each subshell is written as a superscript numeral to the subshell symbol, 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 azimuthal quantum number.
(3) The magnetic quantum number m,
which governs the energies of electrons in an external magnetic Üeld.
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 psubshell (l = 1), m can have values
+1, 0, and –1; i.e. there are three
p-orbitals in the p-subshell, usually
designated p x , p y , and p z . Under normal circumstances, these all have
the same energy level.
(4) The spin quantum number m s ,
which gives the spin of the individual electrons and can have the values
+½ or –½.
According to the *Pauli exclusion
principle, no two electrons in the
atom can have the same set of quantum numbers. The numbers deÜne
the quantum state of the electron,
and explain how the electronic structures of atoms occur. See Chronology: Atomic Theory.
atomic absorption spectroscopy
(AAS) An analytical technique in
which a sample is vaporized and the
nonexcited atoms absorb electromagnetic radiation at characteristic wavelengths.
atomic clock An apparatus for
standardizing time based on periodic
phenomena within atoms or molecules. 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 radiation 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 springloaded 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 nonconducting samples, such as biological specimens.
atomicity The number of atoms in
a given molecule. For example, oxygen (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
–27 kg. This unit superseded both
the physical and chemical mass units
based on oxygen–16 and is sometimes 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 orbital See orbital.
atomic volume The relative
atomic absorption spectroscopy
48
a
www.AzShimi.ir www.AzShimi.com
electrons in each subshell is s
2 , p
6 ,
d
10 , and f
14 . The orbital quantum
number is sometimes called the azimuthal quantum number.
(3) The magnetic quantum number m,
which governs the energies of electrons in an external magnetic Üeld.
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 psubshell (l = 1), m can have values
+1, 0, and –1; i.e. there are three
p-orbitals in the p-subshell, usually
designated p x , p y , and p z . Under normal circumstances, these all have
the same energy level.
(4) The spin quantum number m s ,
which gives the spin of the individual electrons and can have the values
+½ or –½.
According to the *Pauli exclusion
principle, no two electrons in the
atom can have the same set of quantum numbers. The numbers deÜne
the quantum state of the electron,
and explain how the electronic structures of atoms occur. See Chronology: Atomic Theory.
atomic absorption spectroscopy
(AAS) An analytical technique in
which a sample is vaporized and the
nonexcited atoms absorb electromagnetic radiation at characteristic wavelengths.
atomic clock An apparatus for
standardizing time based on periodic
phenomena within atoms or molecules. 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 radiation 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 springloaded 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 nonconducting samples, such as biological specimens.
atomicity The number of atoms in
a given molecule. For example, oxygen (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
–27 kg. This unit superseded both
the physical and chemical mass units
based on oxygen–16 and is sometimes 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 orbital See orbital.
atomic volume The relative
atomic absorption spectroscopy
48
a
www.AzShimi.ir www.AzShimi.com