atomic mass unit(amu) One-twelfth of a mass of an
atom of the carbon-12 (^12 C) isotope; a unit used for stat-
ing atomic and formula weights; also called a dalton.
atomic number The atomic number is equal to the
number of positively charged protons in an atom’s
nucleus and determines which element an atom is. The
atomic number is unique for each element and is desig-
nated by a subscript to the left of the elemental symbol.
The atomic number for hydrogen is 1; it has one pro-
ton. Elements are substances made up of atoms with
the same atomic number. Most of the elements are met-
als (75 percent), and the others are nonmetals.
atomic orbital A one-electron wave function
describing an electron in the effective field provided by
a nucleus and the other electrons present.
See alsoMOLECULAR ORBITAL.
atomic radius Radius of an atom.
atomic weight or mass The total atomic mass (the
weighted average of the naturally occurring isotopes),
which is the mass in grams of one mole of the atom.
The atomic weight is calculated by adding the number
of protons and neutrons together. The atomic weight of
hydrogen is 1.0079 grams per mole.ATP (adenosine triphosphate) An adenine (purine
base), ribose, and three phosphate units containing
nucleoside triphosphate that releases free energy when
its phosphate bonds are hydrolyzed, and also produces
adenosine diphosphate (ADP) and inorganic phospho-
rous. This energy is used to drive endergonic reactions
in cells (chemical reactions that require energy input to
begin). ATP is produced in the cristae of mitochondria
and chloroplasts in plants and is the driving force in
muscle contraction and protein synthesis in animals. It
is the major energy source within cells.ATP synthase(proton translocating ATPase) A pro-
tein complex (a chemiosmotic enzyme), that synthesizes
ATP from ADP and from phosphate coupling with elec-
trochemical ion gradient across the membrane. It is
found in cellular membranes and the inner membrane
of mitochondria, the thylakoid membrane of chloro-
plasts, and the plasma membrane of prokaryotes. The
protein consists of two portions: a soluble fraction that
contains three catalytic sites and a membrane-bound
portion that contains anion channels. It functions in
chemiosmosis (the use of ion gradients across mem-
branes) with adjacent electron transport chains, and it
uses the energy stored across the photosynthetic mem-
brane (a hydrogen-ion concentration gradient) to add
inorganic phosphate to ADP, thereby creating ATP.
This allows hydrogen ions (H+) to diffuse into the
mitochondrion.attachment A TRANSFORMATION by which one
MOLECULAR ENTITY(the SUBSTRATE) is converted into
another by the formation of one (and only one) two-
center BONDbetween the substrate and another molec-
ular entity and which involves no other changes in
CONNECTIVITYin the substrate. For example, consider
the formation of an acyl cation by attachment of car-
bon monoxide to a CARBENIUM ION(R+):R++ CO →(RCO)+20 atomic mass unit
Atom. The smallest part of an element that contains protons, neu-
trons, and electrons