Chemistry - A Molecular Science

Chapter 4 The Ionic Bond

bonding electrons. The metals listed in

Rule 3

all have low ionization energies and give up

all of their outermost electrons.

Rule 4

results because the valence orbital of hydrogen is

higher than most nonmetals (Figure 4.5), so its electron is assigned to nonmetals to produce the +1 oxidation state. However, its va

lence orbital is much lower than those of

the metals in Rule 3, so the bonding electrons in metal-hydrogen bonds are assigned to hydrogen to give it a -1 oxidation state. A hydrogen atom in the -1 oxidation state is called a hydride

ion, and CaH

is calcium hydride. 2

Rule 5

uses the fact that oxygen is the second

most electronegative element, so it is assumed

to gain two electrons to fill its 2p sublevel.

Exceptions

can

occur in compounds with the atoms list

ed in Rules 1 - 4 because fluorine is

more electronegative and is assigned the negative oxidation state in OF

, and oxygen can 2

form both oxides and peroxides (O

2- 2

) with the metals in Rule 3 and with hydrogen. For

example, H

O 2

is hydrogen peroxide and CaO 2

is calcium peroxide. The oxidation state of 2

oxygen in a peroxide is -1.

Rule 6

includes the highly electronegative halogens. They are

assigned the bonding electrons unless they are

bound to a more electronegative element: F

(Rule 1), O (Rule 5), or another more electronegative halogen. Some examples of iodine in an oxidation state other than -1 are IO

1- 4

and IF

, where it is +7 and +3, respectively. 3

Energy

H

1-H

1+H

X

1-X

M

1+M

(a)

(c)

(b)

Figure 4.5 Oxidation states of hydrogen a) The valence orbital of H lies between those of most metals

(M) and most nonmetals (X)

Oxidation states are used to count electrons, so

the sum of the oxidation states of all

atoms must be zero in a molecule, but it must equal the charge on an ion.

The oxidation

state of Ag is zero, but that of Ag

1+ is +1.

b) The electrons would be assigned to the nonmetal in the HX

bond because the valence orbitals

of X are lower. Thus, H

would have to give up an electron to become +1 as in Rule 4.

c) The electrons would be assigned to the H in the MH bond

because the valence orbitals of H are lower. Thus, H would gain an electron to become -1 as required by Rule 3.

* Note that NH

is an example of a formula in which the element with 3

the positive oxidation stat

e is not written first. Hydrogen is written first

in a formula to show that it is acidic. The hydrogen in NH

is not 3

acidic, so it is not written first. Th

is is explained in more detail in

Chapter 12.

Example 4.6

Determine the oxidation state of the underlined atom. Fe

Cl

: 3

The sum of the oxidation stat

es must be zero, and chlorine is a -1 ion (Rule 6), so

0 =

y + (3 Cl)(-1 for each Cl) or

y = +3. The oxidation state of iron is +3 in FeCl

.^3

NH

: 3

Ammonia is a neutral molecule, and each hydrogen atom is +1 from Rule 4. Thus, 0

= y

+ (3 H)(+1 for each H) or

y = -3. The oxidation state of N in NH

* is -3. 3

Cr

O 2

2-: 7

The dichromate ion carries a -2 charge, and oxygen must be a -2 by Rule 5

because Cr does not appear in a prior rule. Thus, -2 = (2 Cr)(

y for each Cr) + (7 O)(-2 for

each oxygen) or 2

y = 12, so each chromium is +6 in the dichromate ion.

Mg

O^2

: Group 2 metals (Rule 3) have priority over

oxygen (Rule 5). Thus, the Mg is +2,

which makes oxygen -1 and the compound a

peroxide

(O

2- 2

), not an oxide.

C^8

H^16

O^8

: H is +1 (Rule 4) and O is -2 (Rule 5), so 8y +16(1) + 8(-2) = 0 or 8y = 0. The

oxidation state of carbon is zero. Note that

elements are not the on

ly substances with

atoms in zero oxidation states.

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