resulting molecular orbital has two
parts, one on each side of the sigma
orbital – this is a pi orbital. It is also
possible for a delta orbital to form by
lateral overlap of two d-orbitals. In
fact, the combination of two atomic
orbitals produces two molecular orbitals with different energies. The
one of lower energy is the bonding
orbital, holding the atoms together;
the other is the antibonding orbital,
which would tend to push the atoms
apart. In the case of valence electrons, only the lower (bonding) orbital is Ülled.
In considering the formation of
molecular orbitals it is often useful
to think in terms of hybrid atomic orbitals. For instance, carbon has in its
outer shell one s-orbital and three porbitals. In forming methane (or
other tetrahedral molecules) these
can be regarded as combining to give
four equivalent sp
3 hybrid orbitals,
each with a lobe directed to a corner
of a tetrahedron. It is these that overlap with the s-orbitals on the hydrogen atoms. In ethene, two p-orbitals
combine with the s-orbital to give
three sp
2 hybrids with lobes in a
plane pointing to the corners of an
equilateral triangle. These form the
sigma orbitals in the C–H and C–C
bonds. The remaining p-orbitals (one
on each carbon) form the pi orbital.
In ethyne, sp
2 hybridization occurs to
give two hybrid orbitals on each
atom with lobes pointing along the
axis. The two remaining p-orbitals on
each carbon form two pi orbitals. Hybrid atomic orbitals can also involve
d-orbitals. For instance, square-planar
complexes use sp
2 d hybrids; octahedral complexes use sp
3
d
2 .
orbital quantum number See
atom.
ORD See optical rotary dispersion.
order In the expression for the rate
of a chemical reaction, the sum of
the powers of the concentrations is
the overall order of the reaction. For
instance, in a reaction
A + B → C
the rate equation may have the form
R = k[A][B]
2
This reaction would be described as
Ürst order in A and second order in B.
The overall order is three. The order
of a reaction depends on the mechanism and it is possible for the rate to
be independent of concentration
(zero order) or for the order to be a
fraction. See also molecularity;
pseudo order.
ore A naturally occurring mineral
from which a metal and certain
other elements (e.g. phosphorus) can
be extracted, usually on a commercial basis. Metals may be present in
ores in the native form, but more
commonly they occur combined as
oxides, sulphides, sulphates, silicates,
etc.
ore dressing See beneficiation.
oregonator A type of chemical reaction mechanism that causes an
*oscillating reaction. It is the type of
mechanism responsible for the *B–Z
reaction, and involves Üve steps of
the form:
A + Y → X
X + Y → C
A + X → 2X + Z
2X → D
Z → Y
Autocatalysis occurs as in the
*Lotka–Volterra mechanism and the
*brusselator. The mechanism was
named after Oregon in America,
where the research group that discovered it is based.
organic chemistry The branch of
chemistry concerned with compounds of carbon.
orbital quantum number
388
o
www.AzShimi.ir www.AzShimi.com
parts, one on each side of the sigma
orbital – this is a pi orbital. It is also
possible for a delta orbital to form by
lateral overlap of two d-orbitals. In
fact, the combination of two atomic
orbitals produces two molecular orbitals with different energies. The
one of lower energy is the bonding
orbital, holding the atoms together;
the other is the antibonding orbital,
which would tend to push the atoms
apart. In the case of valence electrons, only the lower (bonding) orbital is Ülled.
In considering the formation of
molecular orbitals it is often useful
to think in terms of hybrid atomic orbitals. For instance, carbon has in its
outer shell one s-orbital and three porbitals. In forming methane (or
other tetrahedral molecules) these
can be regarded as combining to give
four equivalent sp
3 hybrid orbitals,
each with a lobe directed to a corner
of a tetrahedron. It is these that overlap with the s-orbitals on the hydrogen atoms. In ethene, two p-orbitals
combine with the s-orbital to give
three sp
2 hybrids with lobes in a
plane pointing to the corners of an
equilateral triangle. These form the
sigma orbitals in the C–H and C–C
bonds. The remaining p-orbitals (one
on each carbon) form the pi orbital.
In ethyne, sp
2 hybridization occurs to
give two hybrid orbitals on each
atom with lobes pointing along the
axis. The two remaining p-orbitals on
each carbon form two pi orbitals. Hybrid atomic orbitals can also involve
d-orbitals. For instance, square-planar
complexes use sp
2 d hybrids; octahedral complexes use sp
3
d
2 .
orbital quantum number See
atom.
ORD See optical rotary dispersion.
order In the expression for the rate
of a chemical reaction, the sum of
the powers of the concentrations is
the overall order of the reaction. For
instance, in a reaction
A + B → C
the rate equation may have the form
R = k[A][B]
2
This reaction would be described as
Ürst order in A and second order in B.
The overall order is three. The order
of a reaction depends on the mechanism and it is possible for the rate to
be independent of concentration
(zero order) or for the order to be a
fraction. See also molecularity;
pseudo order.
ore A naturally occurring mineral
from which a metal and certain
other elements (e.g. phosphorus) can
be extracted, usually on a commercial basis. Metals may be present in
ores in the native form, but more
commonly they occur combined as
oxides, sulphides, sulphates, silicates,
etc.
ore dressing See beneficiation.
oregonator A type of chemical reaction mechanism that causes an
*oscillating reaction. It is the type of
mechanism responsible for the *B–Z
reaction, and involves Üve steps of
the form:
A + Y → X
X + Y → C
A + X → 2X + Z
2X → D
Z → Y
Autocatalysis occurs as in the
*Lotka–Volterra mechanism and the
*brusselator. The mechanism was
named after Oregon in America,
where the research group that discovered it is based.
organic chemistry The branch of
chemistry concerned with compounds of carbon.
orbital quantum number
388
o
www.AzShimi.ir www.AzShimi.com