Naming Ionic and Molecular Compounds Practice

What You Need to Know About Naming Ionic and Molecular Compounds

Chemistry students waste hours on this stuff. The naming conventions aren't hard—you just need to know the rules and apply them. This guide cuts through the confusion so you can practice without guessing.

Ionic Compounds: Metals + Nonmetals

These compounds form when a metal gives electrons to a nonmetal. The metal becomes a positive ion, the nonmetal becomes a negative ion, and they stick together through electrostatic attraction.

The Basic Rule

Write the cation name first, then the anion name (with its suffix changed to -ide).

Simple Ionic Compounds

For metals that form only one ion, the naming is straightforward:

• NaCl = Sodium Chloride
• K₂O = Potassium Oxide
• MgS = Magnesium Sulfide
• CaCl₂ = Calcium Chloride

The metal name stays the same. The nonmetal drops its ending and gets -ide.

Transition Metals: The Complication

Metals like iron, copper, and lead can form multiple ions. You must specify the charge using Roman numerals.

• FeO = Iron(II) Oxide (Fe²⁺ + O²⁻)
• Fe₂O₃ = Iron(III) Oxide (Fe³⁺ + O²⁻)
• CuCl = Copper(I) Chloride (Cu⁺ + Cl⁻)
• CuCl₂ = Copper(II) Chloride (Cu²⁺ + Cl⁻)

The Roman numeral tells you the charge on the metal ion, not the number of atoms.

Polyatomic Ions: When -ide Isn't Enough

Some anions have their own names. You don't change these:

• NaOH = Sodium Hydroxide
• CaSO₄ = Calcium Sulfate
• KNO₃ = Potassium Nitrate
• NH₄Cl = Ammonium Chloride

Molecular Compounds: Nonmetals Only

These form when nonmetals share electrons. Both elements need prefixes to indicate how many atoms are present.

The Prefix Rule

Use Greek prefixes for both elements. The first element keeps its name, the second gets -ide.

• CO₂ = Carbon Dioxide (not Carbon Monooxide)
• H₂O = Dihydrogen Monoxide (commonly called Water)
• N₂O₄ = Dinitrogen Tetroxide
• SF₆ = Sulfur Hexafluoride

Drop the prefix on the first element if it's just one atom:

• CO = Carbon Monoxide (not Monocarbon Monoxide)
• N₂O = Dinitrogen Monoxide

Common Prefix Cheat Sheet

• Mono- = 1
• Di- = 2
• Tri- = 3
• Tetra- = 4
• Penta- = 5
• Hexa- = 6
• Hepta- = 7
• Octa- = 8

Ionic vs. Molecular: Side-by-Side Comparison

Common Mistakes Students Make

These errors show up constantly in assignments and exams:

• Forgetting Roman numerals for metals like Fe, Cu, Sn. FeCl₂ is Iron(II), not just Iron Chloride.
• Adding -ide to polyatomic ions. Sulfate is SO₄²⁻, not sulfoxide. You say Calcium Sulfate, not Calcium Sulfate-ide.
• Skipping prefixes on molecular compounds. CO₂ is Carbon Dioxide, not Carbon Oxide.
• Confusing ionic and molecular rules. Ionic uses no prefixes on the metal. Molecular uses prefixes on both.
• Dropping the final "a" in prefixes. It's Dinitrogen Pentoxide, not Dinitrogen Pentaoxide.

How to Practice: Step-by-Step

Reading a Formula and Naming It

1. Identify the elements. Is there a metal? Then it's likely ionic.
2. Check for transition metals. If the metal can have multiple charges, figure out which one from the formula.
3. Name the anion. Nonmetals get -ide. Polyatomic ions keep their names.
4. Add Roman numerals if needed.

Writing a Formula from a Name

1. Identify the ions. Calcium = Ca²⁺. Chloride = Cl⁻.
2. Balance the charges. Ca²⁺ + 2Cl⁻ = CaCl₂.
3. Check your work. Total positive charge equals total negative charge.

Quick Practice Set

Try these without looking at the answers:

1. Name: Al₂O₃
2. Name: PCl₅
3. Name: Fe₂(SO₄)₃
4. Write: Tin(IV) Oxide
5. Write: Diphosphorus Pentoxide

Answers:

1. Aluminum Oxide
2. Phosphorus Pentachloride
3. Iron(III) Sulfate (the sulfate is SO₄²⁻, three of them = -6 total, so iron must be +6 total, or +3 each)
4. SnO₂
5. P₂O₅

What to Do Next

You don't need expensive textbooks or courses. Grab a list of ionic charges, memorize the common polyatomic ions, and practice converting 20 formulas to names and 20 names to formulas daily. After a week, this becomes automatic.

The only way to learn this is by doing it. Start now.