7.1. Ionic Compounds http://www.ck12.org
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. The figure below (Figure7.4) shows several models, and the table below (Table7.5) lists many of the
most common polyatomic ions.
FIGURE 7.4
(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.TABLE7.5: Common Polyatomic Ions
1- 2- 3- 1+ 2+
acetate, CH 3 COO− carbonate, CO 32 − arsenate, AsO 33 − ammonium, NH 4 + dimercury, Hg 22 +
bromate, BrO 3 − chromate, CrO 42 − phosphite, PO 33 −
chlorate, ClO 3 − dichromate,
Cr 2 O 72 −phosphate, PO 43 −chlorite, ClO 2 − hydrogen
phosphate, HPO 42 −
cyanide, CN− oxalate, C 2 O 42 −
dihydrogen
phosphate, H 2 PO 4 −peroxide, O 22 −hydrogen carbonate,
HCO 3 −silicate, SiO 32 −hydrogen sulfate,
HSO 4 −sulfate, SO 42 −hydrogen sulfide,
HS−sulfite, SO 32 −hydroxide, OH−
hypochlorite, ClO−
nitrate, NO 3 −
nitrite, NO 2 −
perchlorate, ClO 4 −
permanganate,
MnO 4 −The vast majority of polyatomic ions are anions, many of which end in–ateor–ite. Notice that in some cases,
such as nitrate (NO 3 −) and nitrite (NO 2 −), there are multiple anions that consist of the same two elements. This is
particularly common for oxoanions, which are binary anions containing one or more oxygen atoms. A given element
may form several oxoanions that all have the same charge but differ in the number of oxygen atoms present. When
there are two common oxoanions for a particular element, the one with the greater number of oxygen atoms gets an
–atesuffix, while the one with the lower number of oxygen atoms gets an–itesuffix. Some elements form more