Category: Dictionary

Polyatomic ion : A molecule composed of two or more covalently bonded atoms which collectively bear a net electric charge and therefore act as an ion.

Pressure : The force applied perpendicular to the surface of an object per unit area. The SI unit for pressure is the pascal (Pa), though many other units of pressure are also commonly used in chemistry.

Photon : A carrier of electromagnetic radiation of all wavelengths (such as gamma rays and radio waves).

Precipitate : The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid.

Potential energy : The stored energy in a body or in a system due to its position in a force field or due to its configuration.

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.

Periodic table of the elements: Also simply called 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.

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

Plasma : One of the four fundamental states of matter, in which very high-energy particles are partially or fully ionized to the point that they display unique properties and behaviors unlike those of the other three states. Plasma does not exist freely on the Earth’s surface under natural conditions.

Pipette : Also spelled pipet.

A laboratory tool commonly used in chemistry, biology, and medicine to transfer and dispense a precisely measured volume of liquid.

Physical chemistry : The branch of chemistry that studies chemical systems in terms of the principles, practices, and concepts of physics, such as motion, energy, force, time, thermodynamics, chemical equilibrium, and statistical mechanics, among others. In contrast to chemical physics, physical chemistry is predominantly (though not entirely) a macroscopic science that studies the physical and chemical interactions of bulk quantities of matter.

Phase transition : 1.  A transformation of a chemical substance between solid, liquid, and gaseous states of matter and, in rare cases, plasma. 2.  The measurable values of the external conditions at which such a transformation occurs.

pH : A logarithmic scale used to specify the acidity or basicity of an aqueous solution. The pH scale approximates the negative of the base-10 logarithm of the molar concentration of hydrogen ions in a solution. At room temperature, pure water is neutral (pH = 7); solutions with a pH less than 7 are acidic and those with a pH greater than 7 are basic.

Phase : A region of space throughout which all physical properties of a substance are essentially uniform, or a region of material that is chemically uniform, physically distinct, and often mechanically separable. The term phase may have several different uses in chemistry contexts; colloquially, it is often used interchangeably with state of matter, but many distinct phases may exist within a single state of matter.

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

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 positive hydrogen ions.

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

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).

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

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

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

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.

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

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.

Number density : A measure of the concentration of countable objects (atoms, molecules, etc.) in space, expressed as the number per unit volume.

Nuclide : A species of atom characterized by its mass number, atomic number, and nuclear energy state, provided that the mean life in that state is long enough to be observable.

Nuclear magnetic resonance spectroscopy : A technique that exploits the magnetic properties of certain nuclei, useful for identifying unknown compounds. Nuclear magnetic resonance is often abbreviated NMR.

 Nuclear chemistry : The branch of chemistry that studies the various processes and properties relevant to atomic nuclei, including radioactivity.

Nuclear : Of or pertaining to the atomic nucleus.

Non-metal : Any chemical element which is not a metal.

Noble gas : Also inert gas. : Any of the six non-metallic elements of Group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). All of the noble gases have outer electron shells that are completely filled in their naturally occurring states, giving them very low chemical reactivity.

Nucleophile : Any atom or molecule which can donate an electron pair to another atom or molecule. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles, by which they are attracted to electron-deficient regions of other species; a chemical reaction involving a nucleophile donating an electron pair to an electrophile may be referred to as nucleophilic attack. Because they donate electrons, nucleophiles are Lewis bases by definition.

Nucleus : The centre of an atom, made up of neutrons and protons and possessing a net positive electric charge.

Neutron : A type of subatomic particle that is electrically neutral, having no net charge.

Nucleon : Either a proton or a neutron, considered in its role as a component of an atomic nucleus.

Neat : Conditions with a liquid reagent or gas performed with no added solvent or cosolvent.

Monatomic : Having only one atom, as opposed to a molecule composed of more than one. Virtually all elements are monatomic in the gas phase at sufficiently high temperatures. Contrast diatomic and polyatomic.

Molecule : A number of atoms that are chemically bonded together and collectively electrically neutral.

Molecular orbital (MO) : Any region in which one or more electrons may be found in a molecule (as opposed to that within an individual atom).

Mole (mol) : A unit (symbol: mol) that measures the amount of a substance in terms of the absolute number of particles or entities composing the substance. A single mole contains approximately 6.022×10²³ particles or entities.

Molar fraction : Also mole fraction.

Molality : Also molal concentration.

A measure of the concentration of a solute in a solution in terms of the amount of the solute per unit mass of the solvent. Molality is typically expressed in units of moles per kilogram (mol/kg); a solution with a concentration of exactly 1 mol/kg is sometimes said to be 1 molal. Contrast molarity.

Molar concentration : Also molarity, amount concentration, or substance concentration.

A measure of the concentration of a chemical species, especially of a solute in a solution, in terms of the amount of the species per unit volume of solution. Molarity is typically expressed in units of moles per litre (mol/L); a solution with a concentration of exactly 1 mol/L is commonly said to be 1 molar, symbolized as 1 M. Contrast molality.

Moiety : Any named characteristic group, branch, or other part of a large molecule that may be identified within other kinds of molecules as well. Functional groups are typically smaller and more generic than moieties, whereas substituents and side chains may often be classified as moieties and vice versa.

Mixture : A material made up of two or more different substances which are mixed physically but are not combined chemically (i.e. a chemical reaction has not taken place which has changed the molecules of the substances into new substances).

Microcentrifuge tube : A small plastic, sealable container that is used to store small volumes of liquid, generally less than 2 milliliters.

Methylene blue : A heterocyclic aromatic compound with the molecular formula C16H18N3SCl.

Metalloid : A chemical element or substance possessing properties of both metals and non-metals.

Melting point : Also liquefaction point.

The temperature at which a substance changes state from a solid to a liquid. It depends on pressure and is usually specified for a given substance under standard conditions. The melting point of a substance is identical to its freezing point.

Melting : The phase transition of a substance from a solid to a liquid.

Metal : Any chemical element which is a good conductor of both electricity and heat and which readily forms cations and ionic bonds with non-metals.

Matter : Any substance that has mass and takes up space by having volume.

Mass spectrometry (MS) : An analytical technique that measures the mass-to-charge ratio of ions in a chemical sample by bombarding the sample with electrons to the point of ionization and then separating the charged fragments by subjecting them to an electric or magnetic field, typically in order to determine the elemental or isotopic signatures of an unknown substance, the masses of its constituent particles, and/or the identities or structures of the molecules within it. The results are presented as a mass spectrum, a plot of the intensity of ion signals as a function of the mass-to-charge ratio.

Mass number (A) : Also atomic mass number or nucleon number.

The total number of protons and neutrons (together known as nucleons) within the nucleus of an atom. It determines the atomic mass of the atom. Mass number varies between different isotopes of the same chemical element, and is often included either after the element’s name (as in carbon-12) or as a superscript to the left of the element’s symbol (as in ¹²C) to identify a specific isotope.

Mass : A property of physical matter that is a measure of its resistance to acceleration when a net force is applied. The SI base unit for mass is the kilogram (kg).

Malleability : See ductility.

Manometer : An instrument used to measure pressure invented by Evangelista Torricelli in 1643.

London dispersion forces : A type of weak intermolecular force.

Liquid : One of the four fundamental states of matter, characterized by nearly incompressible fluid particles that retain a definite volume but no fixed shape.

Light : Also referred to as visible light.

The portion of the electromagnetic spectrum which is visible to the unaided human eye.

Ligand : An ion, functional group, or other molecule that binds to a central metal atom to form a coordination complex. Such bonding can range from covalent to ionic, but generally involves formal donation of one or more of the ligand’s electron pairs to the metal.

Leveling effect : The effect of a solvent on the chemical properties of acids or bases which are dissolved in the solvent. The strength of a strong acid is limited or “leveled” by the basicity of the solvent, and likewise the strength of a strong base is limited by the acidity of the solvent, such that the effective pH of the solution is higher or lower than might be suggested by the acid’s or base’s dissociation constant.

Lattice : The unique arrangement of atoms or molecules in a crystalline liquid or solid.

Kinetic energy : The energy of an object due to its motion.

Lanthanides  : Also lanthanoids.

The periodic series of metallic elements with atomic numbers 57 through 71, from lanthanum through lutetium.

Ketone : An organic compound with a carbonyl group between two carbon atoms.

Kinetics : A subfield of chemistry specializing in reaction rates.

Kelvin (K) : The SI base unit of temperature (symbol: K). The Kelvin scale is an absolute thermodynamic temperature scale that uses absolute zero as its null point.

Joule (J) : The SI unit of energy (symbol: J). One joule is defined as one Newton-meter.

Isotope : A variant of a particular chemical element which differs in the number of neutrons present in the nucleus. All isotopes of a given element have the same number of protons in each atom.

Isomers : Ions or molecules with identical chemical formulas but distinct structures or spatial arrangements. Isomers do not necessarily share similar properties. The two main types of isomers are structural isomers and stereoisomers.

Ionization : The breaking up of a chemical compound into separate ions.

Isoelectronicity : The phenomenon of two or more chemical species (atoms, molecules, ions, etc.) being composed of different elements but having the same number of valence electrons and the same structural arrangement (i.e. the same number of atoms with the same connectivity). Isoelectronic species typically show useful consistency and predictability in their chemical properties.

Ion : A molecule that has gained or lost one or more electrons from its neutral state and therefore possesses a negative or positive electric charge.

Ionic bond : An electrostatic attraction between oppositely charged ions.

International Union of Pure and Applied Chemistry (IUPAC) : An international federation of chemists that is recognized as the world authority in developing standards for chemical nomenclature and other methodologies in chemistry.

Intensive property : A physical quantity whose value does not depend on the size of the system or the quantity of matter for which it is measured. Examples include density, temperature, and pressure. Contrast extensive property.

Inorganic chemistry : The branch of chemistry concerning the chemical properties and reactions of inorganic compounds. Contrast organic chemistry.

Insulator : Any material that resists the flow of an electric current. Contrast conductor.

Inorganic compound : Any chemical compound that does not contain carbon, though there are exceptions. Contrast organic compound.

Induced radioactivity : radioactivity caused by bombarding a stable isotope with elemental particles, forming a radioactive isotope

Indicator : A special compound added to a solution that changes color depending on the acidity of the solution. Different indicators have different colors and are effective within different pH ranges.

Ideal solution : A solution for which the gas phase exhibits thermodynamic properties analogous to those of a mixture of ideal gases.

Ideal gas law : A chemical law which states that the volume of a gas is proportional to the amount of gas and its Kelvin temperature and inversely proportional to its pressure.

Ideal gas constant : Also universal gas constant.

The proportionality constant in the ideal gas law, defined as 0.08206 L·atm/(K·mol).

Hydrolysis : The cleavage of a chemical bond by the addition of water.

Hydrogen bond : A form of electrostatic interaction between an electronegative atom and a hydrogen atom bound to a second electronegative atom. Hydrogen bonding is unique because the small size of the hydrogen atoms permits proximity of the interacting electrical charges, and may occur as an intermolecular or intramolecular force.

Hydrate : Any substance that contains water or its constituent elements, or any compound formed by the addition of water or its elements to another molecule.

Hess’ law of constant heat summation : Also simply called Hess’ law.

A law of physical chemistry which states that the total enthalpy change during the course of a chemical reaction is the same whether the reaction is completed in one step or in multiple steps.

Heat : Energy transferred from one system to another by thermal interaction.

Heat of fusion : See enthalpy of fusion.

Halogen : Any of the five non-metallic elements of Group 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

Hadron : A subatomic particle of a type including the baryons and mesons that can take part in the strong interaction.

Group : Also family.

A vertical column of the periodic table of the elements and the elements that share it. Contrast period.

Ground glass joint : An apparatus designed to quickly and easily fit two pieces of leak-tight glassware together, featuring ground glass surfaces and typically a custom-made conical taper.

Gram-atom : One gram-atom of an element is defined as a collection of 6.023X10^23 atoms.

Gibbs energy : A value that indicates the spontaneity of a reaction. Usually symbolized as G.

Geochemistry : The study of the chemistry and chemical composition of the Earth and geological processes.

Gay-Lussac’s law : A chemical law used for each of the two relationships derived by French chemist Joseph Louis Gay-Lussac and which concern the properties of gases, though the name is more usually applied to his law of combining volumes.

Gas chromatography : A type of chromatography commonly used in analytical chemistry to isolate and analyze chemical compounds that can be vaporized without decomposition. Gas chromatography is often used to test the purity of substances, to identify unknown substances, and to measure the relative amounts of the different components of mixtures.