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Two flow charts and table are shown. The first flow chart is labeled, “Electronegativity difference between bonding atoms.” Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, “Zero,” “Intermediate,” and “Large,” respectively. The second flow chart is labeled, “Bond type.” Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, “Pure covalent,” “Polar covalent,” and “Ionic,” respectively. A double ended arrow is written vertically to the right of the flow charts and labeled, “Covalent character decreases; ionic character increases.” The table is made up of two columns and four rows. The header line is labeled “Bond type” and “Electronegativity difference.” The left column contains the phrases “Pure covalent,” “Polar covalent,” and “Ionic,” while the right column contains the values “less than 0.4,” “between 0.4 and 1.8,” and “greater than 1.8.”
As the electronegativity difference increases between two atoms, the bond becomes more ionic.

A rough approximation of the electronegativity differences associated with covalent, polar covalent, and ionic bonds is shown in [link] . This table is just a general guide, however, with many exceptions. For example, the H and F atoms in HF have an electronegativity difference of 1.9, and the N and H atoms in NH 3 a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent. Likewise, the Na and Cl atoms in NaCl have an electronegativity difference of 2.1, and the Mn and I atoms in MnI 2 have a difference of 1.0, yet both of these substances form ionic compounds.

The best guide to the covalent or ionic character of a bond is to consider the types of atoms involved and their relative positions in the periodic table. Bonds between two nonmetals are generally covalent; bonding between a metal and a nonmetal is often ionic.

Some compounds contain both covalent and ionic bonds. The atoms in polyatomic ions, such as OH , NO 3 , and NH 4 + , are held together by polar covalent bonds. However, these polyatomic ions form ionic compounds by combining with ions of opposite charge. For example, potassium nitrate, KNO 3 , contains the K + cation and the polyatomic NO 3 anion. Thus, bonding in potassium nitrate is ionic, resulting from the electrostatic attraction between the ions K + and NO 3 , as well as covalent between the nitrogen and oxygen atoms in NO 3 .

Electronegativity and bond polarity

Bond polarities play an important role in determining the structure of proteins. Using the electronegativity values in [link] , arrange the following covalent bonds—all commonly found in amino acids—in order of increasing polarity. Then designate the positive and negative atoms using the symbols δ+ and δ–:

C–H, C–N, C–O, N–H, O–H, S–H

Solution

The polarity of these bonds increases as the absolute value of the electronegativity difference increases. The atom with the δ– designation is the more electronegative of the two. [link] shows these bonds in order of increasing polarity.

Bond Polarity and Electronegativity Difference
Bond ΔEN Polarity
C–H 0.4 C δ H δ +
S–H 0.4 S δ H δ +
C–N 0.5 C δ + N δ
N–H 0.9 N δ H δ +
C–O 1.0 C δ + O δ
O–H 1.4 O δ H δ +

Check your learning

Silicones are polymeric compounds containing, among others, the following types of covalent bonds: Si–O, Si–C, C–H, and C–C. Using the electronegativity values in [link] , arrange the bonds in order of increasing polarity and designate the positive and negative atoms using the symbols δ+ and δ–.

Answer:

Bond Electronegativity Difference Polarity
C–C 0.0 nonpolar
C–H 0.4 C δ H δ +
Si–C 0.7 Si δ + C δ
Si–O 1.7 Si δ + O δ
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Key concepts and summary

Covalent bonds form when electrons are shared between atoms and are attracted by the nuclei of both atoms. In pure covalent bonds, the electrons are shared equally. In polar covalent bonds, the electrons are shared unequally, as one atom exerts a stronger force of attraction on the electrons than the other. The ability of an atom to attract a pair of electrons in a chemical bond is called its electronegativity. The difference in electronegativity between two atoms determines how polar a bond will be. In a diatomic molecule with two identical atoms, there is no difference in electronegativity, so the bond is nonpolar or pure covalent. When the electronegativity difference is very large, as is the case between metals and nonmetals, the bonding is characterized as ionic.

Chemistry end of chapter exercises

Why is it incorrect to speak of a molecule of solid NaCl?

NaCl consists of discrete ions arranged in a crystal lattice, not covalently bonded molecules.

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What information can you use to predict whether a bond between two atoms is covalent or ionic?

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Predict which of the following compounds are ionic and which are covalent, based on the location of their constituent atoms in the periodic table:

(a) Cl 2 CO

(b) MnO

(c) NCl 3

(d) CoBr 2

(e) K 2 S

(f) CO

(g) CaF 2

(h) HI

(i) CaO

(j) IBr

(k) CO 2

ionic: (b), (d), (e), (g), and (i); covalent: (a), (c), (f), (h), (j), and (k)

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Explain the difference between a nonpolar covalent bond, a polar covalent bond, and an ionic bond.

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From its position in the periodic table, determine which atom in each pair is more electronegative:

(a) Br or Cl

(b) N or O

(c) S or O

(d) P or S

(e) Si or N

(f) Ba or P

(g) N or K

(a) Cl; (b) O; (c) O; (d) S; (e) N; (f) P; (g) N

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From its position in the periodic table, determine which atom in each pair is more electronegative:

(a) N or P

(b) N or Ge

(c) S or F

(d) Cl or S

(e) H or C

(f) Se or P

(g) C or Si

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From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity:

(a) C, F, H, N, O

(b) Br, Cl, F, H, I

(c) F, H, O, P, S

(d) Al, H, Na, O, P

(e) Ba, H, N, O, As

(a) H, C, N, O, F; (b) H, I, Br, Cl, F; (c) H, P, S, O, F; (d) Na, Al, H, P, O; (e) Ba, H, As, N, O

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From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity:

(a) As, H, N, P, Sb

(b) Cl, H, P, S, Si

(c) Br, Cl, Ge, H, Sr

(d) Ca, H, K, N, Si

(e) Cl, Cs, Ge, H, Sr

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Which atoms can bond to sulfur so as to produce a positive partial charge on the sulfur atom?

N, O, F, and Cl

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Which is the most polar bond?

(a) C–C

(b) C–H

(c) N–H

(d) O–H

(e) Se–H

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Identify the more polar bond in each of the following pairs of bonds:

(a) HF or HCl

(b) NO or CO

(c) SH or OH

(d) PCl or SCl

(e) CH or NH

(f) SO or PO

(g) CN or NN

(a) HF; (b) CO; (c) OH; (d) PCl; (e) NH; (f) PO; (g) CN

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Which of the following molecules or ions contain polar bonds?

(a) O 3

(b) S 8

(c) O 2 2−

(d) NO 3

(e) CO 2

(f) H 2 S

(g) BH 4

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Questions & Answers

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Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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