Download Geometric Figures - U of L Class Index

Geometric Figures
Geometric figures and the angles associated with them form an important foundation for
discussing the shapes of molecules and the bond angles in molecules. You should
become comfortable with the geometric names and angles used in this chapter.
There are 360° as you go around a circle.
The angles change as you cut the circle into different numbers of pieces. The angle of a
straight line is 180°.
Molecules whose atoms have a 180° bond angle are said to have linear geometry.
The bond between the small blue atom and the large gray atom is in the same line as the
bond between the two large gray atoms. The angle between these two bonds is 180°. If
you had to describe the orientation of the blue atom to the two gray atoms, you would say
that they are all on the same line or that they have a linear geometry.
1
The angles at the intersection of three lines equally spaced around a circle are
120°.
Molecules whose bonds are 120° apart have trigonal planar geometry.
The angle between the single bond and the double bond is 120°. The blue, black, and red
atoms are all in the same plane. If you traced a line from the blue atom to the red atom to
the other blue atom and then back to the first blue atom, you would have a triangular
shape. Thus, we describe the geometry as trigonal planar.
The angles at the intersection of two lines perpendicular and crossing each other are 90°.
2
There are no 90° bond angles in common biological molecules. However, there are
many atoms that have four bonds. What angle do the four bonds make? The make 109°
bond angles. Look at the molecules below. Are the bond angles 90° or 109°?
If you look carefully, you will see that the bond angle is more than 90°. You can also see
that the four blue atoms are not in the same plane. Two of the blue atoms are much
smaller than the other two blue atoms because, in space, they are behind the larger blue
atoms. This kind of molecule is NOT planar. If we traced a line around the molecules
going from blue atom to blue atom, we would have a tetrahedral shape. The geometry of
this molecule is tetrahedral.
Geometry Example 1:
The BF3 molecule, shown below, has a shape that is called trigonal planar. What are the
bond angles in the molecule?
Solution:
Since the angles are formed by the intersection of three lines that come together at
the center, the bond angles are 120°.
3
Geometry Example 2:
The BeCl2 molecule below has 180° bond angles. What is the geometry of this molecule?
Solution:
180° bond angles give the molecule linear geometry.
Hints for Using Geometries:
What is the most common mistake in molecular geometry?
Students often think of molecules as flat, planar structures. Once an atom makes 4 bonds,
the geometry becomes very three-dimensional. There are NO 90° bond angles in common
biological molecules. Carbon makes four bonds, but the angle between the bonds is 109°
and the geometry is tetrahedral.
The second most common mistake is not memorizing the possible geometries of biological
molecules. You must have the names and angles memorized in order to understand the
chemical and physical properties of organic and biological molecules.
4