Page 87 - Theory and Problems of BEGINNING CHEMISTRY

76 CHEMICAL BONDING [CHAP. 5

first. (Hydrogen compounds are exceptions. The symbol for hydrogen is written first only for acids. Hydrogen
combined with a halogen atom or with a polyatomic anion forms an acid. In NH 3 , the H is written last despite
its lower electronegativity because NH 3 is not an acid.) You may not even have to consider electronegativity
in making deductions about compounds, for example, in naming compounds (Chap. 6). In fact, the following
generalizations about bonding can be made without reference to electronegativity.
Most binary compounds (compounds of two elements) of metals with nonmetals are essentially ionic. All
+
compounds involving only nonmetals are essentially covalent except for compounds containing the NH 4 ion.
We do not consider compounds of metals with metals in this course.
Practically all tertiary compounds (compounds of three elements) contain covalent bonds. If one or more of
the elements is a metal, there is likely to be ionic as well as covalent bonding in the compound.

Formation of Ions in Solution
When some molecules containing only covalent bonds are dissolved in water, the molecules react with the
water to produce ions in solution. For example, pure hydrogen chloride, HCl, and pure ammonia, NH 3 , consist
◦
of molecules containing only covalent bonds. When cooled to sufficiently low temperatures (−33 C for NH 3 ,
−85 C for HCl), these substances condense to liquids. However, the liquids do not conduct electricity, since they
◦
are still covalent and contain no ions. In contrast, when HCl is dissolved in water, the resulting solution conducts
electricity well. Aqueous solutions of ammonia also conduct, but poorly. In these cases, the following reactions
occur to the indicated extent to yield ions:

+
H −
H Cl + H O H H O H + Cl (100%)
+
H
H N H + H O H H N H + O H − (approximately 1%)
H H

+
H 3 O is often abbreviated H .
+

5.9. DETAILED ELECTRONIC CONFIGURATIONS OF IONS (OPTIONAL)
To get the electronic configuration of ions, a new rule is followed. First we write the electron configuration
of the neutral atom (Chap. 4). Then, for positive ions, we remove the electrons in the subshell with highest
principal quantum number first. Note that these electrons might not have been added last, because of the n + l
rule. Nevertheless, the electrons from the shell with highest principal quantum number are removed first. For
negative ions, we add electrons to the shell of highest principal quantum number. (That shell has the electrons
added last by the n + l rule.)

2+
EXAMPLE 5.9. What is the electronic configuration of each of the following: (a)Mg ,(b)Cl ,(c)Co , and (d)Co ?
3+
−
2+
6
2
2
2
Ans. (a) The configuration of Mg is 1s 2s 2p 3s . For the ion Mg , the outermost (3s) electrons are removed, yielding
2+
2
2
2
2
6
Mg 2+ 1s 2s 2p 3s 0 or 1s 2s 2p 6
5
2
2
6
2
(b) The configuration of Cl is 1s 2s 2p 3s 3p . For the anion, we add an electron for the extra charge:
2
2
6
2
Cl − 1s 2s 2p 3s 3p 6
2
2
6
2
6
2
7
(c) The configuration of Co is 1s 2s 2p 3s 3p 4s 3d . For Co , we remove the two 4s electrons! They are the
2+
electrons in the outermost shell, despite the fact that they were not the last electrons added. The configuration
is
6
6
6
2
6
2
0
2
2
2
2
Co 2+ 1s 2s 2p 3s 3p 4s 3d 7 or 1s 2s 2p 3s 3p 3d 7