Chemistry Reference
In-Depth Information
Snapshot Review
❒
The greater the electronegativity of an atom, the stronger is its
attraction for electrons.
❒
Covalent bonds between atoms of different electronegativity are polar.
ChemSkill Builder 12.1
A. The electrons in each of the following bonds are closer to which atom, if
either?
(a)
O
¬
F
(b)
N
¬
I
(c)
C
¬
Br
13.4
Molecular Shape
We d iscuss the shapes of molecules next, before we examine the effect of polar
bonds on the properties of molecules as a whole. Both the bond polarity and
the shape can have a distinct effect on the molecule's properties.
Molecular shape
is defined by the locations of a molecule's
atoms,
not
from the location of its electrons, because, experimentally, the scientist can
determine the location of the atoms by X-ray diffraction and other studies. We
can predict the shapes of molecules from their formulas by using a few simple
rules. The ability to make such predictions enables us to understand better the
properties of the substances.
To predict the shape of a molecule from its formula, we first draw an elec-
tron dot diagram of the molecule (Sections 5.3 and 5.5). We assume that the
electron groups
around the
central atom
are as far apart as possible while
remaining attached to the central atom. The electron groups are ordinarily the
electrons in the single, double, or triple bonds, or in the lone pairs on the cen-
tral atom, which is the atom to which the other atoms are attached. Then we
bond the other atoms, using some or all of the electron groups, and finally
describe the molecule by the locations of the
atoms
.
As a first example of the process, we consider the molecule experi-
mentally known to be a
linear molecule
with two identical bonds. The electron
dot diagram has only four electrons around the beryllium, two from the outer-
most shell of that atom and one each from the two hydrogen atoms:
Molecular shape is defined by
the location of the atoms.
BeH
2
,
Be
We place the electron pairs as far apart as possible (180°) while still being attached
to the Be atom. Then we add the two hydrogen atoms to form the molecule:
BHH
We have deduced that the molecule is linear because all three atoms lie on a
straight line.
Next, we consider The B atom has three outermost electrons to con-
tribute. The three fluorine atoms each contribute their 1 unpaired electron, which
makes a total of 6 electrons available. These are distributed in three pairs sym-
metrically about the B atom:
BF
3
.
B
