Glossary - Uu Vv
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| Ultraviolet light | (UV) ultraviolet; ultraviolet radiation; ultraviolet region; Electromagnetic radiation with wavelength longer than that of x-rays but shorter than that of visible light. Ultraviolet light can break some chemical bonds and cause cell damage. |
| Uncertainty principle | Heisenberg's uncertainty principle; Heisenberg principle; indeterminacy; indeterminacy principle. The exact momentum and exact location of a particle cannot be specified. Werner Heisenberg stated that the product of uncertainties in location and momentum measurements can never be smaller than h/4  , where h is Planck's constant. |
| Unimolecular reaction | A reaction that involves isomerization or decomposition of a single molecule. |
| Unit | A standard for comparison in measurements. For example, the metre is a standard length which may be compared to any object to describe its length. |
| Unit cell | The simplest arrangement of atoms or molecules that regularly repeats in a crystal structure. |
| Universal indicator | A universal indicator is an indicator which undergoes several colour changes over a wide range of pH. The colour is used to "indicate" pH directly. Universal indicators are usually mixtures of several indicators. |
| Unpaired spin | ( ) unpaired electron. A single electron occupying an orbital. |
| Unsaturated compound | An organic compound with molecules containing one or more double bonds. |
| Unsaturated fat | A lipid containing one or more carbon-carbon double bonds. Unsaturated fats tend to be oily liquids and are obtained from plants. |
| Unsaturated solution | A solution with a concentration lower than its equilibrium solubility. |
| Vacuum | absolute vacuum. A volume which contains no matter. |
| Valence | The number of hydrogen atoms that typically bond to an atom of an element. For example, in H2O, oxygen has a valence of 2; carbon in CH4 has a valence of four. |
| Valence bond | In the valence bond theory, a valence bond is a chemical bond formed by overlap of half-filled atomic orbitals on two different atoms. |
| Valence bond theory | VB theory; A theory that explains the shapes of molecules in terms of overlaps between half-filled atomic orbitals, or half filled "hybridized" orbitals (which are a mixture of atomic orbitals). |
| Valence electron | Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n. For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Germanium (Ge) has the ground state electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2; the 4s and 4p electrons are the valence electrons. |
| Valence shell | The shell corresponding to the highest value of principal quantum number in the atom. The valence electrons in this shell are on average farther from the nucleus than other electrons; they are often directly involved in chemical reaction. |
| Valence shell electron pair repulsion theory | VSEPR. A model that explains the shapes of molecules by assuming that electron pairs arrange themselves around atoms in a way that minimizes electron-electron repulsions. |
| Valine | ((CH3)2CHCH(NH2)COOH) Val;  -aminoisovaleric acid. A naturally occuring aliphatic amino acid with a nonpolar isopropyl side chain. |
| Van der Waals equation | A semiempirical equation that describes the relationship between pressure (P), volume (V), temperature (T), and moles of gas (n) for a real gas. The equation is (P + n2a/V2)(V - nb) = nRT, where a and b are constants that include the effects of molecular attractions and molecular volume. a and b are usually fitted to experimental data for a particular gas. |
| Van der Waals force | A force acting between nonbonded atoms or molecules. Includes dipole-dipole, dipole-induced dipole, and London forces. |
| Van der Waals radius | One half the distance between two nonbonded atoms, when attractive and repulsive forces between the atoms are balanced. |
| Vapour pressure | The partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance. |
| Vapour pressure lowering | Vapour pressure depression A colligative property of solutions. The vapour pressure of a solution is always lower than the vapour pressure of the pure solvent; the ratio of solution to pure solvent vapour pressures is approximately equal to the mole fraction of solvent in the solution. |
| Variable | A quantity that can have many possible values. In designing experiments, variables that affect measurements must be identified and controlled. For example, an experiment that measures reaction rates must control temperature, because temperature is a variable that can change the rate of reaction. |
| Vertical ionization energy | The energy required to remove an electron from an atom, molecule, or ion in the gas phase without moving any nuclei. The vertical ionization energy is greater than or equal to the adiabatic ionization energy. |
| Vinyl | polyethylene. A polymer made by linking ethylene (CH2=CH2) or substituted ethylene molecules together. |
| Viscosity | ( ) coefficient of viscosity. The resistance a liquid exhibits to flow. Experimentally, the frictional force between two liquid layers moving past each other is proportional to area of the layers and the difference in flow speed between them. The constant of proportionality is called "viscosity" or "coefficient of viscosity", and is given the symbol |
| Visible light | Visible light is electromagnetic radiation with a wavelength between 400 and 750 nm. |
| Vitamin | A substance that is critical for proper functioning of a living organism that the organism is unable to produce in sufficient quantities for itself. |
| Volatile | volatility. A solid or liquid material that easily vaporizes. A material with a significant vapour pressure. |
| Volt | (V) The SI unit of electrical potential. One volt equals one joule per coulomb. |
| Voltage | (V) A measured electric potential, in volts. |
| Voltaic cell | galvanic cell. An electrochemical cell that spontaneously generates electrical energy. |
| Voltaic pile | An early battery consisting of disks of dissimilar metals (usually zinc and copper) separated by moist paper or cloth soaked in an electrolyte solution. |
| Voltammeter | An instrument for measuring voltages and amperages. |
| Volume | (V) 1. The amount of space an object takes up. 2. The amount of space a container can hold. The SI unit of volume is the cubic meter (m3); the litre is a popular unit of volume in chemistry. |
| Volume percentage | ((v/v)%) Volume percentages express the concentration of a component in a mixture or an element in a compound. For example, 95% ethanol by volume contains 95 mL of ethanol in 100 mL of solution (NOT in 100 mL of water!). |
| Vulcanization | vulcanization of rubber. A process of combining rubber with sulfur or other substances that causes the polymer chains to crosslink, making them stronger and more elastic. |
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