Chemistry - A Molecular Science

Chapter 4 The Ionic Bond

Example 4.8

Predict formulas for the binary compounds (compounds containing only two different types of atoms) formed between the following elements. Oxygen and calcium

: Ca is a Group 2A metal and forms a +2 ion, and the most common

oxidation state of oxygen is -2 (Rule 5).

LCM = 2, so LCM/OX = 1 for each ion. The

formula is CaO. Chlorine and cobalt

: The chloride ion has a -1 charge (Rule 6). Co is a transition metal

and forms both +2 and +3 ions. If cobalt is +2, two chloride ions are required and the formula is CoCl

. If cobalt is +3, three -1 charges are needed, so the compound is CoCl 2
. 3

Sulfur and aluminum

: Al is +3 (Rule 3), and S is a 6A nonmetal, so its anion is -2. The

LCM of 3 and 2 is 6, so N

pos

= 6/3 = 2 cations and N

neg

= -6/-2 = 3 anions. The compound

is Al

S 2

. Two +3 ions deliver +6 charge and three -2 ions deliver -6 charge. 3

Oxygen and carbon

: O is -2 from Rule 5. C is a 4A nonmetal that must be assigned the

positive oxidation state, which we a

ssume is +4, its group number. The N

pos

= 4/4 = 1 and

Nneg

= -4/-2 = 2. The formula is CO

. 2

4.5

POLYATOMIC IONS

Table 4.1

Some common polyatomic ions

Cations

NH

1+ 4

ammonium ion

H^3

1+ O

hydronium ion*

A number of ionic compounds are composed of

polyatomic ions

, which are charged

groups of covalently bound atoms. While

the polyatomic ion forms ionic bonds with

oppositely charged ions,

the atoms within a polyatomic ion are nonmetals held together by

covalent bonds

. Many of the polyatomic ions are

oxoanions

; i.e.

, they are negative ions

that contain oxygen atoms covalently bound to another element. In the common polyatomic ions listed in Table 4.1, the

only cations are ammonium and hydronium, and

the only anions that are not oxoanions are hydroxide and cyanide.

Anions

Recall that ionic compounds can be identifie

d as those that contain metals because

metals represent almost all of the common monatomic cations. However, ions can also be polyatomic. Ammonium is by far the most common polyatomic cation in compounds. Thus, NH

Cl, NH 4

NO 4

, and (NH 3

) 42

SO

are also ionic compounds. We conclude that 4

Ionic compounds are those that contain either

a metal or a polyatomic cation such as

ammonium.

C^2

H^3

O^2

1- acetate ion

OH

1- hydroxide ion

CO

2- carbonate ion 3

NO

1- nitrate ion 3

ClO

1- perchlorate ion 4

NO

1- nitrite ion 2

ClO

1- chlorate ion 3

MnO

1- permanganate ion 4

ClO

1- chlorite ion 2

O^2

2- peroxide ion

ClO

1- hypochlorite ion

PO

3- 4

phosphate ion

CrO

2- chromate ion 4

SO

2- 4

sulfate ion

Cr

O 2

2- 7

dichromate ion

SO

2- 3

sulfite ion

CN

1- cyanide ion

* The hydronium ion is often used to

represent the association of the

proton with water (H

O + H 2

1+ = H

O 3

1+), but H

1+ is also used to

represent the proton in water.

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