ChemEd DL Glossary

Glossary extended from the original PTL!

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p
p block
Columns 13 through 18 of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns.; the region in the periodic table within which the p atomic orbitals are being filled in the building up of atomic electronic configurations.
para
Describes the relationship between two groups on a benzene-like ring that are attached to carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. that have two other carbon atoms between them; the two groups are at a 180° angle.
paramagnetic
Describes a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. containing unpaired electrons that is attracted into a magnetic field; the strength of attraction can be related to the number of unpaired electrons.
partial pressure
That portion of the total pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container. of a mixtureA combination of two or more substances in which the substances retain their chemical identity. of gases contributed by the presence of one of the component gases.
pascal
The SI unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. for pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container., equal to 1 N m-2 or 1 kg s-2m-1
Pauli exclusion principle
The statement that no two electrons in an atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. can have the same set of four quantum numbers; the principle leads to the rule that only two electrons (having opposite spin) can occupy an atomic orbitalA mathematical function from which electron density around an atom can be calculated. A region in an atom within which there is a high probability that a particular electron will be found..
peptide
A moleculeA set of atoms joined by covalent bonds and having no net charge. of two or more amino acids joined by amide linkages or "peptide bonds."
peptide bond
The amide linkageA chemical bond linkage formed when a carboxylic acid reacts with an amine, often found in polymers such as nylon and proteins; in proteins also called a peptide bond. that joins the carboxylic acidAn organic compound containing the functional group -C(=O)OH. end of one amino acidA carboxylic acid containing an amino group (-NH2). In an alpha amino acid, the amino group is attached to the carbon atom adjacent to the carboxyl group. with the amineAn organic compound formally derived from ammonia by the replacement of one or more hydrogen atoms by alkyl groups. Examples are primary amine: RNH2; secondary amine: R2NH; tertiary amine: R3N. end of another amino acid to form a peptideA molecule of two or more amino acids joined by amide linkages or "peptide bonds." or proteinA biological polymer of amino acids joined by peptide bonds..
percent ionization
The fraction (expressed as a percent) of an electrolyteA substance that dissolves to produce a solution containing ions, which cause the solution to conduct electricity. that is in the form of ions in solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture.; this fraction depends upon both the value of the ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. equilibrium constantThe value of the equilibrium constant expression when equilibrium concentrations are substituted; a value greater than one indicates the position of equilibrium lies toward products (product-favored), and a value less than one indicates the position of equilibrium lies toward reactants (reactant-favored). and the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of the species that ionizes.
period
Those elements from a single row of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns..
periodic table
A chart showing the symbols of the elements arranged in order by atomic numberThe number of protons in the nucleus of an atom; used to define the position of an element in the periodic table; represented by the letter Z. and having chemically related elements appearing in columns.
pH
A logarithmic measure of the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of hydrogen (hydronium) ion; pHA logarithmic measure of the concentration of hydrogen (hydronium) ion; pH = -log10([H+]) or pH = -log10([H3O+]). = -log10([H+]) or pH = -log10([H3O+]).
phase diagram
A diagram showing conditions, most typically of pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container. and temperatureA physical property that indicates whether one object can transfer thermal energy to another object., under which different phases of a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. exist at equilibriumA state in which no net change is occurring, that is, in which the concentrations of reactants and products remain constant; chemical equilibrium is characterized by forward and reverse reactions occurring at the same rate..
photon
A quantum of energyA system's capacity to do work. associated with electromagnetic radiationEnergy in the form of oscillating, mutually perpendicular electric and magnetic fields. The energy is quantized in units called photons.; the quantity of energy is the productA substance produced by a chemical reaction. of the frequencyThe rate at which a periodic event occurs; specifically, the rate at which the waves of electromagnetic radiation pass a point. of light and Planck's constantA proportionality constant between the energy of a photon of light and the frequency of that light; equal to 6.6261 x 10-34 J sec, E = hν.
phototube detector
A device that converts light energyA system's capacity to do work. to an electrical signal. The intensity of light can be measured with a phototube detectorA device that converts light energy to an electrical signal. The intensity of light can be measured with a phototube detector..
physical change
A process, such as meltingThe process of a liquid forming from a solid., in which the composition of all substances remains unchanged.
physical property
A characteristic of a material that can be measured without changing the material's chemical composition.
pi bond
A bond in which the electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. densityThe ratio of the mass of a sample of a material to its volume. lies above and below a plane containing the bonded atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume.; formed by combinations such as the overlap of two p orbitals or the overlap of a p orbital and a d orbital.
pK
A logarithmic measure of the value of the equilibrium constantThe value of the equilibrium constant expression when equilibrium concentrations are substituted; a value greater than one indicates the position of equilibrium lies toward products (product-favored), and a value less than one indicates the position of equilibrium lies toward reactants (reactant-favored). expressed as -log10(K).
Planck's constant
A proportionality constant between the energyA system's capacity to do work. of a photonA quantum of energy associated with electromagnetic radiation; the quantity of energy is the product of the frequency of light and Planck's constant, E = hν. of light and the frequencyThe rate at which a periodic event occurs; specifically, the rate at which the waves of electromagnetic radiation pass a point. of that light; equal to 6.6261 x 10-34 J sec
pOH
A logarithmic measure of the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of hydroxide ion expressed as -log10([OH-]).
polar
Describes a moleculeA set of atoms joined by covalent bonds and having no net charge. that has separated, equal positive and negative charges that consitute a positive and a negative pole; such a molecule tends to assume certain orientations more than others in an electric field.
polarizability
The ability of the electrons in a species to change position in response to the presence of an outside electrical field.
pollution
The contamination of the air, water, and earth by personal, industrial, and farm waste.
polyatomic
Containing two or more atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume..
polymer
A large moleculeA set of atoms joined by covalent bonds and having no net charge. containing a large number of repeating units; a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. formed from such molecules.
polymerization
The process of forming a polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. from monomers.
polymorphism
Existence of a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. in more than one crystalline form.
polypeptide
A polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. of many amino acids joined by amide linkages or "peptide bonds."
polyprotic
Describes an acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond. that can donate two or more hydrogen ions ("protons") to a baseIn Arrhenius theory, a substance that increases the concentration of hydroxide ions in an aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) acceptor. In Lewis theory, a species that donates a pair of electrons to form a covalent bond.. Examples are sulfuric and phosphoric acids.
polysaccharide
A carbohydrateA compound containing carbon, hydrogen, and oxygen, with twice as many hydrogen atoms as oxygen atoms; examples are sugar and starch. that is polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. of many simple sugarA small carbohydrate that either contains five or six carbon atoms or is a dimer of two units, each containing five or six carbon atoms. units. Example polysaccharides are starch and cellulose.
positional isomer
One of two or more compounds that have the same chemical composition but that differ in the location at which functional groups are attached to the backbone of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume..
positron
A positively charged particle having the same massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. and magnitude of charge as an electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass..
potentiometer
An instrument for measuring the voltage of a system by opposing that voltage until no current flows.
precipitation
The formation of a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. within a solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture., often by the combination of cations and anions to form an insolubleUnable to dissolve appreciably in a solvent. ionic compoundA compound containing oppositely charged ions held together by electrostatic attraction. Usually the ions are in a crystal lattice with positive ions surrounded by negative ions and negative ions surrounded by positive ions..
precision
The extent of agreement among several experimental values; reproducibility of results.
pressure
Force per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container.
primary structure
The sequence of amino acids that defines a proteinA biological polymer of amino acids joined by peptide bonds..
product
A substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. produced by a chemical reactionA process in which one or more substances, the reactant or reactants, change into one or more different substances, the products; chemical change involves rearrangement, combination, or separation of atoms. Also called chemical change..
protein
A biological polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. of amino acids joined by peptide bonds.
proton
The positively charged particle in an atomic nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass.; its massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. is similar to the mass of a hydrogen atom.
pseudo-first-order reaction
A reaction in which the concentrations of all but one reactantA substance consumed by a chemical reaction. are more than 10 times larger than the one reactant of interest; because there is appreciable change in only the one reactant concentration, the reaction rate appears to depend directly upon the concentration of only the low-concentration reactant.
pyrophoric
Igniting spontaneously when exposed to air.