# How do I determine the molecular shape of a molecule?

##### 1 Answer
Jan 1, 2014

WARNING. This is a LONG document. It covers all possible shapes for molecules with up to six electron pairs around the central atom.

#### Explanation:

STEPS INVOLVED

There are three basic steps to determining the molecular shape of a molecule:

1. Write the Lewis dot structure of the molecule. That gives you the steric number (SN) — the number of bond pairs and lone pairs around the central atom.

2. Use the SN and VSEPR theory to determine the electron pair geometry of the molecule.

3. Use the VSEPR shape to determine the angles between the bonding pairs.

VSEPR PRINCIPLES:

1. The repulsion between valence electron pairs in the outer shell of the central atom determines the shape of the molecule. You must determine the steric number (SN) — the number of bonding pairs and lone pairs about the central atom.

2. Lone pairs repel more than bond bonding pairs.

A. SN = 2

What is the shape of ${\text{BeCl}}_{2}$?

The Lewis dot structure for ${\text{BeCl}}_{2}$ is

The central $\text{Be}$ atom has two bond pairs in its outer shell (SN = 2).

Repulsion between these two pairs causes the atoms to be as far apart as possible.

The shape of the molecule is linear, and the $\text{Cl-Be-Cl}$ bond angle is 180°.

B. SN = 3

There are two possibilities.

i. ${\text{AX}}_{3}$ — Three bonding pairs

What is the shape of ${\text{BF}}_{3}$?

The Lewis dot structure ${\text{BF}}_{3}$ is

The $\text{B}$ atom has three bond pairs in its outer shell.

Minimizing the repulsion causes the $\text{F}$ atoms to form an equilateral triangle about the $\text{B}$ atom, as shown below.

The shape of the molecule is trigonal planar.

All the atoms are in the same plane, and the $\text{F-B-F}$ bond angles are all 120°.

ii. $\text{AX"_2"E}$ — two bond pairs and one lone pair

What is the shape of ${\text{SO}}_{2}$?

The Lewis dot structure ${\text{SO}}_{2}$ is

The central atom, $\text{S}$, has three groups bonded to it, two oxygen atoms and a lone pair.

The electron pair geometry of ${\text{SO}}_{2}$ is trigonal planar.

It would be drawn as

The molecular shape of ${\text{SO}}_{2}$ is not trigonal planar.

In determining the molecular shape, we consider only the positions of the atoms, not the lone pairs.

Hence, the molecular shape of ${\text{SO}}_{2}$ is bent and is represented as

The lone pair of electrons occupies a relatively large volume, since they are held by only one atom.

They compress the bond angle between the oxygens and sulfur to about 119.5°.

C. SN = 4

There are four possibilities.

i. ${\text{AX}}_{4}$ — four bond pairs

What is the shape of ${\text{CH}}_{4}$?

The Lewis dot structure ${\text{CH}}_{4}$ is

The shape of this molecule, however, is not planar, as you might think from the way we draw this dot structure.

The four bond pairs are arranged about the $\text{C}$ atom, pointing toward the corners of a regular tetrahedron.

This shape minimizes the repulsion between the bond pairs.

The 109.5° angle is the same for all $\text{H-C-H}$ bond angles and is called the tetrahedral bond angle.

The shape of the ${\text{CH}}_{4}$ molecule is tetrahedral.

ii. $\text{AX"_3"E}$ — three bond pairs and one lone pair.

What is the molecular geometry of ${\text{NH}}_{3}$?

The Lewis dot structure of ${\text{NH}}_{3}$ is

The central atom, $\text{N}$, has four groups bonded to it: three hydrogen atoms and a lone pair.

The electron pair geometry of ${\text{NH}}_{3}$ is tetrahedral.

It is drawn as shown below:

Remember that, in determining the molecular shape, we consider only the positions of the atoms, not the lone pairs.

If we look only at the atoms, we see a short, rather distorted tetrahedron.

This is called a pyramid.

The ${\text{NH}}_{3}$ pyramid has a triangular base.

Hence the shape is trigonal pyramidal.

The greater repulsion of the lone pair causes the $\text{H}$ atoms in ${\text{NH}}_{3}$ to be bent closer together than the normal tetrahedral angle of 109.5°.

In ${\text{NH}}_{3}$ the observed $\text{H-N-H}$ bond angle is 107.3°.

iii. ${\text{AX"_2"E}}_{2}$ — two bond pairs and two lone pairs

What is the molecular geometry of $\text{H"_2"O}$?

The Lewis dot structure of $\text{H"_2"O}$ is

The central atom, $\text{O}$, has four groups bonded to it, two hydrogen atoms and two lone pairs.

The electron pair geometry of $\text{H"_2"O}$ is tetrahedral.

It is drawn as shown below:

The shape is called bent.

The $\text{H-O-H}$ bond angle is less than that in ${\text{NH}}_{3}$, partly because of the greater repulsions caused by two lone pairs.

In water, the observed $\text{H-O-H}$ bond angle is 104.5°.

All bond angles in ${\text{AX"_2"E}}_{2}$ molecules are significantly less than 109.5°.

D. SN = 5

There are four possibilities.

i. ${\text{AX}}_{5}$ — five bond pairs

What is the molecular shape of ${\text{PCl}}_{5}$?

The Lewis dot structure of ${\text{PCl}}_{5}$ is

Atoms past $\text{Si}$ in the Periodic Table can “expand their octet” and have more than eight valence electrons.

Here, the $\text{P}$ atom has ten valence electrons.

If you view the $\text{P}$ atom at the centre of a sphere like the earth, you have one $\text{Cl}$ atom at the North Pole, one $\text{Cl}$ atom at the South Pole, and three $\text{Cl}$ atoms spread evenly around the equator.

Note that the $\text{Cl}$ atoms occupy two types of positions.

The two $\text{Cl}$ atoms that are on a straight line that passes through the $\text{P}$ nucleus are said to occupy axial positions.

The other three $\text{Cl}$ atoms are in equatorial positions.

If you join the $\text{Cl}$ atoms by straight lines, the diagram looks like two trigonal pyramids joined together at the base.

The ${\text{PCl}}_{5}$ molecule has a trigonal bipyramidal shape.

There are two different bond angles in the molecule.

The axial $\text{Cl}$ atoms are at angles of 90° to the equatorial $\text{Cl}$ atoms, while the equatorial $\text{Cl}$ atoms are at angles of 120° to each other.

ii. $\text{AX"_4"E}$ — Four bond pairs and one lone pair.

What is the molecular shape of ${\text{SF}}_{4}$?

The Lewis dot structure of ${\text{SF}}_{4}$ is

The five electron pairs assume a trigonal bipyramidal geometry.

The lone pair occupies an equatorial position, because that gets it furthest away from the other electron pairs.

If you turn the molecule on its side, it looks like a see-saw, with the axial $\text{F}$ atoms at the ends, and the equatorial $\text{F}$ atoms acting as the pivot.

The shape is called a see-saw.

The axial $\text{F–S–F}$ bond angle is 173.1° rather than 180°, because of the lone pair of electrons in the equatorial plane.

The equatorial $\text{F-S-F}$ bond angle is compressed to 101.6° from its normal value of 120°.

iii. ${\text{AX"_3"E}}_{2}$ — tree bonding pairs and two lone pairs

What is the molecular shape of ${\text{ClF}}_{3}$?

The Lewis dot structure of ${\text{ClF}}_{3}$ is

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The five electron pairs assume a trigonal bipyramidal geometry.

The two lone pairs occupy the equatorial positions in order to minimize repulsions.

The bonding pairs form a T-shape, and that is the shape of the molecule.

Actually, the molecule has a “distorted-T” shape because the two lone pairs reduce the $\text{F-S-F}$ bond angle from 90° to 86.98°, and the $\text{F-S-F}$ bond angle across the top of the T is reduced from 180° to 173.96°.

iv. ${\text{AX"_2"E}}_{3}$ — two bonding pairs and three lone pairs

What is the molecular shape of ${\text{XeF}}_{2}$?

The Lewis dot structure of ${\text{XeF}}_{2}$ is

The five electron pairs assume a trigonal bipyramidal geometry.

The three lone pairs occupy the three equatorial positions, so what you have is a xenon atom with three lone pairs pointing toward the corners of an equilateral triangle, with one $\text{F}$ atom below the triangle and another above.

The result is a linear shape for the ${\text{XeF}}_{2}$ molecule, and the $\text{F-Xe-F}$ bond angle is 180°.

E. SN = 6

There are five possibilities.

i. ${\text{AX}}_{6}$ — six bonding pairs

What is the molecular shape of ${\text{SF}}_{6}$?

The Lewis dot structure of ${\text{SF}}_{6}$ is

The six bonding pairs arrange themselves with four equatorial bond pairs and one more pair at each of the polar locations.

The shape is called octahedral.

Every $\text{F-S-F}$ bond angle is 90°.

ii. $\text{AX"_5"E}$ — five bonding pairs and one lone pair

What is the structure of ${\text{IF}}_{5}$?

The Lewis dot structure of ${\text{IF}}_{5}$ is

The electron pairs arrange themselves at the corners of an octahedron with a lone pair occupying one of these positions.

Four of the $\text{F}$ atoms are at the corners of a square, and one is directly above the $\text{I}$ atom.

Lines connecting the $\text{F}$ atoms form a pyramid with a square base, so the shape is square pyramidal.

The $\text{F-I-F}$ angles between neighbouring $\text{F}$ atoms are 90°.

iii. ${\text{AX"_4"E}}_{2}$ — four bond pairs and two lone pairs

What is the molecular shape of ${\text{XeF}}_{4}$?

The Lewis dot structure is

The electron pairs arrange themselves at the corners of an octahedron.

The four bonding pairs point toward the corners of a square, and the lone pairs occupy the axial positions above and below the plane of the square.

Since the four $\text{F}$ atoms are at the corners of a square, the molecular shape is square planar.

The $\text{F-Xe-F}$ bond angles between neighbouring $\text{F}$ atoms are 90°.

iv. ${\text{AX"_3"E}}_{3}$ — three bonding pairs and three lone pairs

There are no molecules that belong to the ${\text{AX"_3"E}}_{3}$ system.

However, the electron geometry is octahedral, with the three lone pairs occupying equatorial positions.

The remaining bond pairs are arranged in a T-shape with bond angles of 90°.

v. ${\text{AX"_2"E}}_{4}$ — two bonding pairs and four lone pairs

No stable ${\text{AX"_2"E}}_{4}$ molecules are known.

However, we predict the molecular shape to be linear with bond angles of 180°.

SUMMARY

To predict the shape of a molecule:

1. Write the Lewis dot structure for the molecule.

2. Determine the steric number of the central atom.

3. Decide on the electron pair orientation based on the steric number.

4. Consider the placement of lone pairs and any distortions from "regular" shapes.

5. Name the shape based on the location of atoms attached to the central atom

The table below summarizes all the molecular shapes.