Which structural formula represents a linear nonpolar molecule containing two polar bonds

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To determine if a molecule is polar or nonpolar, it is frequently useful to look at Lewis structures. Nonpolar compounds will be symmetric, meaning all of the sides around the central atom are identical - bonded to the same element with no unshared pairs of electrons. Notice that a tetrahedral molecule such as \(\ce{CCl_4}\) is nonpolar Figure (\(\PageIndex{1}\). Another non polar molecule shown below is boron trifluoride, BF3. BF3 is a trigonal planar molecule and all three peripheral atoms are the same.

Polar molecules are asymmetric, either containing lone pairs of electrons on a central atom or having atoms with different electronegativities bonded. This works pretty well - as long as you can visualize the molecular geometry. That's the hard part. To know how the bonds are oriented in space, you have to have a strong grasp of Lewis structures and VSEPR theory. Assuming you do, you can look at the structure of each one and decide if it is polar or not - whether or not you know the individual atom electronegativity. This is because you know that all bonds between dissimilar elements are polar, and in these particular examples, it doesn't matter which direction the dipole moment vectors are pointing (out or in).

A polar molecule is a molecule in which one end of the molecule is slightly positive, while the other end is slightly negative. A diatomic molecule that consists of a polar covalent bond, such as \(\ce{HF}\), is a polar molecule.

As mentioned in section 4.7, because the electrons in the bond are nearer to the F atom, this side of the molecule takes on a partial negative charge, which is represented by δ− (δ is the lowercase Greek letter delta). The other side of the molecule, the H atom, adopts a partial positive charge, which is represented by δ+. The two electrically charged regions on either end of the molecule are called poles, similar to a magnet having a north and a south pole. A molecule with two poles is called a dipole (see figure below). Hydrogen fluoride is a dipole.

For molecules with more than two atoms, the molecular geometry must also be taken into account when determining if the molecule is polar or nonpolar. The figure below shows a comparison between carbon dioxide and water. Carbon dioxide \(\left( \ce{CO_2} \right)\) is a linear molecule. The oxygen atoms are more electronegative than the carbon atom, so there are two individual dipoles pointing outward from the \(\ce{C}\) atom to each \(\ce{O}\) atom. However, since the dipoles are of equal strength and are oriented this way, they cancel out and the overall molecular polarity of \(\ce{CO_2}\) is zero.

Water is a bent molecule because of the two lone pairs on the central oxygen atom. The individual dipoles point from the \(\ce{H}\) atoms toward the \(\ce{O}\) atom. Because of the shape, the dipoles do not cancel each other out and the water molecule is polar. In the figure below, the net dipole is shown in blue and points upward.

Three other polar molecules are shown below with the arrows pointing to the more electron dense atoms. Just like the water molecule, none of the bond moments cancel out.

To summarize, to be polar, a molecule must:

1. Contain at least one polar covalent bond.
2. Have a molecular structure such that the sum of the vectors of each bond dipole moment does not cancel.

Depending on the relative electronegativities of the two atoms sharing electrons, there may be partial transfer of electron density from one atom to the other. When the electronegativities are not equal, electrons are not shared equally and partial ionic charges develop.

The greater the electronegativity difference, the more ionic the bond is. Bonds that are partly ionic are called polar covalent bonds.

• A bond between 2 nonmetal atoms that have the same electronegativity and therefore have equal sharing of the bonding electron pair
• Example: In H-H each H atom has an electronegativity value of 2.1, therefore the covalent bond between them is considered nonpolar

Polar Covalent Bond

• A bond between 2 nonmetal atoms that have different electronegativities and therefore have unequal sharing of the bonding electron pair
• Example: In H-Cl, the electronegativity of the Cl atom is 3.0, while that of the H atom is 2.1
• The result is a bond where the electron pair is displaced toward the more electronegative atom. This atom then obtains a partial-negative charge while the less electronegative atom has a partial-positive charge.This separation of charge or bond dipole can be illustrated using an arrow with the arrowhead directed toward the more electronegative atom.

• Within a molecule each polar bond has a bond dipole
• A polar molecule always contains polar bonds, but some molecules with polar bonds are nonpolar.

Polar Molecule

• A molecule in which the bond dipoles present do not cancel each other out and thus results in a molecular dipole.(see below). Cancellation depends on the shape of the molecule or Stereochemistry and the orientation of the polar bonds.

Molecular Dipole

• A result of the bond dipoles in a molecule.
• Bond dipoles may or may not cancel out thereby producing either molecules that are nonpolar, if they cancel, or polar, if they do not cancel
• Examples:
• CO2 is a linear molecule with 2  bond dipoles that are equal and oppositely directed therefore the bond polarities cancel and the molecule is nonpolar.

 
• HCN is a linear molecule with 2 bond dipoles that are in the same direction and are not equal therefore the bond polarities do not cancel and the molecule is polar

• More examples can be found on the Table: Stereochemistry of Some Common Molecules

 

STEREOCHEMISTRY OF SOME COMMON MOLECULES
Compound	Lewis Diagram	# of  Lone Pairs Around  Central Atom	# of  Bonding Electron Groups  Around Central Atom	Name of Shape	*Shape Diagram and  Bond Dipoles	Polar
C2H2	H : C ::: C : H	0	2	linear		no
C2H4		0	3	trigonal planar		no
CH4		0	4	tetrahedral		no
NH3		1	3	pyramidal		yes
H2O		2	2	v-shaped (bent)		yes

• For the shape diagrams:
• Solid lines represent bonds that are in the same plane as that of the page
• Dashed lines represent bonds that are directed into the plane of the page
• A wedge indicates bonds that are directed out of the plane of the page
• When determining the shapes of molecules the electron pairs of a multiple bond count as a group since the bonds formed are all in the same direction

Is a linear molecule with two nonpolar bonds polar?

Which of the following linear molecules is a nonpolar molecule containing polar bonds?

Which molecules have both polar and non

Is nh3 polar or nonpolar?