Polyatomic ions

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.

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, CO32-

– Sulfate, SO42-

• Common 3- ions

– Phosphate, PO43-

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+)

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 = PO43-

• Balance Charges

– 3 calciums (6+ total) required to balance 2 phosphates (6-total)

– Ca3(PO4)2

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

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

Pause and Practice

• Write the formulas of the following ions:

– Ammonium

– Carbonate

– Nitrate

– Phosphate

– Hydroxide

– Bicarbonate

– Sulfate

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-

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

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 PO43-

• Na3PO4

• sodium phosphate– Magnesium and sulfate ions

• Mg2+ and SO42-

• 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

Pause and Practice

• What are the formulas of the following:

– Sodium sulfate

– Magnesium phosphate

– Ammonium sulfide

– Calcium nitrate

Pause and Practice

• What are the formulas of the following:

– Sodium sulfate

• Na+ and SO42-

• Na2SO4

– Magnesium phosphate

• Mg2+ and PO43-

• Mg3(PO4)2

– Ammonium sulfide

• NH4+ and S2-

• (NH4)2S

– Calcium nitrate

• Ca2+ and NO3-

• Ca(NO3)2

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 – SO42-

• Nitrate – NO3-

• Phosphate – PO43-

Try the exercises.