1. Which of the structures below represent a linear molecule?
QUESTION:
Which of the structures below represent a linear molecule?
A. 1 and 2, B. 2 and 3, C. 3 and 4, D. 1 and 4
A triatomic molecule or ion is either linear or bent.
Linear if three atoms are on a straight line.
Linear if steric number of central atom is 2, 5; bent if 3, 4
Steric number = # of regions of high electron density around the atom = (# of bonds) + (# lone pairs)
SCRIPT:
Which of the structures shown here represent a linear molecule?
A. 1 and 2, B. 2 and 3, C. 3 and 4, D. 1 and 4
PAUSE CLICK
If a molecule or ion has only three atoms, then its shape can be described only two ways ---- linear or bent.
CLICK
The shape is linear you can draw a straight line through all three atoms. Otherwise, it’s bent.
We can use the Lewis structure to determine if the shape is linear by determining the steric number for the central atom.
If the steric number is 2 or 5. If the steric number is 3 or 4, it’s bent.
The steric number of an atom is the number of regions of high electron density around the atom. It’s the total number of bonds and lone pairs.
Each single bond counts as one region. Each lone pair also counts as one region. A double bond counts as just one region. So does a triple bond.
Let’s examine the structures….
The central atoms for our four structures are….
Carbon for hydrogen cyanide
Carbon for carbon dioxide
CLICK Oxygen for water
And sulfur for sulfur dioxide
Now, let ‘s determine the steric number.
For the carbon atom structure 1, the steric number is 2.
The single bond counts as one region of high electron density.
HIGHLIGHT
And the triple bond counts as another.
TEXTBOX “S.N.=2” left of structure 1
How about structure 2?
This double bond on the left counts as one region of high electron density.
And this double bond counts as another region.
Therefore the steric number for the carbon atom in structure 2 is 2.
There are two groups of electrons sticking out of the carbon atom.
TEXTBOX “S.N.=2” right of structure 2.
For structure 4, the steric number is 4.
There are four groups of electrons sticking out of the oxygen.
One…
HIGHLIGHT single bond
Two…
HIGHLIGHT other single bond
Three…
HIGHLIGHT lone pair
Four…
HIGHLIHGT other lone pair.
The steric number of the oxygen atom in water is 4.
TEXTBOX “S.N.=4” next to structure 3.
Fnially, for the sulfur atom in structure four, the steric number is 3.
HIGHLIGHT double bond
TEXTBOX “S.N.=3” next to structure 4
Therefore, the correct answer is A. Of the four molecules given here, only hydrogen cyanide and carbon dioxide are linear.
CONTINUED ON NEXT SLIDE

2. Q. Why is a molecule or ion linear if steric number of central atom is 2? Why is it bent if steric number is 3?
VSEPR (valence shell electron pair repulsion) theory: “regions of high electron density are oriented as far away from one another as possible” Be H H
Let’s try to understand how the steric number determines the shape of a triatomic molecule.
The explanation is actually quite simple.
CLICK We know that electrons repel each other…. Therefore,
We expect regions of high electron density around an atom to be as far away from each other as possible.
This idea is known as the VSEPR theory. VSEPR stands for valence shell electron pair repulsion.
Let’s examine the possible cases where the steric number is 2.
CLICK CLICK
Here, we have three possible ways that an atom ---- X---- can have a steric number of two.
In order for two regions of high electron density to be as far away from each other as possible, then they
must be oriented 180 degrees apart. In other words, they must be sticking out of the atom in
completely opposite directions.
It can have two single bonds and no lone pair.
An example of this is beryllium hydride.
CLICK CLICK CLICK
Or it can have two double bonds…
An example is carbon dioxide.
Or it can have a single bond and a triple bond.
An example is hydrogen cyanide
Let’s examine what happens if the steric number is three.
There will be three groups of electrons sticking out of the central atom.
CLICK
In order to be as far away from each other as possible, the groups must be oriented 120 degrees apart, if the three groups are identical.
If they’re not, as in the case shown here, the angles would still be very close to 120 degrees.
We say that the central atom in this case has a trigonal planar geometry.
CALLOUT “Trigonal planar center” pointing to “X”
The groups of electrons sticking out of the central atom are all on the same plane and are pointing towards the corners of a triangle.
CLICK Since a triatomic molecule has only three atoms, only two of the three groups can have an atom on the other side
….and one of the groups must be a lone pair
HIGHLIGHT lone pair
For example, in sulfur dioxide, where the sulfur atom is the central atom
CLICK The two oxygen atoms would be located here …
CLICK and here…
CLICK If we were to trace a path from one oxygen through sulfur
to the other oxygen,
CLICK we can see that the atoms are not all in one line. This molecule, we say, is bent.
CONTINUED ON NEXT SLIDE C N S O H O O C O

3. Why is a triatomic molecule or ion bent if the steric number of central atom is 4?
VSEPR Theory: four regions of high electron density stay as far away from one another by being oriented towards the corners of a tetrahedron.
Ideal: deg.
Water: deg. H O
Why is a triatomic molecule or ion bent if the steric number of the central atom is 4?
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.
Since we’re dealing here with a triatomic molecule. Only two of these corners would have an atom.
CLICK CLICK
If we trace a path from ANY corner of the tetrahedron through the central atom…
CLICK
To another corner…
CLICK We can see that that the the three atoms are not on a straight line. For a perfect tetrahedron,
the angle that we trace is, in fact, very close to degrees. We get a perfect tetrahedron
if all four groups sticking out of the central atom are identical. In the case of water, they’re not.
Two are single bonds, and the other two are lone pairs. The lone pairs exert a stronger repulsion on neighboring groups than the single bonds. As a result, the H-O-H angle in water is degrees.
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 our cube. You can see that it is a perfectly symmetrical pyramid --- the four faces are equilateral triangles.
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 through each of these faces.
CLICK CLICK PAUSE
CLICK CLICK PAUSE CLICK CLICK PAUSE H
Tetra = 4
Hedron = side

4. Three “equatorial groups” – on same plane, 120 deg. apart Cl
Why is a triatomic molecule or ion LINEAR if the steric number of central atom is 5?
VSEPR Theory:
five regions of high electron density stay as far away from one another by being oriented towards the corners of a trigonal bipyramid.
Three “equatorial groups” – on same plane, 120 deg. apart Cl I
Two axial groups perpendicular to plane of the equatorial groups
Why is is the molecule linear if the steric number of the central atom is 5?
CLICK According to VSEPR theory, if you have five groups of electrons sticking out of an atom,
They will be oriented towards the corners of a trigonal bipyramid.
CLICK Here’s what it looks like.
CLICK
Three of the groups are on the same plane, 120 degrees apart.
CLICK CLICK CLICK
These point towards the corner of a triangle.
Two of the groups are pernpendicular to to the equaltorial groups. One directed upward,.
CLICK the other downward.
An example of a ion with this geometry is ICl2 with a –1 charge.
This ion has five groups sticking out of the central atom.
Two single bonds
And
Three lone pairs.
Because lone pairs repel other groups more strongly than single bonds, they take up the equatorial positions.
The equatorial positions are spaced farther apart.
So, if you imagine iodine in the center of the earth.
CLICK The lone pairs will be sticking out towards the equator.
The single bonds to the chlorine atoms will be directed toward the north pole.
CLICK and the south pole.
IF we were to trace a path from one chlorine atom, through the central atom, towards the other chlorine atom.
CLICK … we can see that this ion is linear.
CLICK PAUSE END RECORDING Cl
Ex. ICl2-
Two single bonds (I-Cl)
Three lone pairs (equatorial positions)

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