Molecular Polarity: Videos & Practice Problems

Understanding molecular polarity is crucial in chemistry as it influences the physical and chemical properties of molecules. Molecular polarity arises from the unequal sharing of electrons between atoms, which is determined by the difference in electronegativity. Nonpolar molecules typically include hydrocarbons (composed only of carbon and hydrogen) and molecules with a "perfect shape." A perfect shape occurs when a molecule's central atom is surrounded by identical elements with no lone electron pairs (perfect shape one) or when it is linear or square planar (perfect shape two). Conversely, polar molecules do not fit these criteria and have an uneven electron distribution. Identifying the molecular geometry and the presence of lone pairs on the central atom is essential. If a molecule does not conform to perfect shape one or two, it is polar by default. This distinction is key to predicting molecule behavior in different environments.

Molecular Polarity

When it comes to the idea of molecular polarity, we have to take into account perfect shapes versus non-perfect shapes or imperfect shapes we're going to say here. First, recall that polarity of chemical bonds arises from unequal sharing of electrons between atoms based on electronegativity. Remember, elements that are more electronegative will tend to attract electrons more towards themselves.

Now we're going to say the molecular polarity itself is just the polarity that arises for an entire molecule. And we're going to say here we have nonpolar molecules and polar molecules as a result. Now when we say nonpolar molecule, we're saying this is any hydrocarbon so composed of only carbons and hydrogens and any non-hydrocarbon with the perfect shape.

Now when we say perfect shape, we break it down into perfect shape one versus perfect shape two. Now for perfect shape one, this is when our central element has the same surrounding elements and it has zero lone pairs. So your central element has no lone pairs on it. Our perfect shape two, this happens when our central element still has the same surrounding elements and it's either linear or square, planar or planar. So that's the additional caveat for perfect shape two.

We're going to say here that a polar molecule is any Lewis structure that doesn't have a perfect shape. So if you're able to identify a perfect shape automatically, it's going to be nonpolar. And if you have a molecule that doesn't fit perfect shape one or two, it's by default polar.

Molecular Polarity Criteria

So remember, if you are a perfect shaped one or a perfect shape 2, you're going to be nonpolar by default. If we take a look here, we have our number of electron groups around our central element. Remember, an electron group is either a surrounding element or a lone pair on that central element. They're both classified as electron groups and our central element can either have 0 lone pairs all the way up to 3 lone pairs.

Now if we take a look here, all those that are shaded in green represent perfect shape ones. All of them have a central element that is the black sphere, and all of them have the same surrounding elements around them. And most importantly, the central element has no lone pairs. OK, so everyone here in this green column would be nonpolar.

If we look at these two orange backgrounds, we're going to say those are also nonpolar because they represent our perfect shape twos. They have our central element in the center and we have the same surrounding elements. So we're going to assume that these spheres here are the same surrounding elements, and they are either linear, so this would be a linear shape, or they are square planar or planar. So they fit the whole idea of perfect shape two. That's why they're also nonpolar.

The other ones that are not highlighted. If you have those shapes, there's no way we can make them nonpolar. They're always going to be polar molecules, all right. So if you get anything that's within this part here which is not shaded in green or in orange, they're always going to be polar. So keep that in mind. Look to see if your molecule has a perfect shape one or a perfect shape 2. If it does and fits the criteria, it's nonpolar. If it doesn't, then it's going to be polar.

Carbon Tetrachloride Polarity Example

Here on this example question it says determine if carbon tetrachloride which is CCl₄ is polar or nonpolar. So here remember when we draw Lewis down structures we place the least electronegative element in the center. In this case it would be carbon. Carbon here is in Group 4A so it has four valence electrons. So we're going to draw our four valence electrons.

Our chlorines are in Group 7A, so they each have 7 valence electrons. And remember carbon wants to make four bonds or must make four bonds. So each chlorine will form a single bond with our carbon. Now if we take a look at the structure, we're going to say that our central element has the same surrounding elements and it has no lone pairs on it.

It would be defined as a perfect shape one and as a result of being a perfect shape it would be nonpolar. So here we're going to say that carbon tetrachloride is a nonpolar molecule.

Determine if the compound of BCl₂F is polar or nonpolar.

• A. polar
• B. nonpolar
• C. cannot be determined

Answer: (A)

Determine if phosphorus trihydride, PH₃, is polar or nonpolar.

• A. Polar
• B. Nonpolar
• C. Cannot be determined

Answer: (A)

Which of the following compounds is/are nonpolar?
I. COF₂                    II. ICl₂^–                    III. XeF₄                    IV. C₈H₁₈

• A. I only
• B. II, IV
• C. I, III
• D. II, III, IV
• E. All of the above

Answer: (D)

Determine if disulfur dichloride, S₂Cl₂, is polar or nonpolar.

• A. Polar
• B. Nonpolar
• C. Cannot be determined

Answer: (A)

Here’s what students ask on this topic:

How does the molecular structure of a water molecule affect its polarity?

The molecular structure of a water molecule significantly contributes to its polarity. Water (H₂O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom is more electronegative than the hydrogen atoms, meaning it has a stronger tendency to attract electrons. As a result, the shared electrons in the covalent bonds spend more time closer to the oxygen atom than to the hydrogen atoms.

This uneven distribution of electron density creates a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms. Moreover, the water molecule has a bent shape due to the two lone pairs of electrons on the oxygen atom, which repel the hydrogen atoms. This angular shape prevents the charges from canceling out, resulting in a dipole moment where one end of the molecule is negatively charged, and the other end is positively charged. This dipole moment is what makes water a polar molecule, leading to its unique properties, such as high surface tension, solvent capabilities, and an unusually high boiling point for its molecular weight.

How do you determine molecular polarity?

To determine molecular polarity, you first need to consider the individual bond polarities within the molecule. This is determined by the electronegativity difference between the atoms; a greater difference means a more polar bond. If the molecule has polar bonds, you then assess the shape of the molecule to see if the bond polarities cancel out or if there's an asymmetrical distribution of charge.

Here's a step-by-step approach:

1. Identify Polar Bonds: Look for differences in electronegativity between bonded atoms. If there's a significant difference, the bond is polar.
2. Molecular Geometry: Determine the shape of the molecule using VSEPR (Valence Shell Electron Pair Repulsion) theory. This will help you understand how the bonds are arranged in three-dimensional space.
3. Symmetry: Check if the molecule is symmetrical. If it is, and the polarities are evenly distributed, they may cancel out, resulting in a nonpolar molecule.
4. Dipole Moment: If the molecule has an asymmetrical shape, the dipole moments (vector quantity) of the polar bonds may not cancel out. This results in an overall dipole moment, making the molecule polar.

In summary, a molecule is polar if it contains polar bonds and has a geometry that doesn't allow for the cancellation of these polarities.

Which phrase describes the molecular polarity and distribution of charge?

Molecular polarity refers to the uneven distribution of charge across a molecule. This occurs when the electrons within the molecule are not shared equally among the atoms. In a polar molecule, one end (pole) of the molecule has a slight negative charge, while the opposite end has a slight positive charge. This is often the result of a significant difference in electronegativity between the atoms involved in a bond. Electronegativity is a measure of how strongly an atom can attract electrons in a chemical bond.

When atoms with different electronegativities form a bond, the more electronegative atom pulls the shared electrons closer to itself, creating a partial negative charge (δ^-) on that atom, and a partial positive charge (δ⁺) on the less electronegative atom. This separation of charge within the molecule leads to a dipole moment, a vector quantity that has both magnitude and direction, representing the polarity of the molecule.

Nonpolar molecules, on the other hand, have an even distribution of charge due to the equal sharing of electrons between atoms with similar electronegativities, resulting in no permanent dipole moment.