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9.16: Bond Polarity

Last updated

Mar 21, 2025
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- 9.15: Molecular Shapes - Lone Pair(s) on Central Atom
- 9.17: Polar Molecules

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What makes people share?

Have you ever spent time with someone you really didn't like? You had nothing in common with them and did not want to have anything to do with them. On the other hand, there are people that you enjoy being with. You have a lot in common and like to share with them. Atoms work the same way. If there are strong differences in their attraction of electrons, one atom gets the electrons and the other atom loses them. If they are similar, they share electrons to form a covalent bond.

Bond Polarity

Electronegativity is defined as the ability of an atom to attract electrons when the atoms are in a compound. Electronegativities of elements are shown in the periodic table below.

Periodic table highlighting electronegativity values. Elements are color-coded: blue for low, orange for medium, and green for high electronegativity. — Figure $\PageIndex{2}$ : Electronegativities of elements. (CC BY-NC 3.0; Christopher Auyeung via CK-12 Foundation)

The degree to which a given bond is ionic or covalent is determined by calculating the difference in electronegativity between the two atoms involved in the bond.

As an example, consider the bond that occurs between an atom of potassium and an atom of fluorine. Using the table, the difference in electronegativity is equal to $4.0 - 0.8 = 3.2$ . Since the difference in electronegativity is relatively large, the bond between the two atoms is ionic. Since the fluorine atom has a much larger attraction for electrons than the potassium atom does, the valence electron from the potassium atom is completely transferred to the fluorine atom. The diagram below shows how difference in electronegativity relates to the ionic or covalent character of a chemical bond.

Chart showing bond types based on electronegativity difference: Mostly ionic (≥1.7), polar covalent (0.4-1.7), mostly covalent (<0.4). Includes visual examples of electron sharing, with positive and negative charges. — Figure $\PageIndex{3}$ : Bond type is predicted on the difference in electronegativity of the two elements involved in the bond. (CC BY-NC 3.0; Zachary Wilson via CK-12 Foundation)

Nonpolar Covalent Bonds

A bond in which the electronegativity difference is less than 1.7 is considered to be mostly covalent in character. However, at this point we need to distinguish between two general types of covalent bonds. A nonpolar covalent bond is a covalent bond in which the bonding electrons are shared equally between the two atoms. In a nonpolar covalent bond, the distribution of electrical charge is balanced between the two atoms.

Diagram of a Cl2 molecule showing nonpolar covalent bonding. Two green spheres labeled Cl are connected. Electronegativity (EN) values are 3.0 for both, and the difference is 0. — Figure $\PageIndex{4}$ : A nonpolar covalent bond is one in which the distribution of electron density between the two atoms is equal. (CC BY-NC 3.0; Jodi So via CK-12 Foundation)

The two chlorine atoms share the pair of electrons in the single covalent bond equally, and the electron density surrounding the $\ce{Cl_2}$ molecule is symmetrical. Note that molecules in which the electronegativity difference is very small (<0.4) are also considered nonpolar covalent. An example would be a bond between chlorine and bromine ( $\Delta$ EN $=3.0 - 2.8 = 0.2$ ).

Polar Covalent Bonds

A bond in which the electronegativity difference between the atoms is between 0.4 and 1.7 is called a polar covalent bond. A polar covalent bond is a covalent bond in which the atoms have an unequal attraction for electrons, and so the sharing is unequal. In a polar covalent bond, sometimes simply called a polar bond, the distribution of electrons around the molecule is no longer symmetrical.

A diagram of a diatomic molecule with two differently colored spheres connected by a bond. The left sphere is blue, and the right sphere is red, each surrounded by a gradient halo. — Figure $\PageIndex{5}$ : In the polar covalent bond of $\ce{HF}$ , the electron density is unevenly distributed. There is a higher density (red) near the fluorine atom, and a lower density (blue) near the hydrogen atom. (Public Domain; Ben Mills (Wikimedia: Benjah-bmm27) via Commons Wikimedia, Hydrogen fluoride elpot transparent 3D balls [commons.wikimedia.org])

An easy way to illustrate the uneven electron distribution in a polar covalent bond is to use the Greek letter delta $\left( \delta \right)$ .

Chemical reaction showing diatomic hydrogen (H2) plus diatomic fluorine (F2) forming two molecules of hydrogen fluoride (HF). — Figure $\PageIndex{6}$ : Use of $\delta$ to indicate partial charge. (CC BY-NC 3.0; Joy Sheng via CK-12 Foundation)

The atom with the greater electronegativity acquires a partial negative charge, while the atom with the lesser electronegativity acquires a partial positive charge. The delta symbol is used to indicate that the quantity of charge is less than one. A crossed arrow can also be used to indicate the direction of greater electron density.

Illustration of a chemical reaction showing hydrogen molecules (H2) reacting with iodine molecules (I2) to form hydrogen iodide (HI). Arrows indicate the direction of the reaction. — Figure $\PageIndex{7}$ : Use of crossed arrow to indicate polarity. (Credit: Joy Sheng Source: CK-12 Foundation; License: CC BY-NC 3.0)

Simulation

If it wasn't for our understanding of bond polarity, we'd have a really tough time cleaning our clothing. Use this simulation to understand how the unique polarity of soap molecules helps to clean our clothing!

Summary

The electronegativity of an atom determines how strongly it attracts electrons to itself.
The polarity of a bond is affected by the electronegativity values of the two atoms involved in that bond.

Review

If two atoms bonded together have an electronegativity difference of 1.9, what is the bond type?
What would be the bond type for BH₂?
Your friend tells you that the LiF bond is covalent. Are they correct? Why or why not.