1. Which molecule has the smallest bond angles? A. CH4, B. NH3, C. H2O
QUESTION:
Which molecule has the smallest bond angles?
A. CH4, B. NH3, C. H2O
Steric Number = number of regions around an atom with high electron density; number of bonds and lone pairs
SCRIPT:
Which molecule has the smallest bond angles?
Methane…. Ammonia…., or water
PAUSE
The first thing we need to do to answer this question is draw the Lewis structures of methane, ammonia, and water.
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Then we determine the steric number for the central atom in each molecule.
CLICK The steric number is the number of regions around an atom with high electron density. This is just equal to the number of bonds
and lone pairs. Each lone pair counts as one region. A single bond counts as one region.
Let’s examine the structures….
The central atoms for our structures are….
Carbon for methane
HIGHLIGHT C
Nitrogen for ammonia
HIGHLIGHT N
and Oxygen for water
HIGHLIGHT O
Now, let ‘s determine the steric number for the central atoms.
For the carbon atom in methane, the steric number is 4.
Each single bond counts as one region of high electron density.
Here we have four single bonds
One…
HIGHLIGHT single bond
Two…
HIGHLIGHT other single bond
Three…
HIGHLIGHT third single bond
Four…
HIGHLIGHT lfourth single bond
How about ammonia?
Here we also have a steric number of 4 for the nitrogen atom.
HIGHLIHGT lone pair.
For water, the steric number of the central atom is also 4.
There are four groups of electrons sticking out of the oxygen.
HIGHLIGHT lone pair
HIGHLIHGT other lone pair.
The steric number of the oxygen atom in water is 4.
CONTINUED ON NEXT SLIDE

2. VSEPR Theory: four regions of high electron density stay as far away from one another by being oriented towards the corners of a tetrahedron.
NH3 107 deg.
H2O deg.
CH4
109o 28’ or 109.5o H H H H
According to the VSEPR theory, four regions of high electron density would need to be oriented towards the corners of a tetrahedron
in order to be as far away from each other as possible.
But what’s a tetrahedron?
Imagine a cube….
CLICK IF we put our central atom in the middle of this cube.
CLICK Then the four corners of the tetrahedron are indicated here by the green circles.
What we just drawn in a 3-dimensional representation of the shape of methane. Carbon is in the middle of the tetrahedron, and
each of the four corners of the tetrahedron has an oxygen atom.
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If we trace a path from ANY corner of the tetrahedron through the central atom…
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To another corner…
CLICK and measure the angle, we find that this angle is 109 degrees and 28 minutes.
To the nearest tenth of a degree, it’s degrees.
This angle is called the “tetrahedral angle”.
And we say that the central atom is a “tetrahedral center”
When all the groups sticking out of the central atom are identical, which is the
case in methane, we get a perfect tetrahedron. This is not the case with ammonia and water.
In ammonia, one of the groups is a lone pair, so there’s no atom at the other end of that group.
CLICK CLICK A lone pair repels other groups more strongly than a single bond. Therefore, the single bonds in ammonia
end up being slightly closer together. The actual bond angle is around 107 degrees, instead of
In water, two of the groups are lone pairs.
and the single bonds slight closer still. The actual bond angle is around degrees.
To summarize,
CLICK The central atom in CH4 has a perfect tetrahedral geometry around it.
Because of lone pairs, the central atoms in NH3 and H2O have a distorted tetrahedral geometry around them.
CONTINUED ON NEXT SLIDE
The central atom in CH4 has a perfect tetrahedral geometry around it. Because of lone pairs, the central atoms in NH3 and H2O have a distorted tetrahedral geometry around them.

3. Which molecule has the smallest bond angles? A. CH4, B. NH3, C. H2O
QUESTION:
Which molecule has the smallest bond angles?
A. CH4, B. NH3, C. H2O
The bond angles are degrees for methane,
CALLOUT
107 for ammonia
And degrees for water
Therefore, the correct answer is C.
In case you’re wondering about the origin of the term tetrahedron, here’s what a tetrahedron looks like if we
carve it out of a cube. You can see that it is a perfectly symmetrical pyramid --- the four faces are equilateral triangles.
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Tetra is the greek for the number 4. And hedron is greek for sides. A tetrahedron is a four-sided figure.
Let’s relate this to the cube. To find the one corner of a tetrahedron, pick any one of the corners of the cube.
You’ll notice that three faces of the cube intersect at this corner. To get to the other three corners of the tetrahedron,
simply imagine a diagonal line starting from the corner you picked and go across each of these faces.
CLICK CLICK PAUSE
CLICK CLICK PAUSE CLICK CLICK PAUSE
CONTINUED ON NEXT SLIDE
Tetra = 4
Hedron = side

4. NH3 Trigonal pyramidal H2O bent CH4 tetrahedral
Here are some three-dimensional representations of methane, ammonia, and water.
The methane molecule’s shape is tetrahedral. The carbon atom is in the center of the tetrahedron and the four hydrogen atoms
are at the four corners of the tetrahedron
In ammonia, the nitrogen atom is in the middle of a tetrahedron and
three hydrogen atoms are at three of the four corners of a tetrahedron. The other fourth has is empty. The shape
is called trigonal pyramidal, with the nitrogen atom at the top of the pyramid and the three hydrogen atoms forming the triangular base.
In water, only two of the tetrahedral corners are occupied by hydrogen atom. The molecule’s shape is simply described as bent.
CLICK PAUSE END RECORDING

5. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08