kb/data/en.wikipedia.org/wiki/Glossary_of_chemistry_terms-13.md

title	chunk	source	category	tags	date_saved	instance
Glossary of chemistry terms	14/20	https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms	reference	science, encyclopedia	2026-05-05T07:59:20.572989+00:00	kb-cron

octet rule Also Lewis octet rule. A classical rule for describing the electron configuration of atoms in certain molecules: the maximum number of electron pairs that can be accommodated in the valence shell of an element in the first row of the periodic table is four (or eight total electrons). For elements in the second and subsequent rows, there are many exceptions to this rule.

olefin A trivial (non-IUPAC) name for any alkene.

optical activity

orbital Any region of an atom or molecule in which one or more electrons can be found. The term may refer to either an atomic orbital or a molecular orbital.

orbital hybridisation Also orbital hybridization.

order of reaction

organic acid Any organic compound with acidic properties. Contrast organic base.

organic base Any organic compound with basic properties. Contrast organic acid.

organic chemistry The branch of chemistry concerned with the chemical properties and reactions of organic compounds. Contrast inorganic chemistry.

organic compound Any chemical compound that contains one or more carbon atoms. Contrast inorganic compound.

organic redox reaction

organosulfur compound Any chemical compound which contains both carbon and sulfur atoms.

osmole

osmosis The spontaneous net movement or diffusion of molecules of a solvent (e.g. water) through a selectively permeable membrane separating two solutions with different concentrations of dissolved solutes, in the direction that tends to equalize the solute concentrations on the two sides, i.e. from the more dilute solution to the more concentrated solution, or, equivalently, from a region of high water potential to a region of low water potential. Because the solute is unable to cross the membrane, the tendency towards equilibration compels the solvent to cross the membrane instead. This continues until an equilibrium is reached, where neither side of the membrane is more or less concentrated than the other.

osmotic concentration Also osmolarity.

osmotic pressure

other metal Any of the metallic elements in the p-block, which are characterized by having a combination of relatively low melting points (all less than 950 K) and relatively high electronegativity values (all more than 1.6, revised Pauling).

oxidation The increase in the oxidation state of a chemical species in a redox reaction, generally by losing electrons. Contrast reduction.

oxidation state Also oxidation number.

1. The degree of oxidation of an individual atom in a chemical compound, measured as the decrease in the number of electrons relative to the atom's naturally occurring elemental state.
2. The hypothetical electric charge (positive, negative, or zero) that an atom would have if all bonds to atoms of different elements were 100% ionic, with no covalent component.

oxidizing agent Also oxidant, oxidizer, or electron acceptor.

1. A chemical species that gains or accepts one or more electrons from another species, called the reducing agent, in a redox reaction, thereby causing the oxidation of the other species and in turn being itself reduced. The oxidizing agent's oxidation state decreases, while the reducing agent's increases.
2. A chemical species that transfers strongly electronegative atoms, usually oxygen, to a substrate.

oxoacid Also oxyacid or oxacid.

1. Any acid having oxygen in the acidic group.
2. Any compound which contains oxygen, at least one other element, and at least one hydrogen atom bound to oxygen, and which produces a conjugate base by the loss of positively charged hydrogen protons.

oxygen

== P ==

p-block

paired electron One of two electrons that together form a valence bond between two atoms. Contrast unpaired electron.

paraffin

1. A trivial (non-IUPAC) name for any alkane.
2. Another name for kerosene.

partial pressure

partition coefficient

pascal (Pa)

passivation The process of coating a substance with a thin layer of a protective material, often a metal oxide, to create a shield against corrosion or other chemical reactions with the environment, thereby rendering the coated substance "passive" or less susceptible to undesirable reactions.

passivity A state of chemical inactivity, especially of a metal that is relatively resistant to corrosion due to natural or induced loss of chemical reactivity (as with passivation). See also inert.

pendant group An offshoot or side chain of the backbone of a polymeric molecule, especially one which is itself neither oligomeric or polymeric.

pentabasic (of a chemical compound) Having five hydrogen atoms which may be replaced by metals or bases.

pentoxide Any binary compound containing five atoms of oxygen, e.g. iodine pentoxide (I2O5).

pentyl Also amyl. An alkyl functional group containing five carbon atoms, with the generic chemical formula –C5H11. It is the substituent form of the alkane pentane.

per- A prefix in IUPAC chemical nomenclature meaning complete, exhaustive, or extreme, as in a completely substituted hydrocarbon; or indicating the presence of a peroxy group.

peracid An acid containing an acidic peroxy group (–O–O–); e.g. periodic acid.

period A horizontal row of the periodic table of the elements and the elements that share it. Contrast group.

periodic table of the elements Also simply the periodic table. A tabular arrangement of the chemical elements organized by their atomic number, electron configuration, and other chemical properties, whose adopted structure shows periodic trends and is used by chemists to derive relationships between various elements as well as predict the properties and behaviors of undiscovered or newly synthesized elements. The first periodic table of the elements was published by Russian chemist Dmitri Mendeleev in 1869.

peroxide

1. A class of compounds which contain a peroxy group, having the generic structural formula R–O–O–R, where R is any element or functional group; e.g. hydrogen peroxide (empirically H2O2, structurally H–O–O–H).
2. Another name for the peroxy group itself.
3. A salt of the anion O2−2.

peroxy Also peroxide and sometimes peroxo. A functional group consisting of two oxygen atoms directly connected to each other by a single bond and each also connected to one other atom. Peroxides have the general structural formula –O–O–.