(16) Molecular Shape and Polarity

Lesson 10

(16) Molecular Shape and Polarity

Obj. 16. From the name of a molecular chemical, determine the shape and polarity of the molecule.

To work on this objective, we have the chemicals inexercise 16 and we can also use all of the chemicals in exercise 15 as well.

The general approach for simple molecules is to start with a Lewis diagram for the molecule. If there are more than two atoms, focus on the central atom in the molecule. Count the number of atoms bonded to the central atom and count the number of unbonded pairs of electrons on the central atom. Determine the molecular shape accordingly. Other shapes exist but we will not deal with them in this course.

Central Atom		Molecular Shape
Bonded Atoms	Unbonded Electron Pairs	Molecular Shape
1	any number	linear
2	none	linear
2	1 or more	angular
3	none	flat triangular
3	1	triangular pyramid
4	none	tetrahedral

Determining the polarity of the molecule requires an additional step. After determining the shape, look at whether the atoms surrounding the central atom are all identical to one another. (For some shapes this additional step is not necessary.)

Molecular Shape	Are all "outside" atoms identical?	Molecular Polarity
linear	yes	nonplar
linear	no	polar
angular	-	polar
flat triangular	yes	nonpolar
flat triangular	no	polar
triangular pyramid	-	polar
tetrahedral	yes	nonpolar
tetrahedral	no	polar

Exercises

For each of the following molecular compounds, determine the shape and polarity of the molecules.

a. water

b. ammonia

c. dichlorine monoxide

d. carbon(IV) oxide

e. sulfur(IV) oxide

f. carbon tetrachloride

g. hydrogen chloride

Worked-Out Examples (15a,15b,16f)

(15a) Hydrogen bromide has the formula HBr and it has two atoms. Any molecule that has only two atoms is linear. A linear molecule might be polar, it might be nonpolar. In this case, if we look at the two ends of the molecule, the hydrogen end has a different attraction for electrons than the bromine end. Therefore, the molecule will be polar.

렁
H : Br :
렁

two atoms

linear

polar

(15b) The electron dot diagram was determined earlier. Now focus on the electrons around the central sulfur atom and look at how they are grouped. In this particular case we have a variety. The sulfur has a double bond to one oxygen, a single bond to the other oxygen, and also, an unbonded pair of electrons. Altogether that's three groups of electrons. Having three groups of electrons around the central atom results in a flat triangular arrangement of electrons. Those three groups include bonds to two atoms and an unbonded pair of electrons, so we get an angular molecule. The shape of the molecule is dictated by the arrangement of the electrons, but the name of that shape is dictated by the position of the atoms with respect to one another. We have a triangular arrangement of electrons, but we have an angular arrangement of atoms.

To determine the polarity or nonpolarity of this molecule, we can look at those three corners of the triangular electron arrangement. There are oxygen atoms at two corners and a pair of unbonded electrons at the third corner. This results in an asymmetric arrangement of electron pulling, and the molecule will be polar. You can also note that this is an angular molecule and all angular molecules are polar (for the reasons just described).

렁 렁 렁
O : : S : O :
렁 렁

S is bonded to 2 atoms.

S has 1 unbonded pair
of electrons.

angular

polar

(16f) Carbon tetrachloride has one carbon and four chlorine atoms. The electron dot diagram for that has a carbon in the center and four chlorine atoms around it. The carbon atom has its own four valence electrons and gains one from each of the four chlorine atoms. Each chlorine atom in return gains one of those four valence electrons from the carbon.

렁
: Cl :
렁 렁 렁
: Cl : C : Cl :
렁 렁 렁
: Cl :
렁

To determine shape and polarity we have to look at the electron arrangement around the central atom; in this case that's the carbon atom, and the carbon atom has four groups of electrons, so it has a tetrahedral arrangement of electrons. Each of those four groups of electrons is a bond to a chlorine atom. There are no unbonded pairs of electrons, so the shape of the molecule is the same as the shape of the electron arrangement. It is a tetrahedral molecule.

C bonded to 4 atoms.

C has no unbonded electron pairs.

tetrahedral

Regarding the polarity of this molecule, it is true that each carbon-to-chlorine bond is a polar bond in which the electrons are going to be pulled away from the carbon toward the chlorine. This is because chlorine is more electronegative. However, the molecule is symmetric. There is a three-dimensional symmetry in this molecule that has the electrons pulled by the chlorine being pulled out equally in all directions. That symmetry cancels out the polarity of the bonds and carbon tetrachloride has nonpolar molecules.

All "outside' atoms are the same.

symmetric

nonpolar

Answers to Exercises

15. For each of the following molecule compounds, draw the Lewis (electron dot) diagram for it.

a. hydrogen bromide molecules are linear and polar

b. sulfur dioxide molecules are angular and polar

c. sulfur trioxide molecules are flat triangular and nonpolar

d. phosphorus trichloride molecules are triangular pyramid and polar

e. hydrogen sulfide molecules are angular and polar

f. carbon monoxide molecules are linear and polar

g. carbon disulfide molecules are linear and nonpolar

16. For each of the following molecular compounds, determine the shape and polarity of the molecules.

a. water molecules are angular and polar

b. ammonia molecules are triangular pyramid and polar

c. dichlorine monoxide molecules are angular and polar

d. carbon(IV) oxide molecules are linear and nonpolar

e. sulfur(IV) oxide molecules are angular and polar

f. carbon tetrachloride molecules are tetrahedral and nonpolar

g. hydrogen chloride molecules are linear and polar

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