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4.7: Unequal Sharing- Polar Covalent Bonds

  • Page ID
    153798
  • Learning Objectives

    • Define electronegativity
    • Determine the polarity of a covalent bond.

    Electronegativity

    The ability of an atom in a molecule to attract shared electrons is called electronegativity. When two atoms combine, the difference between their electronegativities is an indication of the type of bond that will form. If the difference between the electronegativities of the two atoms is small, neither atom can take the shared electrons completely away from the other atom and the bond will be covalent. If the difference between the electronegativities is large, the more electronegative atom will take the bonding electrons completely away from the other atom (electron transfer will occur) and the bond will be ionic. This is why metals (low electronegativities) bonded with nonmetals (high electronegativities) typically produce ionic compounds.

    A bond may be so polar that an electron actually transfers from one atom to another, forming a true ionic bond. How do we judge the degree of polarity? Scientists have devised a scale called electronegativity, a scale for judging how much atoms of any element attract electrons. Electronegativity is a unitless number; the higher the number, the more an atom attracts electrons. A common scale for electronegativity is shown in Figure \(\PageIndex{1}\).

    alt
    Figure \(\PageIndex{1}\) Electronegativities of the Elements. Electronegativities are used to determine the polarity of covalent bonds.

    Bond Polarity

    The polarity of a covalent bond can be judged by determining the difference of the electronegativities of the two atoms involved in the covalent bond, as summarized in the following table:

    Electronegativity Difference Bond Type
    0 nonpolar covalent
    0–0.4 slightly polar covalent
    0.5–2.0 definitely polar covalent
    >2.0 likely ionic

    Nonpolar Covalent Bonds

    A bond in which the electronegativity difference is less than 1.9 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.

    Figure \(\PageIndex{2}\) A nonpolar covalent bond is one in which the distribution of electron density between the two atoms is equal.

    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. Also note that molecules in which the electronegativity difference is very small (<0.5) 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 0.5 and 2.0 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.

    Figure \(\PageIndex{3}\) 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.

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

    Figure \(\PageIndex{4}\) Use of \(\delta\) to indicate partial charge.

    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.

    Figure \(\PageIndex{5}\) Use of crossed arrow to indicate polarity

    Electronegativity differences in bonding using Pauling scale. Using differences in electronegativity to classify bonds as covalent, polar covalent, or ionic.

    Example \(\PageIndex{1}\): Bond Polarity

    What is the polarity of each bond?

    1. C–H
    2. O–H

    Solution

    Using Figure \(\PageIndex{1}\), we can calculate the difference of the electronegativities of the atoms involved in the bond.

    1. For the C–H bond, the difference in the electronegativities is 2.5 − 2.1 = 0.4. Thus we predict that this bond will be non polar covalent.
    2. For the O–H bond, the difference in electronegativities is 3.5 − 2.1 = 1.4, so we predict that this bond will be definitely polar covalent.

    Exercise \(\PageIndex{1}\)

    What is the polarity of each bond?

    1. Rb–F
    2. P–Cl
    Answer a

    likely ionic

    Answer b

    polar covalent

    Summary

    • The type of bond (polar covalent,non polar covalent or ionic) between two atoms is determined by the differences in electronegativity.
    • For atoms sharing a polar covalent bond, the atom with the greater electronegativity acquires a partial negative charge, while the atom with the lesser electronegativity acquires a partial positive charge.

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