Polarity - Polar & Nonpolar
Covalent
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C HAPTER
Chapter 1. Polarity - Polar & Nonpolar Covalent
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Polarity - Polar & Nonpolar
Covalent
Lesson Objectives
• Describe how the electronegativity difference between two atoms in a covalent bond results in the formation
of a nonpolar covalent, polar covalent, or ionic bond.
Lesson Vocabulary
• diatomic element
• nonpolar covalent bond
• polar covalent bond
Check Your Understanding
Recalling Prior Knowledge
• How do electrons behave during the formation of an ionic bond, and what properties of ionic compounds
result from the bonding?
• What is meant by the electronegativity of an element?
In an ionic bond, one or more electrons are transferred from one atom to another. In a covalent bond, one or more
pairs of electrons are shared between atoms. However, bonding between atoms of different elements is rarely purely
ionic or purely covalent. The true nature of a chemical bond often falls somewhere in between these two extremes.
What is the difference in electron behavior with ionic vs covalent bonds?
Bond Polarity
Recall from the earlier chapter, The Periodic Table, that electronegativity is defined as the relative ability of an atom
to attract electrons when present in a compound. The electronegativities of most elements are shown below ( Figure
1.1).
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 4.0 – 0.8 = 3.2. Because the difference in electronegativity is relatively large, the
bond between the two atoms is primarily ionic. Since the fluorine atom has a much larger attraction for electrons than
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FIGURE 1.1
Electronegativities of the elements.
the potassium atom does, the valence electron from the potassium atom is considered to have completely transferred
to the fluorine atom. The figure below ( Figure 1.2) shows how the difference in electronegativity relates to the ionic
or covalent character of a chemical bond.
According to Figure 1.2, a difference in electronegativity (∆EN) greater than 1.7 results in a bond that is mostly
ionic in character.
What is the definition of electronegativity? What is the electronegativity difference between
ionically bonded elements?
Nonpolar Covalent Bonds
A bond in which the electronegativity difference is less than 1.7 is considered to be mostly covalent in character.
However, a distinction is often made 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 1.3).
How are electrons shared in a non polar covalent bond?
The two chlorine atoms share the pair of electrons in the single covalent bond equally, and the electron density
surrounding the Cl2 molecule is symmetrical. Any diatomic molecule in which the two atoms are the same element
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Chapter 1. Polarity - Polar & Nonpolar Covalent
FIGURE 1.2
The difference in electronegativity between the two elements involved in a
chemical bond is predictive of the type of
bond made by those two atoms. A small
difference (<0.4) results in a nonpolar covalent bond, an intermediate difference
(0.4-1.7) results in a polar covalent bond,
and a large difference (>1.7) results in an
ionic bond.
must be joined by a nonpolar covalent bond.
Why are diatomic bonded elements considered non polar covalent in nature? (refer to their
electronegativity difference.)
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FIGURE 1.3
A nonpolar covalent bond is one in
which the shared electrons are distributed
equally between the two atoms.
There are seven diatomic elements, which are elements whose natural form is of a diatomic molecule. They are
hydrogen (H2 ), nitrogen (N2 ), oxygen (O2 ), fluorine (F2 ), chlorine (Cl2 ), bromine (Br2 ), and iodine (I2 ). By forming
a diatomic molecule, both atoms in each of these molecules satisfy the octet rule, resulting in a structure that is much
more stable than the isolated atoms.
Notice from the figure above ( Figure 1.2) 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 (∆EN =
3.16 – 2.96 = 0.20).
An animation of a nonpolar covalent bond formation can be found at http://www.dlt.ncssm.edu/core/Chapter9-Bondi
ng_and_Geometry/Chapter9-Animations/CovalentBonding.html .
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, so
the sharing is unequal. In a polar covalent bond, sometimes simply called a polar bond, the distribution of shared
electrons within the molecule is no longer symmetrical ( Figure 1.4).
FIGURE 1.4
In the polar covalent bond of 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.
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Chapter 1. Polarity - Polar & Nonpolar Covalent
How are electrons shared in a polar covalent bond?
The hydrogen fluoride molecule has an electronegativity difference of 1.9, which places it in the category of being
slightly ionic. However, the hydrogen ion (H+ ) is so very small that it is not capable of adopting the crystal
lattice structure of an ionic compound. Hydrogen fluoride is a highly polar molecule. Because of its greater
electronegativity, the electron density around the fluorine atom is much higher than the electron density around
the hydrogen atom.
An easy way to illustrate the uneven electron distribution in a polar covalent bond is to use the Greek letter, delta (δ)
along with a positive or negative sign to indicate that an atom has a partial positive or negative 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.
Sample Problem 9.3: Identifying Bond Types
Which type of bond will form between each of the following pairs of atoms?
1. C and O
2. Na and N
3. B and H
Step 1: List the known quantities and plan the problem.
Known
Using the electronegativity chart:
1. C = 2.5, O = 3.5
2. Na = 0.9, N = 3.0
3. B = 2.0, H = 2.1
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Step 2: Solve.
Calculate the difference and use the diagram above ( Figure 1.2) to identify the bond type.
1. 3.5 – 2.5 = 1.0 → C-O bond is polar covalent
2. 3.0 – 0.9 = 2.1 → Na-N bond is ionic
3. 2.1 – 2.0 = 0.1 → B-H bond is nonpolar covalent
Step 3: Think about your result.
Bonds between nonmetal atoms are generally covalent in nature (A and C), while bonds between a metal atom and
a nonmetal atom are generally ionic.
Practice Problems
1. Place the following bonds in order from least polar to most polar.
a.
b.
c.
d.
Fe-N
H-Cl
Ca-O
C-S
ww.youtube.com/watch?v=rCbtapphNwA&feature=player_embedded .
i
An exciting demonstration of the poor conductivity of covalent bonds in a rubber stopper can be seen at http://educa
tion.jlab.org/frost/stopper.html .
Lesson Summary
• The difference in electronegativity between the two atoms in a bond determines the type of bond that is formed.
A small difference means that the sharing of electrons is equal, and the bond is nonpolar covalent. A larger
difference means that sharing is unequal, and the bond is polar covalent. A very large difference means that
one or more electrons are transferred, and the bond is ionic.
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Chapter 1. Polarity - Polar & Nonpolar Covalent
Lesson Review Questions
Reviewing Concepts
Please Answer the questions below.
What range of electronegativity differences between the two bonded atoms are classified as polar covalent?
What is a diatomic molecule? Give an example.
What is meant by the symbols δ+ and δ− in a structural formula?
Arrange the following bonds in order from least polar to most polar: Al−N, Br−C, P−H, Cl−F, K−O.
Further Reading / Supplemental Links
• Electronegativity (http://www.chemguide.co.uk/atoms/bonding/electroneg.html )
• Intermolecular Bonding – van der Waals Forces (http://www.chemguide.co.uk/atoms/bonding/vdw.html )
• Intermolecular Bonding – Hydrogen Bonds (http://www.chemguide.co.uk/atoms/bonding/hbond.html )
References
1.
2.
3.
4.
Christopher Auyeung. CK-12 Foundation .
Zachary Wilson. CK-12 Foundation . CC BY-NC 3.0
Jodi So. CK-12 Foundation . CC BY-NC 3.0
Ben Mills (User:Benjah-bmm27/Wikimedia Commons). http://commons.wikimedia.org/wiki/File:Hydrogenfluoride-elpot-transparent-3D-balls.png . Public Domain
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