Name: ______________________________ Per: _____
Ch. 5 Enrichment ~ Polyatomic Ions Reading and Questions ~
So far, we have studied that ions are single atoms that have lost or gained electrons in order to become
stable. Yet, in nature, many ionic compounds contain polyatomic ions, which are ions made up or more than one
atom. The atoms within a polyatomic ion are usually very tightly bound together, so the ion retains its identity
within ionic compounds and over the course of many chemical reactions (the atoms do not easily separate).
The table below lists the formulas and the charges for several polyatomic ions.
The charge given to a polyatomic ion applies to the entire group of atoms. Although an ionic compound
containing one or more polyatomic ions contains more than two atoms, the polyatomic ion acts as an individual
ion. Therefore, the chemical formula for the compound can be written following the same rules used for a
compound containing only two ions.
Because a polyatomic ion exists as a unit, never change the subscripts of the atoms within the ion. If more
than one polyatomic ion is needed, place parentheses around the ion and write the appropriate subscript
outside the parentheses. For example, the formula for magnesium chlorate is Mg(ClO3)2. Note that the
ammonium ion is the only common polyatomic positive ion (cation).
How can you determine the formula unit for an ionic compound containing a polyatomic ion? Let’s consider the
compound formed from the ammonium ion and the chloride ion.
NH4+ (ammonium ion)
Cl- (chloride ion)
Because the sum of the charges on the ions is zero, the ions are in a one-to-one ratio. The correct formula
unit for this compound is NH4Cl.
Group
(or column)
1
2
15
16
17
Common Ions Based on Groups
Atoms that commonly form ions
Charge on ions
H, Li, Na, K, Rb, Cs
Be, Mg, Ca, Sr, Ba
N, P, As
O, S, Se, Te
F, Cl, Br, I
1+
2+
321-
1) Look at the chart “Common Ions Based on Groups.” Why do the atoms in group 1 form a positive ion?
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2) Using the same chart, consider: Why do the atoms in group 15 form ions that have a 3- charge?
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In ionic compounds, the charges of each ion need to be balanced in such a way as to have the overall charge of
zero. For instance,
Na+ and Cl-
The +1 and -1 charge cancel each other out and the net charge of the ionic compound is zero.
K+ and S2-
Since potassium and sulfur do not have equal, opposite charges, another potassium must be
added in order to make the charge of the ionic compound zero (neutral). Therefore, the ionic
compound formula is K2S.
3) Look at the list of the common polyatomic ions (on the previous page) and write the formulas for the
following elements:
Ions
Charges of ions/polyatomic ion
Ionic Compound Formula
+
Sodium and hydroxide
Na and OH
NaOH
Lithium and nitrite
Hydrogen and cyanide
Sodium and perchlorate
Hydrogen and sulfate
Magnesium and nitrite
Barium and chlorate
Hydrogen and acetate
Strontium and peroxide
Cesium and sulfite
Hydrogen and arsenate
Barium and phosphate
Aluminum and chromate
Potassium and chromate
Magnesium and carbonate
Calcium and chlorate
4) In 2-3 sentences, summarize what you learned from this reading and exercise:
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