Slide presentation
Naming Salts with Polyatomic Ions
General Chemistry • Chemical Compounds
Topic 1 · Ionic naming
Naming Salts with Polyatomic Ions
Polyatomic-ion salts are ionic compounds made from cations and multi-atom anions or cations. The ion names stay intact while charge balance controls the formula.
Learning target
Name salts containing polyatomic ions and write formulas from names using ion charges, parentheses, and Roman numerals when needed.
calcium nitrate
Why it matters
Many real salts are built from polyatomic ions.
Fertilizers, minerals, antacids, cleaning products, and laboratory reagents often contain ions such as nitrate, sulfate, carbonate, phosphate, ammonium, and hydroxide.
Chemical identity
NaNO3 and NaNO2 are different salts because nitrate and nitrite are different ions.
Formula accuracy
Parentheses in Al2(SO4)3 show three sulfate ions, not only three oxygen atoms.
Charge logic
A neutral salt forms when total positive charge equals total negative charge.
Core concept
Name the ions; balance the formula.
The name of a salt usually gives the cation first and the anion second. The formula must be neutral, so subscripts come from charge balance.
1. Identify cation
Use the metal name or ammonium for NH4+.
2. Identify anion
Keep the polyatomic ion name such as nitrate or sulfate.
3. Balance charge
Use the lowest whole-number ratio of ions.
4. Use parentheses
Use them when a polyatomic ion appears more than once.
Pattern: cation name + optional Roman numeral + polyatomic ion name.
Vocabulary
Polyatomic ions have names and fixed charges to remember.
A polyatomic ion is a charged group of atoms that behaves as one unit in ionic formulas and names.
| Ion | Name | Charge | Example salt |
|---|---|---|---|
| NH4+ | ammonium | 1+ | NH4Cl, ammonium chloride |
| NO3− | nitrate | 1− | KNO3, potassium nitrate |
| SO42− | sulfate | 2− | CaSO4, calcium sulfate |
| CO32− | carbonate | 2− | Na2CO3, sodium carbonate |
| PO43− | phosphate | 3− | AlPO4, aluminum phosphate |
| OH− | hydroxide | 1− | Mg(OH)2, magnesium hydroxide |
Main relationship
A salt formula must have zero net charge.
Use ion charges to determine the smallest whole-number ratio of cations to anions. Parentheses show repeated polyatomic ions.
For Al3+ and SO42−, the least common charge magnitude is 6. Two Al3+ ions and three sulfate ions give Al2(SO4)3.
Parentheses needed
Ca(NO3)2 has two nitrate ions, so nitrate is placed in parentheses.
Parentheses not needed
NaNO3 has only one nitrate ion, so no parentheses are used.
Interactive simulation
Choose ions and build a neutral salt.
Select a cation and a polyatomic anion. The builder balances charges, writes the formula, and creates the correct name.
Polyatomic salt builder
Formula and name
Ca(NO₃)₂ — calcium nitrate
Ca2+ needs two nitrate ions, so parentheses are used.
Static fallback model
Ca2+ needs two NO3− ions. The formula is Ca(NO3)2, and the name is calcium nitrate.
Cation
Ca2+
Anion
2 NO₃−
Dynamic relationship
Charge balance chooses the ion ratio.
The visual compares total positive and negative charge after the formula is balanced. Switch views to see the naming path.
This visual updates from the selected cation and anion in the builder.
Worked example
Name Fe2(SO4)3.
The formula contains iron and sulfate. Since iron can form more than one charge, use the sulfate charge to find the Roman numeral.
Identify the anion
SO42− is sulfate. Three sulfate ions give a total charge of −6.
Find the metal charge
Two iron ions must total +6, so each iron ion is Fe3+.
Name the compound
Fe3+ is iron(III), and SO42− is sulfate.
Final answer: Fe2(SO4)3 is iron(III) sulfate.
Common mistake
Do not change polyatomic ion endings like binary anions.
Polyatomic ion names such as nitrate, sulfate, carbonate, and phosphate are not changed to -ide in normal salt naming. Keep the ion name intact.
Incorrect reasoning
“NaNO3 should be sodium nitride because the anion comes second.”
Nitride is N3−, not NO3−.
Correct reasoning
NO3− is nitrate, so NaNO3 is sodium nitrate.
Practice check
Name Al2(CO3)3 and write ammonium phosphate.
Question: What is the correct name of Al2(CO3)3, and what formula represents ammonium phosphate?
Show answer
Name Al2(CO3)3
Aluminum is Al3+, and CO32− is carbonate. The name is aluminum carbonate.
Write ammonium phosphate
Ammonium is NH4+, and phosphate is PO43−. Three ammonium ions are needed: (NH4)3PO4.
Reasonableness check
Parentheses mean multiple copies of the entire polyatomic ion: (NH4)3 contains three ammonium ions.
Apply the topic
Use ion identity first, then formula balance.
When you see a polyatomic ion, keep its name together. When you write a formula, balance charges before adding subscripts or parentheses.
Open the calculator
Practice naming salts with polyatomic ions and writing balanced formulas.
Try related questions
Check your understanding of ion names, parentheses, charge balance, and Roman numerals.
Recognize ions
Identify cation and anion names.
Read charges
Use known ion charges.
Balance ratio
Make net charge equal zero.
Write cleanly
Use parentheses only for repeated polyatomic ions.
Final summary
Polyatomic-ion salts are named by ion identity and charge balance.
Keep ion names intact.
Nitrate, sulfate, carbonate, phosphate, ammonium, and hydroxide keep their standard names.
Balance charge.
Use the lowest whole-number ratio of cations and anions.
Use parentheses carefully.
Parentheses are used only when a polyatomic ion appears more than once.
Use Roman numerals when needed.
Variable-charge metals such as Fe and Cu require Roman numerals in the name.
Key idea: A correct salt name tells the ions; a correct salt formula shows the neutral ion ratio.