Video Transcript
Molecules with a steric number of
four have bond angles that differ by approximately 2.5 degrees and have a
tetrahedral, trigonal pyramidal, or bent, nonlinear, shape. What is the bond angle around the
central atom in a molecule with a tetrahedral shape?
First of all, the steric number
identifies the number of groups around the central atom in a molecule. These groups may be bonded electron
pairs or nonbonded electron pairs, otherwise known as lone pairs. If a molecule has a steric number
of four, then it has four groups around the central atom. You may already be familiar with
the AXE notation, which includes the central atom, number of bonded pairs, and
number of nonbonded pairs in a molecule. The sum of the bonded pairs and
nonbonded pairs, or 𝑚 plus 𝑛, must equal four for the steric number to be
four.
Now, let’s discuss three shapes of
molecules with a steric number of four. Molecules in the AX2E2 group have a
bent or nonlinear shape. Molecules in the AX3E group have a
trigonal pyramidal shape. Finally, molecules in the AX4 group
have a tetrahedral shape.
Let’s discuss the bent shape
first. Water is an example of a molecule
with a bent shape. We can see that the molecule has
two bonded electron pairs. Each pair is shared in a covalent
bond between the central oxygen atom and a hydrogen atom. Water also has two lone pairs,
which are valence electrons that are not shared in a covalent bond.
Now, let’s examine the trigonal
pyramidal shape. Ammonia is an example of a molecule
that has a trigonal pyramidal shape. The molecule has three bonded
electron pairs. Each of these pairs is shared in a
covalent bond between the central nitrogen atom and a hydrogen atom. Ammonia also has one lone pair.
Finally, let’s discuss the
tetrahedral shape by looking at a molecule of methane. Methane has four bonded electron
pairs, and each pair is shared between the central carbon atom and a hydrogen
atom. Methane does not have any lone
pairs on the central atom.
Now we’re ready to determine the
bond angles around the central atom in each molecule. In methane, the four bonded pairs
have equal repulsive forces and all HCH bond angles are identical, with a value of
109.5 degrees. In ammonia, the lone pair occupies
more space than each bonded pair, so the HNH angle is only 107 degrees. Finally, in water, the two lone
pairs take up even more space and repel each other more strongly than the bonded
pairs. Therefore, the HOH bond angle is
even less and has a value of 104.5 degrees.
In conclusion, the bond angle
around the central atom in a molecule with a tetrahedral shape is 109.5 degrees.
