Chemistry Reference
In-Depth Information
H
Figure 13.5
Tetrahedral
Orientation of Electron Pairs and
Hydrogen Atoms
(a) Tetrahedral orientation of the
electron pairs around the carbon
atom. (b) A tetrahedral molecule is
produced when the four hydrogen
nuclei are attached. (c) The molecule
is usually shown as a ball-and-stick
model, in which each stick represents
an electron pair.
C
H
C
H
H
(a)
(b)
(c)
We add the three fluorine atoms and again get a symmetrical molecule:
F
F
B
F
In this case, we call the molecular shape
trigonal planar
because the triangle
of fluorine atoms and the boron atom all lie in the same plane.
Our next example is methane. The 4 outermost carbon electrons plus
the 4 electrons from the hydrogen atoms total 8. These are distributed in four
pairs as far apart as possible. In this case, the distribution is toward the corners
of a tetrahedron (Figure 13.5a). Note that the electrons are not limited to a single
plane. The addition of the hydrogen atoms produces a
tetrahedral molecule
(Figure 13.5b and c).
Ammonia, is our next case. The 5 outermost electrons in the nitro-
gen atom, plus the 3 electrons from the three hydrogen atoms, again make 8.
Once more, these are distributed toward the corners of a tetrahedron:
CH
4
,
NH
3
,
N
This time, however, only three hydrogen atoms are to be attached. Although the
electrons
are located toward the corners of a tetrahedron, the molecular shape
is called
trigonal pyramidal,
not tetrahedral, because the
atoms
lie at the cor-
ners of a triangular pyramid:
Molecular geometry is defined
by the locations of the atoms.
N
H
H
H
EXAMPLE 13.8
Deduce the electronic arrangement and the molecular shape of the water mole-
cule.
