each subshell is written as a super-
script numeral to the subshell sym-
bol, and the maximum number of
electrons in each subshell is s^2 , p^6 ,
d^10 , and f^14. The orbital quantum
number is sometimes called the az-
imuthal quantum number.
(3) The magnetic quantum number m,
which governs the energies of elec-
trons in an external magneticÜeld.
This can take values of +l, +(l – 1), ...
1, 0, –1, ... –(l – 1), –l. In an s-subshell
(i.e. l = 0) the value of m = 0. In a p-
subshell (l = 1), m can have values
+1, 0, and –1; i.e. there are three
p-orbitals in the p-subshell, usually
designated px, py, and pz. Under nor-
mal circumstances, these all have
the same energy level.
(4) The spin quantum number ms,
which gives the spin of the individ-
ual electrons and can have the values
+½or –½.
According to the *Pauli exclusion
principle, no two electrons in the
atom can have the same set of quan-
tum numbers. The numbers deÜne
the quantum state of the electron,
and explain how the electronic struc-
tures of atoms occur. See Chronol-
ogy: Atomic Theory.atomic absorption spectroscopy
(AAS)An analytical technique in
which a sample is vaporized and the
nonexcited atoms absorb electromag-
netic radiation at characteristic wave-
lengths.atomic clockAn apparatus for
standardizing time based on periodic
phenomena within atoms or mol-
ecules. See ammonia clock; caesium
clock.atomic emission spectroscopy
(AES)An analytical technique in
which a sample is vaporized and the
atoms present are detected by their
emission of electromagnetic radia-
tion at characteristic wavelengths.atomic force microscope (AFM)
A type of microscope in which a
small probe, consisting of a tiny chip
of diamond, is held on a spring-
loaded cantilever in contact with the
surface of the sample. The probe is
moved slowly across the surface and
the tracking force between the tip
and the surface is monitored. The
probe is raised and lowered so as to
keep this force constant, and a
proÜle of the surface is produced.
Scanning the probe over the sample
gives a computer-generated contour
map of the surface. The instrument
is similar to the scanning tunnelling
microscope, but uses mechanical
forces rather than electrical signals.
It can resolve individual molecules
and, unlike the scanning tunnelling
microscope, can be used with non-
conducting samples, such as biologi-
cal specimens.atomicityThe number of atoms in
a given molecule. For example, oxy-
gen (O 2 ) has an atomicity of 2, ozone
(O 3 ) an atomicity of 3, benzene (C 6 H 6 )
an atomicity of 12, etc.atomic mass unit (a.m.u.)A unit
of mass used to express *relative
atomic masses. It is equal to 1/12 of
the mass of an atom of the isotope
carbon–12 and is equal to 1.660 33 ×
10 –27kg. This unit superseded both
the physical and chemical mass units
based on oxygen–16 and is some-
times called the uniÜed mass unit or
the dalton.atomic number (proton number)
Symbol Z. The number of protons in
the nucleus of an atom. The atomic
number is equal to the number of
electrons orbiting the nucleus in a
neutral atom.atomic orbitalSee orbital.atomic volumeThe relativeatomic absorption spectroscopy 48a