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.
© by
North
Carolina
State
University