cal compound contains an excess of
an element, e.g. a peroxide.
perchlorate See chlorates.
perchloric acid See chloric(vii)
acid.
perdisulphuric acid See peroxosulphuric(vi) acid.
perfect gas See ideal gas; gas.
perfect solution See raoult’s law.
pericyclic reaction A type of concerted chemical reaction that proceeds through a cyclic conjugated
transition state. Pericyclic reactions
include *cheletropic reactions, some
*cycloadditions, *sigmatropic reactions, and *electrocyclic reactions.
They can be understood using *frontier orbital theory or the *Woodward–
Hoffmann rules.
peridot See olivine.
period 1. The time taken for one
complete cycle of an oscillating system or wave. 2. See periodic table.
period doubling A mechanism for
describing the transition to *chaos in
certain dynamical systems. If the
force on a body produces a regular
orbit with a speciÜc *period a sudden
increase in the force can suddenly
double the period of the orbit and
the motion becomes more complex.
The original simple motion is called
a one-cycle, while the more complicated motion after the period doubling is called a two-cycle. The
process of period doubling can continue until a motion called an n-cycle
is produced. As n increases to inÜnity
the motion becomes non-periodic.
The period-doubling route to chaos
occurs in many systems involving
nonlinearity, including lasers and
certain chaotic chemical reactions.
The period-doubling route to chaos
was postulated and investigated by
the US physicist Mitchell Feigenbaum in the early 1980s. Routes to
chaos other than period doubling
also exist.
periodic acid See iodic(vii) acid.
periodic law The principle that
the physical and chemical properties
of elements are a periodic function of
their proton number. The concept
was Ürst proposed in 1869 by Dimitri
*Mendeleev, using relative atomic
mass rather than proton number, as
a culmination of efforts to rationalize
chemical properties by Johann
Döbereiner (1817), John Newlands
(1863), and Lothar Meyer (1864). One
of the major successes of the periodic
law was its ability to predict chemical and physical properties of undiscovered elements and unknown
compounds that were later conÜrmed
experimentally. See periodic table.
periodic table A table of elements
arranged in order of increasing proton number to show the similarities
of chemical elements with related
electronic conÜgurations. (The original form was proposed by Dimitri
Mendeleev in 1869 using relative
atomic masses.) In the modern short
form, the *lanthanoids and *actinoids are not shown. The elements
fall into vertical columns, known as
groups. Going down a group, the
atoms of the elements all have the
same outer shell structure, but an increasing number of inner shells. Traditionally, the alkali metals were
shown on the left of the table and
the groups were numbered IA to
VIIA, IB to VIIB, and 0 (for the noble
gases). All the elements in the middle
of the table are classiÜed as *transition elements and the nontransition
elements are regarded as main-group
elements. Because of confusion in
the past regarding the numbering of
groups and the designations of subgroups, modern practice is to number the groups across the table from
403
periodic table
p
www.AzShimi.ir www.AzShimi.com
an element, e.g. a peroxide.
perchlorate See chlorates.
perchloric acid See chloric(vii)
acid.
perdisulphuric acid See peroxosulphuric(vi) acid.
perfect gas See ideal gas; gas.
perfect solution See raoult’s law.
pericyclic reaction A type of concerted chemical reaction that proceeds through a cyclic conjugated
transition state. Pericyclic reactions
include *cheletropic reactions, some
*cycloadditions, *sigmatropic reactions, and *electrocyclic reactions.
They can be understood using *frontier orbital theory or the *Woodward–
Hoffmann rules.
peridot See olivine.
period 1. The time taken for one
complete cycle of an oscillating system or wave. 2. See periodic table.
period doubling A mechanism for
describing the transition to *chaos in
certain dynamical systems. If the
force on a body produces a regular
orbit with a speciÜc *period a sudden
increase in the force can suddenly
double the period of the orbit and
the motion becomes more complex.
The original simple motion is called
a one-cycle, while the more complicated motion after the period doubling is called a two-cycle. The
process of period doubling can continue until a motion called an n-cycle
is produced. As n increases to inÜnity
the motion becomes non-periodic.
The period-doubling route to chaos
occurs in many systems involving
nonlinearity, including lasers and
certain chaotic chemical reactions.
The period-doubling route to chaos
was postulated and investigated by
the US physicist Mitchell Feigenbaum in the early 1980s. Routes to
chaos other than period doubling
also exist.
periodic acid See iodic(vii) acid.
periodic law The principle that
the physical and chemical properties
of elements are a periodic function of
their proton number. The concept
was Ürst proposed in 1869 by Dimitri
*Mendeleev, using relative atomic
mass rather than proton number, as
a culmination of efforts to rationalize
chemical properties by Johann
Döbereiner (1817), John Newlands
(1863), and Lothar Meyer (1864). One
of the major successes of the periodic
law was its ability to predict chemical and physical properties of undiscovered elements and unknown
compounds that were later conÜrmed
experimentally. See periodic table.
periodic table A table of elements
arranged in order of increasing proton number to show the similarities
of chemical elements with related
electronic conÜgurations. (The original form was proposed by Dimitri
Mendeleev in 1869 using relative
atomic masses.) In the modern short
form, the *lanthanoids and *actinoids are not shown. The elements
fall into vertical columns, known as
groups. Going down a group, the
atoms of the elements all have the
same outer shell structure, but an increasing number of inner shells. Traditionally, the alkali metals were
shown on the left of the table and
the groups were numbered IA to
VIIA, IB to VIIB, and 0 (for the noble
gases). All the elements in the middle
of the table are classiÜed as *transition elements and the nontransition
elements are regarded as main-group
elements. Because of confusion in
the past regarding the numbering of
groups and the designations of subgroups, modern practice is to number the groups across the table from
403
periodic table
p
www.AzShimi.ir www.AzShimi.com