Chemistry Reference
In-Depth Information
1
Bonding and Isomerism
Chapter Summary
∗
An
atom
consists of a nucleus surrounded by
electrons
arranged in
orbitals
. The electrons
in the outer shell, or the
valence electrons
, are involved in bonding.
Ionic bonds
are
formed by electron transfer from an
electropositive
atom to an
electronegative
atom.
Atoms with similar electronegativities form
covalent
bonds
by sharing electrons. A
single
bond
is the sharing of one electron pair between two atoms. A covalent bond has specific
bond
length
and
bond
energy
.
Carbon, with four valence electrons, mainly forms covalent bonds. It usually forms
four such bonds, and these may be with itself or with other atoms such as hydrogen,
oxygen, nitrogen, chlorine, and sulfur. In pure covalent bonds, electrons are shared equally,
but in
polar
covalent
bonds
, the electrons are displaced toward the more electronegative
element.
Multiple
bonds
consist of two or three electron pairs shared between atoms.
Structural
(or
constitutional
)
isomers
are compounds with the same
molecular
formulas
but different
structural
formulas
(that is, different arrangements of the atoms in
the molecule).
Isomerism
is especially important in organic chemistry because of the
capacity of carbon atoms to be arranged in so many different ways: continuous chains,
branched chains, and rings. Structural formulas can be written so that every bond is shown,
or in various abbreviated forms. For example, the formula for
n
-pentane (
n
stands for
normal) can be written as:
H
H
H
H
H
H
C
C
C
C
C
H
or
CH
3
CH
2
CH
2
CH
2
CH
3
or
HH
H
H
H
Some atoms, even in covalent compounds, carry a
formal
charge
, defined as the
number of valence electrons in the neutral atom minus the sum of the number of unshared
electrons and half the number of shared electrons.
Resonance
occurs when we can write
two or more structures for a molecule or ion with the same arrangement of atoms but
different arrangements of the electrons. The correct structure of the molecule or ion is a
resonance
hybrid
of the
contributing
structures
, which are drawn with a double-headed
arrow (
↔
) between them. Organic chemists use a curved arrow (
) to show the movement
of an electron pair.
A
sigma
(
σ
)
bond
is formed between atoms by the overlap of two atomic orbitals
along the line that connects the atoms. Carbon uses
sp
3
-hybridized orbitals
to form four
such bonds. These bonds are directed from the carbon nucleus toward the corners of a
tetrahedron. In
methane
, for example, the carbon is at the center and the four hydrogens
are at the corners of a regular tetrahedron with H-C-H bond angles of 109.5
°
.
∗
In the chapter summaries, terms whose meanings you should know appear in boldface type.
