P-type Semiconductor
A p-type semiconductor is a semiconductor doped with acceptor impurities that create "holes" (positive charge carriers) in the valence band. In silicon, p-type doping is achieved b…
Chemical Glossary — Letter P
101 chemicals found starting with "P"
Pairing
Electron pairing refers to the occupation of an atomic or molecular orbital by two electrons with opposite spins (↑↓), as required by the Pauli exclusion principle. Pairing energy …
Pairing Energy
The energy cost of placing two electrons in the same orbital, where they experience electron-electron repulsion. In transition metal complexes, pairing energy competes with crystal…
Palladium
Palladium is a soft, silvery-white platinum group metal, atomic number 46. Notable for its ability to absorb large volumes of hydrogen (up to 900 times its own volume). Key catalys…
Parachor
The parachor is a physico-chemical quantity that relates the surface tension of a liquid to its density, introduced by Sugden (1924). It is approximately constant and additive for …
Paramagnetic
Describes materials with one or more unpaired electrons that are weakly attracted to a magnetic field. Paramagnetic species align their unpaired electron spins with an external fie…
Paramagnetism
Paramagnetism is a form of magnetism in which a material is weakly attracted to an externally applied magnetic field. It arises from unpaired electrons in atoms, ions, or molecules…
Partial Ionic Character
Partial ionic character describes the degree to which a chemical bond has ionic character, as a percentage. Most bonds between different elements are neither purely covalent nor pu…
Partial Pressure
The partial pressure of a gas in a mixture is the pressure that gas would exert if it alone occupied the total volume of the mixture at the same temperature. According to Dalton's …
Partial Pressure
The pressure exerted by one component of a gas mixture as if it alone occupied the container. According to Dalton's law, the total pressure of a gas mixture equals the sum of parti…
Particulate Matter
Particulate matter (PM) refers to tiny solid particles and liquid droplets suspended in the atmosphere, classified by aerodynamic diameter: PM₁₀ (particles ≤10 µm) and PM₂.₅ (fine …
Parts Per Million
A concentration unit (ppm) expressing the number of parts of a substance per million parts of the mixture. Used for trace quantities of contaminants in water, air, and solutions. 1…
Pauli Exclusion Principle
The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers (n, l, mₗ, mₛ). As a consequence, each atomic orbital can hold a…
Pauli Exclusion Principle
The fundamental quantum mechanical principle stating that no two electrons in an atom can have the same set of four quantum numbers (n, l, mₗ, ms). As a result, each orbital can ho…
Pauling Scale
The Pauling scale is the most widely used scale for measuring electronegativity of elements, developed by Linus Pauling in 1932. Electronegativity on this scale ranges from 0.7 (Fr…
Peptide
A peptide is a molecule consisting of two or more amino acids linked together by peptide bonds (amide bonds between the carboxyl group of one amino acid and the amino group of the …
Peptide Bond
A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with loss of water. Peptide bonds link amino acids together to form polypeptides…
Percent By Mass
Percent by mass (mass percent) is a concentration unit expressing the mass of a solute as a percentage of the total mass of the solution, calculated as (mass of solute / mass of so…
Percent Composition
Percent composition is the percentage by mass of each element in a compound. It is calculated as: % element = (mass of element in 1 mol / molar mass of compound) × 100%. Percent co…
Percent Composition
The percentage by mass of each element in a compound. Calculated as (mass of element / molar mass of compound) × 100%. Used to determine empirical formulas and to verify compound i…
Percent Purity
Percent purity is the percentage by mass of a desired substance in a sample that also contains impurities, calculated as (mass of pure substance / mass of sample) × 100%. It is use…
Percent Yield
The ratio of actual yield to theoretical yield, expressed as a percentage: % yield = (actual/theoretical) × 100%. Values less than 100% result from incomplete reactions, side react…
Percentage Ionization
Percentage ionization is the fraction of an electrolyte (acid or base) that has undergone ionization in solution, expressed as a percentage: (concentration of ionized form / initia…
Period
In the periodic table, a period is a horizontal row of elements. All elements in the same period have the same number of electron shells. Moving left to right across a period, the …
Period
A horizontal row in the periodic table. Elements in the same period have the same number of electron shells (principal quantum number n). Properties change systematically across a …
Periodic Law
The periodic law states that the physical and chemical properties of the elements are periodic functions of their atomic numbers; when elements are arranged in order of increasing …
Periodic Table
The periodic table is a tabular arrangement of all known chemical elements, organised by increasing atomic number and grouped by similar electronic structure and chemical propertie…
Periodic Table
The tabular arrangement of the chemical elements ordered by atomic number, electron configuration, and recurring chemical properties. Elements in the same column (group) have simil…
Periodicity
Periodicity refers to the repeating trends in physical and chemical properties of elements across periods and down groups in the periodic table. Periodic trends include atomic radi…
Periodicity
The regular, repeating variation in properties of elements as atomic number increases. Periodic trends include atomic radius, ionization energy, electronegativity, and electron aff…
Peroxide
Peroxides are compounds containing the peroxide ion O₂²⁻ (in ionic peroxides such as Na₂O₂) or the –O–O– covalent linkage (in organic peroxides and hydrogen peroxide H₂O₂). The oxy…
pH
A logarithmic scale measuring the acidity or alkalinity of an aqueous solution: pH = -log[H⁺]. A pH of 7 is neutral (at 25°C), below 7 is acidic, and above 7 is basic. The scale ty…
pH
A measure of the hydrogen ion concentration in a solution, defined as pH = -log₁₀[H⁺]. In pure water at 25°C, [H⁺] = 10⁻⁷ M and pH = 7. Values below 7 are acidic; values above 7 ar…
Phase Diagram
A phase diagram is a graphical representation showing the physical states (phases) of a substance under different conditions of temperature and pressure. It typically shows the reg…
Phase Diagram
A graph showing the physical states of a substance as a function of temperature and pressure. It contains regions for solid, liquid, and gas phases separated by boundary lines. The…
Phase Rule
The Phase Rule (Gibbs Phase Rule) is a relationship that gives the number of degrees of freedom (F) in a thermodynamic system at equilibrium, based on the number of components (C) …
Phase Transition
A change in the physical state of matter, such as melting, freezing, vaporization, condensation, sublimation, or deposition. Phase transitions involve energy changes (latent heat) …
Phenol
Phenol (C₆H₅OH, hydroxybenzene) is an aromatic organic compound consisting of a hydroxyl group directly attached to a benzene ring. It is a weak acid (pKa ≈ 10) due to resonance st…
Phosphoric Acid
Phosphoric acid (H₃PO₄) is a triprotic inorganic oxyacid that is a moderate weak acid, ionizing in three steps with pKa values of approximately 2.1, 7.2, and 12.4. It is manufactur…
Phosphorus
Phosphorus is a non-metal, atomic number 15, with several allotropes. White phosphorus (P4) is extremely reactive and toxic; red phosphorus is stable. Essential for life - forms ba…
Phosphorus Oxychloride
Phosphorus oxychloride (POCl₃, phosphoryl chloride) is an inorganic compound with a tetrahedral molecular geometry in which a phosphorus atom is bonded to three chlorine atoms and …
Photocatalysis
Photocatalysis is the acceleration of a chemical reaction by a photocatalyst — a substance that absorbs light and uses that energy to drive a chemical reaction while itself being r…
Photochemical Oxidants
Photochemical oxidants are secondary air pollutants formed in the atmosphere by the action of ultraviolet sunlight on primary pollutants such as nitrogen oxides and volatile organi…
Photochemical Smog
Photochemical smog is a type of air pollution formed when primary pollutants (primarily nitrogen oxides and volatile organic compounds from vehicle exhaust and industry) react in t…
Photochemistry
The branch of chemistry concerned with chemical reactions and physical processes initiated by the absorption of light (photons). Photochemical reactions include photosynthesis, pho…
Photoelectric Effect
The photoelectric effect is the emission of electrons from a material surface when it is illuminated by light of sufficient frequency (above the threshold frequency), demonstrated …
Photoionization
Photoionization (photoelectric effect in atoms) is the process in which a photon of sufficient energy is absorbed by an atom or molecule, ejecting an electron and creating a positi…
Photon
A photon is a quantum of electromagnetic radiation — the fundamental particle of light and all electromagnetic waves. Photons are massless, travel at the speed of light (3×10⁸ m/s)…
Photon
The quantum of electromagnetic radiation, carrying energy E = hν, where h is Planck's constant and ν is frequency. Photons have zero rest mass and travel at the speed of light. In …
Physical Change
A physical change is a change in a substance's physical properties (such as shape, size, phase, or state of matter) without any change in its chemical composition or molecular stru…
Physical Change
A change that alters the form or physical properties of a substance without changing its chemical composition. Examples include melting, boiling, dissolving, and crushing. Physical…
Physical Property
A characteristic of a substance that can be measured without changing its chemical identity. Physical properties include melting point, boiling point, density, color, and solubilit…
Pi Bond
A covalent bond formed by sideways overlap of p orbitals, creating regions of electron density above and below the internuclear axis. Pi bonds are weaker than sigma bonds and canno…
pKa
The negative logarithm of the acid dissociation constant Ka: pKa = -log(Ka). A lower pKa indicates a stronger acid. The pKa is the pH at which an acid is exactly half-dissociated. …
Plasma
Plasma is a high-energy state of matter consisting of a gas of ions and free electrons, not bound into atoms. It is sometimes called the fourth state of matter. Plasma forms when a…
Plasma
The fourth state of matter, consisting of a gas of ions and free electrons at very high temperatures. Plasma conducts electricity and responds to magnetic fields. It occurs in star…
Platinum
Platinum is a dense, malleable, silvery-white precious metal, atomic number 78. Highly resistant to corrosion and oxidation. An excellent catalyst - essential in catalytic converte…
Plutonium
Plutonium is a radioactive actinide metal first synthesised in 1940. Plutonium-239 is fissile and is used in nuclear weapons and as reactor fuel. It is one of the most studied of a…
Polar Bond
A polar bond (polar covalent bond) is a covalent bond in which the electron density is unequally shared between the two bonded atoms due to a difference in their electronegativitie…
Polar Bond
A covalent bond between two atoms of different electronegativities in which electrons are unequally shared. The more electronegative atom carries a partial negative charge (δ-), an…
Polar Covalent Bond
A polar covalent bond is a covalent bond between two atoms of different electronegativities, resulting in unequal sharing of the bonding electrons. The more electronegative atom at…
Polar Molecule
A molecule with a net dipole moment resulting from the vector sum of bond dipoles. A molecule can have polar bonds but be nonpolar if the bond dipoles cancel due to molecular symme…
Polarimeter
A polarimeter is an optical instrument used to measure the angle of rotation of plane-polarized light as it passes through an optically active substance. It consists of a light sou…
Polarity
The separation of electric charge across a bond or molecule, resulting from differences in electronegativity. Polar bonds have partial positive and negative charges (δ+ and δ-). Mo…
Polarization
In chemistry, polarization refers to the distortion of the electron cloud of an ion or molecule by a neighboring ion or electric field. According to Fajans' rules, cations with hig…
Polonium
Polonium is a highly radioactive metalloid, atomic number 84. Discovered by Marie and Pierre Curie in 1898 and named after Poland. Polonium-210 is an intense alpha emitter used as …
Polyatomic Ion
An ion consisting of two or more atoms covalently bonded together and carrying a net electric charge. Common examples include hydroxide (OH⁻), sulfate (SO₄²⁻), nitrate (NO₃⁻), and …
Polydentate
Polydentate ligands (also called multidentate or chelating ligands) are ligands that can bind to a central metal ion through two or more donor atoms simultaneously, forming ring st…
Polyene
Polyenes are organic compounds containing multiple conjugated carbon-carbon double bonds (C=C) alternating with single bonds in a chain. The extended pi system of polyenes results …
Polymer
A polymer is a large molecule (macromolecule) composed of many repeating structural units called monomers, connected by covalent bonds. Polymers can be natural (proteins, cellulose…
Polymer
A large macromolecule composed of many repeating structural units (monomers) linked by covalent bonds. Natural polymers include proteins, DNA, and cellulose. Synthetic polymers inc…
Polymerisation
The chemical reaction in which monomer molecules join together to form a polymer. Addition polymerization occurs without loss of atoms; condensation polymerization releases small m…
Polymerization
Polymerization is the chemical process by which small molecules (monomers) are covalently linked together to form large, chain-like macromolecules (polymers). The two main types ar…
Polymorphous
Polymorphism in chemistry refers to the ability of a solid material to exist in more than one crystalline form (polymorph) with the same chemical composition but different crystal …
Polyprotic Acid
A polyprotic acid is an acid that can donate more than one proton (H⁺) per molecule in successive ionisation steps. Diprotic acids (H₂SO₄, H₂CO₃) can donate two protons; triprotic …
Polyprotic Acid
An acid that can donate more than one proton per molecule. Sulfuric acid (H₂SO₄) is diprotic; phosphoric acid (H₃PO₄) is triprotic. Each ionization step has its own Ka value (Ka1 >…
Positron
A positron (β⁺) is the antimatter counterpart of the electron, having the same mass as an electron (9.109 × 10⁻³¹ kg) but carrying a positive charge of +1. It is emitted in positro…
Potassium
Potassium is a soft, silvery-white alkali metal, atomic number 19. Highly reactive with water. Essential element for all living organisms - maintains cellular electrical potential …
Potential Energy
Potential energy is the stored energy of a system due to its configuration or position relative to a reference state. In chemistry, potential energy is associated with the position…
Potentiometry
An electroanalytical technique that measures the potential (voltage) of an electrochemical cell to determine the concentration of a species in solution. The pH meter uses potentiom…
Praseodymium
Praseodymium is a soft, silvery, yellowish rare earth metal, atomic number 59. Its name comes from Greek for green twin (prasios didymos) - it produces green compounds. Used in hig…
Precipitate
A precipitate is an insoluble solid that forms and separates from a solution when two soluble solutions are mixed, or when the solubility limit of a compound is exceeded. Precipita…
Precipitate
An insoluble solid formed when two solutions are mixed or when conditions change, causing a dissolved substance to come out of solution. Precipitate formation is observed as turbid…
Precipitation
The formation of an insoluble solid (precipitate) from ions in solution. Precipitation occurs when the ion product exceeds the solubility product (Ksp). Used in qualitative analysi…
Pressure
The force exerted per unit area, measured in pascals (Pa) in SI units. Atmospheric pressure is 101,325 Pa (1 atm). Gas pressure results from molecular collisions with container wal…
Primary Standard
A primary standard is a highly pure, stable substance with known composition that is used to prepare standard solutions of accurately known concentration (for calibration in titrat…
Primary Standard
A highly pure, stable substance of known composition used to prepare standard solutions for calibrating titrations. Properties: high purity, stability in air, non-hygroscopic, high…
Primary Voltaic Cells
Primary voltaic cells are electrochemical cells that convert chemical energy into electrical energy through spontaneous redox reactions and cannot be recharged once the reactants a…
Principal Quantum Number
The quantum number (n) that determines the energy level and size of an atomic orbital. Takes positive integer values (1, 2, 3...). Higher n values correspond to orbitals farther fr…
Products
The substances formed in a chemical reaction, appearing on the right side of a chemical equation. Products are the result of bond breaking and forming during the reaction. In an ex…
Promethium
Promethium is a radioactive rare earth metal, atomic number 61. The only lanthanide with no stable isotopes. Produced in nuclear reactors. Named after the Greek Titan Prometheus. P…
Protactinium
Protactinium is a rare, highly toxic and radioactive actinide metal. Its name means 'parent of actinium' as it decays into actinium. It is one of the rarest and most expensive natu…
Proton
A proton is a subatomic particle in the atomic nucleus with a positive elementary charge (+1.602×10⁻¹⁹ C) and a mass of approximately 1.673×10⁻²⁷ kg (1 amu). The atomic number (Z) …
Proton
A subatomic particle in the nucleus with a positive charge of +1.6 × 10⁻¹⁹ coulombs and a mass of 1.673 × 10⁻²⁷ kg. The number of protons (atomic number) defines the element. In ac…
Proton Affinity
Proton affinity (PA) is a measure of the intrinsic basicity of a molecule — specifically, the negative of the enthalpy change when a proton (H⁺) is added to a neutral molecule in t…
Proton NMR
Nuclear magnetic resonance spectroscopy using the ¹H nucleus. Provides information about the number of chemically distinct hydrogen environments, their relative numbers, and coupli…
Proton Transfer
The fundamental process in Brønsted-Lowry acid-base reactions in which a proton (H⁺) moves from an acid (proton donor) to a base (proton acceptor). Proton transfer reactions are ra…
Pseudobinary Ionic Compounds
Pseudobinary ionic compounds are ionic compounds that appear to be binary (two-component) but actually contain polyatomic ions, such that the compound behaves chemically like a bin…
Pulverization
The process of grinding or crushing a solid material into fine powder or particles. Pulverization increases surface area, which increases reaction rates and improves mixing. Used i…
Purification
The process of separating a desired substance from contaminants. Techniques include recrystallization, distillation, chromatography, and extraction. Purity is assessed by melting p…
Pyrolysis
The chemical decomposition of organic materials at elevated temperatures in the absence of oxygen or other oxidants. Pyrolysis produces smaller molecules from larger ones. Applicat…
About Chemicals Starting With P
This page lists all chemicals in our database beginning with the letter P. 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.