Lesson Explainer: Polar Bonding Chemistry

In this explainer, we will learn how to identify polar bonds within molecules and assess their effect on the polarity of the molecule as a whole.

Polar molecules have an asymmetrical distribution of electron density because one of the covalently bonded atoms has a higher electronegativity value than the other. Electronegativity measures the tendency of an atom to attract a bonding pair of electrons from a chemical bond. In 1932, Linus Pauling famously introduced an accurate electronegativity scale that could be used to quantify the ability of any element to withdraw electron density from bonding pairs of electrons.

The following diagram shows a single molecule of hydrogen chloride. The image shows that the bonding electron density (colored cloud) is concentrated around the more electronegative chlorine atom.

Definition: Polar Bonding

Polar bonding is a type of covalent bonding where the bonding electrons are distributed unequally between the bonding atoms.

Pauling electronegativity values can be used to understand why some atoms form simple molecular compounds, while other atoms bond together and form giant ionic lattices. Elements will usually bond together and form simple covalently bonded compounds when they have the same or very similar electronegativity values. Elements will usually bond ionically and form giant ionic structures when one of the elements has a very low electronegativity value and the other has a much higher electronegativity value.

Definition: Electronegativity

Electronegativity measures the tendency of an atom to attract a bonding pair of electrons from a chemical bond.

Example 1: Understanding How Chemists Define the Capacity of an Atom to Withdraw a Bonding Pair of Electrons

Fill in the blank: The ability of an atom to attract a pair of electrons in a chemical bond is known as .

  1. nuclear affinity
  2. electronegativity
  3. electron affinity
  4. conductivity
  5. ionization energy

Answer

Electronegativity measures the tendency of an atom to attract a bonding pair of electrons from a chemical bond. Linus Pauling famously introduced an accurate electronegativity scale for most of the chemical elements. We can use these statements to determine that option B is the correct answer for this question.

The absolute difference of electronegativity values can be defined as Δ𝐸𝑁=|πΈβˆ’πΈ| where 𝐸 and 𝐸 are the electronegativity values of two chemically bonded elements. Elements usually bond together and form covalent bonds when the difference of electronegativity values is less than 1.7. Elements usually bond together and form more ionic bonds when the difference of electronegativity values is greater than 1.7.

Definition: Absolute Difference of Electronegativity Values (Δ𝐸𝑁)

The absolute difference of electronegativity values can be defined as Δ𝐸𝑁=|πΈβˆ’πΈ| where 𝐸 and 𝐸 are the electronegativity values of two chemically bonded elements.

The absolute difference of electronegativity values also determines if simple molecular compounds will form nonpolar or polar covalently bonded molecules. Elements will usually bond together and make nonpolar covalent molecules when the difference of their electronegativity values is less than 0.4. Elements will usually bond together and make polar covalent molecules when the difference of their electronegativity values is greater than 0.4 but less than 1.7.

Bond TypeElectronegativity DifferenceExample Molecules
Pure covalent0Hydrogen gas (H2)
Nonpolar covalentLess than 0.4Methane (CH4)
Polar covalentBetween 0.4 and 1.7Hydrogen fluoride (HF)
IonicGreater than 1.7Sodium fluoride (NaF)

Example 2: Understanding What Chemical Properties Determine the Polarity of a Bond Between Two Elements

Which of the following determines the polarity of a bond between two elements?

  1. The difference in electronegativity between the two elements
  2. The atomic size of the two elements
  3. The number of valence electrons in the two elements
  4. The type of bond between the two elements
  5. The difference in first ionization energy between the two elements

Answer

Electronegativity measures the tendency of an atom to attract a bonding pair of electrons from a chemical bond. The absolute difference of electronegativity values (Δ𝐸𝑁) can be used to determine if two chemically bonded elements will form a covalently or ionically bonded compound.

The absolute difference of electronegativity values can also be used to determine if a simple molecular compound will form a nonpolar or polar covalently bonded molecule. These statements can be used to determine that option A has to be the correct answer for this question.

Bonding electrons are distributed unequally in ionic compounds but are shared slightly more equally in polar covalently bonded compounds. Bonding pairs of electrons are shared even more equally in nonpolar covalent compounds, and they are shared completely evenly in pure covalent compounds. Pure covalent compounds always contain atoms that have the same electronegativity values.

Bond TypeBonding Electrons Distribution
Pure covalentCompletely even
Nonpolar covalentSomewhat uneven
Polar covalentNoticeably uneven
IonicExtremely uneven

Definition: Pure Covalent Bond

The bonding pairs of electrons are shared completely evenly in pure covalent bonds.

For example, hydrogen gas molecules (H2) contain two identical hydrogen atoms that both have Pauling electronegativity numbers of 2.20. Neither one of the hydrogen atoms is capable of withdrawing more electron density than the other hydrogen atom. The electron density, as shown below, is shared completely evenly between the two bonded hydrogen atoms, and the HH bond is described as being pure covalent or completely nonpolar.

Example 3: Understanding How Chemists Define Completely Nonpolar Covalent Bonds

Which of the following represents the type of bond formed when two electrons in a chemical bond are equally shared?

  1. Ionic bond
  2. Pure covalent bond
  3. Polar covalent bond
  4. Hydrogen bond

Answer

Hydrogen bonds are some of the strongest intermolecular forces that are generated between electron lone pairs and hydrogen atoms that are covalently bonded to atoms that have some of the highest possible electronegativity values.

Ionic bonds are intramolecular forces that are established when the valence electrons of one metal atom are effectively transferred to another nonmetal atom. The process creates positively charged metal ions and negatively charged nonmetal ions that are attracted through electrostatic forces. We can use these statements to determine that D and A cannot be correct answers for this question. We now have to compare the pure and polar covalent bonds to determine if option B or C is the correct answer for this question.

Polar covalent bonds are made up of atoms that have different electronegativity values. One of the bonded atoms attracts a disproportionate amount of electron density, and the electron density becomes concentrated on one side of the chemical bond. Pure covalent bonds are made up of atoms that have the same electronegativity values, and the electrons are shared completely evenly between the two bonded atoms. Neither one of the bonded atoms is capable of withdrawing more bonding-pair electron density than the other bonded atom. We can use these statements to determine that answer B has to be the correct answer for this question.

Bonding pairs of electrons are shared unequally in polar covalent compounds, such as hydrogen chloride (HCl) molecules. Hydrogen has a relatively low electronegativity number of 2.20, and chlorine has a much higher electronegativity number of 3.16. In a molecule of HCl, the chlorine atom is much more effective at withdrawing the bonding pair of electrons, and the HCl covalent bond is almost always highly polar. The negatively charged electron density shifts over toward the chlorine atom and it ends up having a partial negative electrostatic charge (π›Ώβˆ’), while the hydrogen atom ends up having a partial positive electrostatic charge (𝛿+).

Example 4: Understanding How Electron Density Is Distributed in Hydrogen Chloride Molecules

Which of the following diagrams shows a correct representation of the electron cloud that surrounds an HCl molecule?

Answer

Hydrogen has an electronegativity number of 2.20, and chlorine atoms have a much higher electronegativity number of 3.16. The negatively charged electron density shifts over toward the highly electronegative chlorine atom and it ends up having a partial negative electrostatic charge (π›Ώβˆ’), while the covalently bonded hydrogen atom ends up having a partial positive electrostatic charge (𝛿+). A larger cloud of electron density would therefore be expected around the chlorine atom with a smaller cloud of electron density around the hydrogen atom. Such a distribution is shown best in option C.

The electron density distribution is asymmetric about the center of the HCl covalent bond. One side of the hydrogen chloride molecule has a partial positive electrostatic charge, while the other side has a partial negative electrostatic charge. There is a large difference in electron density at the two ends of the hydrogen chloride molecules, and there is an electric dipole moment that runs from the hydrogen atom through to the chlorine atom. The quantitative measure of the dipole is called the dipole moment, and it is represented by the letter πœ‡.

It is important to realize that a molecule cannot automatically be assumed to be polar just because it contains at least one polar covalent bond. Bonded atoms with significantly different electronegativity values will generate unidirectional electric dipole moments, but molecules can contain other unidirectional electric dipole moments, and two or more of these vectors can cancel each other out.

The following figure shows how the three polar covalent bonds of boron trifluoride molecules can cancel each other out. The molecule has three polar bonds but no overall dipole moment because the molecule has a highly symmetric shape. The polar bonds effectively oppose each other, and the molecule ends up having no overall dipole moment. Molecules will usually not have an overall dipole moment if they have highly symmetric shapes

Definition: Dipole Moment

The dipole moment measures the polarity of a chemically bonded molecule.

Chemists usually use dipole arrows to show which direction the bonding pair of electrons is being pulled toward in polar molecules. The β€œ+” sign shows which atom has the partial positive electrostatic charge, and the arrowhead shows which side has the partial negative electrostatic charge. The following figure shows how a single dipole arrow can be used to represent the electric dipole moment that runs along the HCl bond in hydrogen chloride.

HΞ΄+Clδ–

Definition: Dipole Arrow

Dipole arrows are used to show which direction the bonding pair of electrons is being pulled toward in polar molecules.

We must consider molecular geometries when we are trying to determine the overall polarity of a chemical compound that contains at least one highly electronegative atom and at least one weakly withdrawing atom. Water (HO2) and carbon dioxide (CO2) are both simple molecular compounds that contain three atoms each, but only one of these molecules has an overall dipole moment because the molecules have very different shapes. Both of these compounds and their dipoles are shown in the image below.

COOOHH

Carbon dioxide is a symmetric molecule, and the two opposing CO dipole moment arrows cancel each other out. Water has an angular (bent) structure, and the two OH bond dipole moment arrows reinforce each other, and this produces one large dipole moment that runs from the hydrogen atoms through to the oxygen atom’s lone pair electrons. Water molecules end up having an overall dipole moment, and carbon dioxide molecules end up having an overall dipole moment of zero.

Example 5: Understanding How Electrons Are Distributed in Simple Diatomic and Triatomic Molecules

Partial charges are created from the asymmetric distribution of the electrons in chemical bonds within molecules.

  1. In the following diagram, identify which atom has a delta positive charge.
    1. Chlorine
    2. Hydrogen
  2. In the following diagram, identify which atom has a delta negative charge.
    1. Hydrogen
    2. Oxygen

Answer

Part 1

Dipole arrows are used to show which direction the bonding pair of electrons is being pulled toward in polar molecules. The arrow indicates that the electron density is being pulled away from hydrogen atoms, and so the delta positive charge (𝛿+) will be on the hydrogen atom. We can use these statements to determine that option B is the correct answer for this question.

Part 2

Dipole arrows are used to show which direction the bonding pair of electrons is being pulled toward in polar molecules. The arrow indicates that the electron density is being pulled toward the oxygen atom, and so the delta negative charge (π›Ώβˆ’) will be on the oxygen atom. We can use these statements to determine that option B is the correct answer for this question.

The same line of reasoning can be applied to medium-sized simple molecular compounds to determine if they have an overall dipole moment and to calculate the size of any dipole moment that does exist. Carbon tetrachloride (CCl4) contains one carbon atom that is covalently bonded to four other highly electronegative chlorine atoms. The CCl4 molecule, illustrated below, has four identical CCl covalent bonds, each of which is a polar bond. Even though carbon tetrachloride has four polar covalent bonds, it does not have an overall dipole moment because the four opposing CCl covalent-bond dipole moment arrows cancel each other out. The molecule does not have an overall dipole moment because of its highly symmetric shape.

Key Points

  • Electronegativity measures the tendency of an atom to attract a bonding pair of electrons from a chemical bond.
  • The absolute difference of electronegativity values (Δ𝐸𝑁) can be used to determine if two chemically bonded elements will form a covalently or ionically bonded compound.
  • The absolute difference of electronegativity values (Δ𝐸𝑁) can be used to determine if a covalent bond will be polar or nonpolar.
  • Polar covalently bonded compounds contain at least one atom that has a partial negative electrostatic charge (π›Ώβˆ’) and at least one atom that has a partial positive electrostatic charge (𝛿+).
  • Dipole arrows are used to show which direction the bonding pair of electrons is being pulled toward in polar molecules.
  • The shape of a polar molecule can determine if it has an overall dipole moment or if separate dipole moment arrows cancel each other out.

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