2. Purpose
• Not all ions consist of a single atom like Na+ or S2-. Some ions are
combinations of atoms (polyatomic) such as the sulfate ion - SO4
2-. In this
polyatomic ion, one sulfur is bound to four oxygens with covalent bonds.
When this polyatomic ion formed, it gained two electrons thus the 2-
charge it has. In this lesson, you will learn the names and formulas of
several polyatomic ions.
3. Common Polyatomic ions
Memorize the names and the formulas
• Common 1+ ions
– Ammonium, NH4
+
• Common 1- polyatomic ions
– Hydroxide, OH-
– Nitrate, NO3
-
– Bicarbonate, HCO3
-
• Common 2- polyatomic ions
– Carbonate, CO3
2-
– Sulfate, SO4
2-
• Common 3- ions
– Phosphate, PO4
3-
4. Misconception Alert
• Some students mistake the subscript for the superscript and want to apply
it to the charge.
– For example: NH4
+
• Contains 1 nitrogen
• Contains 4 hydrogens
• Has a charge of 1+ (not 4+)
5. Polyatomic ions in compounds
• When you join a polyatomic ion with another ion in a compound, the
same rules apply as with other ions – determine the simplest ratio to
balance the charges.
– For example: What is the formula of the ionic compound made with
the calcium ion and the phosphate ion?
• Identify ions
– Calcium ion = Ca2+
– Phosphate ion = PO4
3-
• Balance Charges
– 3 calciums (6+ total) required to balance 2 phosphates (6-
total)
– Ca3(PO4)2
6. Note about the parentheses used.
• In the previous example, Ca3(PO4)2 , we used parentheses around
phosphate to indicate two phosphates. Without the parentheses, it would
look like this Ca3PO42 with 42 oxygens.
• Only use parentheses when you have more than one polyatomic ion in a
formula
– For example:
• Sodium nitrate is NaNO3
• Magnesium nitrate is Mg(NO3)2
7. Note about the naming
• In the previous example, we called the substances sodium nitrate and
magnesium nitrate. Notice how we did not call them nitride. Nitride is
the N3- ion. Recall that monatomic non-metal ions end in –ide.
• We do have one exception to be careful about with the –ide suffix that is
not a monatomic ion. It is the hydroxide ion – OH-
– Recognize the difference between the following
• O2- oxide
• H- hydride
• OH- hydroxide
8. Pause and Practice
• Write the formulas of the following ions:
– Ammonium
– Carbonate
– Nitrate
– Phosphate
– Hydroxide
– Bicarbonate
– Sulfate
9. Pause and Practice Answers
• Write the formulas of the following ions:
– Ammonium
• NH4
+
– Carbonate
• CO3
2-
– Nitrate
• NO3
-
– Phosphate
• PO4
3-
– Hydroxide
• OH-
– Bicarbonate
• HCO3
-
– Sulfate
• SO4
2-
10. Pause and Practice
• Write the formulas and names of the compounds made with the following
combinations of ions: (Hint: Remember to determine the charges on the
ions before combining them.)
– Sodium and phosphate ions
– Magnesium and sulfate ions
– Lithium and nitrate ions
– Calcium and hydroxide ions
– Beryllium and bicarbonate ions
11. Pause and Practice Answers
• Write the formulas and names of the compounds made with the following combinations of
ions:
– Sodium and phosphate ions
• Na+ and PO4
3-
• Na3PO4
• sodium phosphate
– Magnesium and sulfate ions
• Mg2+ and SO4
2-
• MgSO4
• Magnesium sulfate
– Lithium and nitrate ions
• Li+ and NO3
-
• LiNO3
• Lithium nitrate
– Calcium and hydroxide ions
• Ca2+ and OH-
• Ca(OH)2
• Calcium hydroxide
– Beryllium and bicarbonate ions
• Be2+ and HCO3
-
• Be(HCO3)2
• Beryllium bicarbonate
12. Pause and Practice
• What are the formulas of the following:
– Sodium sulfate
– Magnesium phosphate
– Ammonium sulfide
– Calcium nitrate
13. Pause and Practice
• What are the formulas of the following:
– Sodium sulfate
• Na+ and SO4
2-
• Na2SO4
– Magnesium phosphate
• Mg2+ and PO4
3-
• Mg3(PO4)2
– Ammonium sulfide
• NH4
+ and S2-
• (NH4)2S
– Calcium nitrate
• Ca2+ and NO3
-
• Ca(NO3)2
14. Misconception Alert
• Many students fail to recognize the difference between ions such as
sulfide and sulfate, nitride and nitrate, phosphide and phosphate.
– Remember that almost all ions that end in –ide are the monatomic
ion:
• Sulfide – S2-
• Nitride – N3-
• Phosphide – P3-
– Ions that end in –ate are always polyatomic
• Sulfate – SO4
2-
• Nitrate – NO3
-
• Phosphate – PO4
3-
