7.1. Ionic Compounds http://www.ck12.org
- HgO (composed of Hg^2 +and O^2 −)
- Fe 2 O 3 (composed of Fe^3 +and O^2 −)
- MnO 2 (composed of Mn^4 +and O^2 −)
Answers:
- copper (I) chloride
- mercury (II) oxide
- iron (III) oxide
- manganese (IV) oxide
In the above example, we gave you the charges of the cations, but what if all you had was the formula? Ionic
compounds must be electrically neutral, so if the charge on the anion is known, the charge of the cation can be
determined from the ratio given by the formula. For example, we know that chlorine forms an ion with a charge of
-1. If we see the formula CuCl, we know that copper must have a charge of +1, because in order for the charges
to cancel, we would need to combine these ions in a 1:1 ratio. Similarly, the formula CuCl 2 , we would know that
copper has a charge of +2, because two Cl−ions are required to balance out the charge of each Cu^2 +ion. In general,
the charge on the anion can be determined from inspection, and the charge on the cation can be indirectly determined
from the ratio by which the cation and anion combined.
Polyatomic Ions
Apolyatomic ionis an ion composed of more than one atom. For example, the ammonium ion consists of one
nitrogen atom and four hydrogen atoms. Together, they comprise a single ion with a 1+ charge and a formula of
NH 4 +. The carbonate ion consists of one carbon atom and three oxygen atoms, and it carries an overall charge of
2 −. The formula of the carbonate ion is CO 32 −. The atoms of a polyatomic ion are tightly bonded together, so the
entire ion behaves as a single unit.Figure7.2 shows several models, andTable7.3 lists many of the most common
polyatomic ions.
FIGURE 7.2
(A) The ammonium ion (NH 4 +) is a nitro-
gen atom (blue) bonded to four hydrogen
atoms (white). (B) The hydroxide ion
(OH−) is an oxygen atom (red) bonded to
a hydrogen atom. (C) The carbonate ion
(CO 32 −) is a carbon atom (black) bonded
to three oxygen atoms.TABLE7.3: Common Polyatomic Ions
1- 2- 3- 1+
acetate, CH 3 COO− carbonate, CO 32 − arsenate, AsO 33 − ammonium, NH 4 +
bromate, BrO 3 − chromate, CrO 42 − phosphite, PO 33 −
chlorate, ClO 3 − dichromate, Cr 2 O 72 − phosphate, PO 43 −