third law of thermodynamics If all the thermal
motion of molecules (kinetic energy) could be removed,
absolute zero would occur (a temperature of 0 Kelvin
or –273.15°C).
See alsoFIRST LAW OF THERMODYNAMICS; SECOND
LAW OF THERMODYNAMICS.
thorium-lead dating A method to measure the age
of rocks and other materials that contain thorium and
lead through the use of the natural radioactive decay of
(^232) Th (HALF-LIFEabout 4.7 billion years) as it decays to
(^208) Pb. Because of the very long half-life, this dating
technique is usually restricted to ages greater than 10
million years.
three-center bond Bonding of three atoms by a pair
of electrons in a molecular orbital formed from the
overlap of three atomic orbitals.
three-dimensional quantitative structure-activity
relationship (3D-QSAR) The analysis of the quanti-
tative relationship between the biological activity of a
set of compounds and their spatial properties using sta-
tistical methods.
thylakoid Enclosed membrane structure inside
CHLOROPLASTs and PHOTOSYNTHETICbacteria.
tight ion pair SeeION PAIR.
titration A method in which one solution is added to
another until the reaction between the two is complete.
T-jump SeeCHEMICAL RELAXATION.
topliss tree A topliss tree is an operational scheme
for ANALOGdesign.
torquoselectivity The term refers to the preference
for “inward” or “outward” rotation of substituents in
conrotatory or disrotatory electrocyclic ring-opening
reactions.
torr A unit of measure of pressure. At sea level, 760
torr is standard pressure.
torsion angle, dihedral angle The relative position,
or angle, between the A–X bonds and the B–Y bonds
when considering four atoms connected in the order
A–X–Y–B; also is the angle between two planes defined
as A–X–Y and X–Y–B.
total energy The sum of both kinetic and potential
energy.
total ionic equation Equation for a chemical reaction
to show the predominant form of all species in aqueous
solution or in contact with water. For example:
CaCO 3 (s) + 2H+(aq) + 2Cl–(aq) Ca 2 +(aq) +
2Cl–(aq) + H 2 O(l) + CO 2 (g)
toxicity The action of poisons (including XENOBI-
OTICs) on biochemical reactions or processes in living
organisms or ecological systems. A study of this action
is the subject matter of toxicology.
toxin A poisonous material that can cause damage to
living tissues.
trace elements Elements required for physiological
functions in very small amounts that vary for different
organisms. Included among the trace elements are Co,
Cu, F, Fe, I, Mn, Mo, Ni, Se, V, W, and Zn. Excess
mineral intake may produce toxic symptoms.
tracer A radioactively tagged compound used to pro-
duce a nuclear image, often used in medicine.
trans- In inorganic nomenclature, a structural prefix
designating two groups directly across a CENTRAL
trans- 267