Page 86 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 5] CHEMICAL BONDING 75

charge. (For example, see Fig. 5-1.) In a solid ionic compound, it is incorrect to speak of a bond between specific
pairs of ions, and ionic compounds do not form molecules.
In contrast, covalent bonding involves the sharing of electron pairs between two specific atoms, and it is
possible to speak of a definite bond. For example, in molecules of HCl and CH 4 there are one and four covalent
bonds per molecule, respectively.
Polyatomic ions such as OH , ClO 3 , and NH 4 possess covalent bonds as well as an overall charge.
+
−
−
+
_ − H
O H O Cl O H N H
O
H
The charges on polyatomic ions cause ionic bonding between these groups of atoms and oppositely charged ions.
In writing electron dot structures, the distinction between ionic and covalent bonds must be clearly indicated.
For example, the electron dot diagram for the compound NH 4 ClO 3 is

+
H −
O Cl O
H N H
O
H

5.8. PREDICTING THE NATURE OF BONDING IN COMPOUNDS
Electronegativity
Electronegativity is a semiquantitative measure of the ability of an atom to attract electrons involved in
covalent bonds. Atoms with higher electronegativities have greater electron-attracting ability. Selected values of
electronegativity are given in Table 5-1. The greater the electronegativity difference between a pair of elements,
the more likely they are to form an ionic compound; the lower the difference in electronegativity, the more likely
that if they form a compound, the compound will be covalent.

Table 5-1 Selected Electronegativities

H He
2.1
Li Be B C N O F Ne
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Na Mg Al Si P S Cl Ar
0.9 1.2 1.5 1.8 2.1 2.5 3.0
K Ca Ga Ge As Se Br Kr
0.8 1.0 1.6 1.8 2.0 2.4 2.8
Rb Sr In Sn Sb Te I Xe
0.8 1.0 1.7 1.8 1.9 2.1 2.5
Cs Ba Tl Pb Bi Po At Rn
0.7 0.9 1.8 1.9 1.9 2.0 2.2

You need not memorize values of electronegativity (although those of the second-period elements are very
easy to learn). You may generalize and state that the greater the separation in the periodic table, the greater the
electronegativity difference. Also, in general, electronegativity increases to the right and upward in the periodic
table. Compounds are generally named and formulas are written for them with the less electronegative element