromic acid (HBrO4) is the oxyacid of bromine in its highest oxidation state. It is a powerful oxidizer and very unstable; encountered mainly in solution or produced by acidifyi…

Perchlorate (ClO4−): chemistry, uses, health and environmental issues Perchlorate Perchlorate is the ClO4− anion found in stable salts and strong oxidizers. This article covers its chemistry, common salts, uses (including propellants), environmental behavior, an…

Perchloric acid (HClO4): properties, production, uses, and hazards Perchloric acid Perchloric acid (HClO4) is a strong inorganic acid and oxidizer used to make perchlorate salts. It is highly corrosive and regulated because of explosive hazards and environmental…

Pericyclic reaction: concerted, cyclic rearrangements in organic chemistry Pericyclic reaction Pericyclic reactions are concerted organic reactions with cyclic transition states. This article explains types (electrocyclic, cycloaddition, sigmatropic, etc.), orbital symmetry…

Period (periodic table row) Period (periodic table) A period is a horizontal row in the periodic table. Periods group elements with the same number of electron shells and show repeating chemical and physical trends across increasing…

Period 1 elements — hydrogen and helium Period 1 element Overview of the first period of the periodic table, explaining why it contains only hydrogen and helium, their electron structure, chemical behavior, isotopes, classification issue…

Period 2 elements (lithium through neon) Period 2 element Overview of the second row of the periodic table: properties, electron structure, common uses, and how lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon differ…

Period 3 elements — sodium to argon Period 3 element Elements that occupy the third row of the periodic table (Na to Ar), their electronic structure, general trends across the period, common uses, and notable chemical properties.

Period 4 elements (fourth row of the periodic table) Period 4 element Elements in the fourth period (row) of the periodic table, spanning potassium to krypton; their electron shells, general properties, common uses and notable features of the 3d tran…

Period 5 element An element found in the fifth row (period 5) of the periodic table. Period 5 contains 18 elements from rubidium to xenon and is defined by electrons occupying the 5s, 4d and 5p orb…

Period 6 elements (sixth row of the periodic table) Period 6 element The 32 elements of period 6, from caesium to radon, including the lanthanide series; their electronic structure, major subgroups, uses and notable chemical features.

Period 7 elements (seventh period of the periodic table) Period 7 element The seventh period contains 32 heavy elements from francium to oganesson. This article explains their electron structure, chemistry, discovery history, uses and notable distinction…

Periodate: the iodine(VII) oxyanion and its chemistry Periodate Periodate is the oxyanion of iodine in its +7 oxidation state, found as metaperiodate (IO4-) and orthoperiodate (IO6 5-). It is a strong oxidant widely used in organic and biologic…

Periodic acid (HIO4, H5IO6) — properties, reactions and uses Periodic acid Periodic acid is an iodine oxoacid existing as HIO4 or H5IO6; a strong oxidizer used in organic chemistry and histology for cleavage of vicinal diols and carbohydrate analysis. Cor…

Periodic table: organization, trends, history and uses Periodic table A concise, structured overview of the periodic table: how elements are arranged, major patterns and classifications, historical development, and why the table remains central to ch…

Periodic table: structure, trends, history, and uses Periodic table (big) Comprehensive overview of the periodic table: its layout, major element groups and blocks, periodic trends, historical development, practical uses, and notable variations in modern…

Periodic table: metals, nonmetals and metalloids Periodic table (metals and non-metals) Overview of how elements are classified on the periodic table into metals, nonmetals and metalloids, their typical properties, locations, historical origin and key uses.

Permanganate (MnO4⁻): properties, reactions, uses and safety Permanganate An overview of the permanganate ion, covering its structure and color, redox chemistry, common salts and preparations, major applications in chemistry and water treatment, and safe…

Peroxide (O2 2−) — structure, types, uses and safety Peroxide Peroxide denotes the O2 2− ion or compounds with an O–O bond. Often strong oxidizers, peroxides occur as inorganic salts and organic derivatives (including hydrogen peroxide) with…

Petrochemical: Definition, Feedstocks, Processes and Uses Petrochemical Petrochemicals are industrial chemicals derived from petroleum, natural gas or other hydrocarbons used to make plastics, solvents, synthetic fibres, detergents, pharmaceuticals and…

Petroleum coke (petcoke) Petroleum coke Carbon-rich solid residue from crude oil refining used as fuel and industrial feedstock; available in fuel and specialty grades and associated with sulfur and metal contamination r…

Petroleum product: refined oil products, uses, and production Petroleum product A petroleum product is any useful substance obtained by refining crude oil. This article explains common products, how they are made, historical development, main uses, and distinc…

Pewter (tin-based alloy) Pewter Pewter is a low-melting tin alloy traditionally used for tableware and decorative objects. Modern pewter is typically lead-free and hardened with antimony and copper for strength a…