The bottle of bleach under the sink says sodium hypochlorite; the safety sheet says NaClO. A chemist reads either one and knows exactly which ions are inside, because names and formulas follow a small set of rules.

This lesson gives you those rules: how ion charges fix an ionic formula, how to name ionic compounds, molecular compounds and acids, and how to go back from a name to a formula (Figure 1).

1. Formula or namewhat are the elements?
2. Q1metal ion + anion, two nonmetals, or H + anion?
3. Ionic, molecular or acidthe kind decides the rule
4. rulecharges, prefixes, or -ic and -ous
5. Name or formulachecked both ways

Ionic: the charges must total zero; no prefixes.Molecular: prefixes count atoms; no charges.

Learning Objectives

After this lesson, you should be able to:

1. Write formulas of ionic compounds from ion charges, including polyatomic ions.
2. Name and write formulas for ionic compounds, binary molecular compounds and common acids.

Key Ideas

1. Formulas of Ionic Compounds from Ion Charges

An ionic compound is made of positive ions (cations, usually metal ions) and negative ions (anions). It has no overall charge, so the positive and negative charges must cancel. Write the cation first, then use the smallest whole numbers of each ion that make the total charge zero (Figure 2):

Here $n_{\text{c}}$ and $n_{\text{a}}$ are the numbers of cations and anions, and $z_{\text{c}}$ and $z_{\text{a}}$ are their charges. For magnesium chloride, one Mg²⁺ needs two Cl⁻, so the formula is MgCl₂.

A polyatomic ion is a group of atoms that carries one charge, such as nitrate, NO₃⁻, or ammonium, NH₄⁺. It behaves as one unit: when a formula needs more than one of it, put it in parentheses and write the number outside, as in Ca(NO₃)₂. Learn the common ones in Figure 3.

Key idea: the charges decide the subscripts. Cross-check by adding up the charges: they must total zero.

2. Naming Ionic Compounds

Name the cation first, then the anion. A monatomic anion takes the stem of the element plus -ide (chloride, oxide, nitride); a polyatomic ion keeps its own name (sulfate, nitrate). Ionic names never use prefixes such as di- or tri-, because the charges already fix the numbers.

Metals of Groups 1A and 2A, aluminum, zinc, silver and cadmium form only one common ion, so their names need nothing more. Most transition metals, tin and lead form more than one ion, so the name gives the charge as a Roman numeral in parentheses: Cu₂O is copper(I) oxide and CuO is copper(II) oxide (Figure 4).

Key idea: a Roman numeral is the charge of one metal ion, never the number of metal ions in the formula.

3. Binary Molecular Compounds

Two nonmetals share electrons instead of forming ions, so there are no charges to balance. The name counts the atoms with Greek prefixes: mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6, hepta- 7, octa- 8, nona- 9, deca- 10 (Figure 5).

• Name the element written first with its prefix, but never with mono-: CO₂ is carbon dioxide.
• Name the second element with its prefix, its stem and -ide: PCl₃ is phosphorus trichloride.
• Drop the final a or o of a prefix before oxide: N₂O₄ is dinitrogen tetroxide, CO is carbon monoxide.

IUPAC also accepts the full prefixes (tetraoxide, pentaoxide). This lesson uses the shorter forms, as most textbooks do.

4. Naming Acids

An acid here is a compound of hydrogen with an anion that releases H⁺ in water. Its formula has one H for each negative charge of the anion. The name comes from the anion name (Figure 6):

• An -ide anion gives hydro- + stem + -ic acid: HCl(aq) is hydrochloric acid.
• An -ate anion gives stem + -ic acid: H₂SO₄ (sulfate) is sulfuric acid.
• An -ite anion gives stem + -ous acid: H₂SO₃ (sulfite) is sulfurous acid. The prefixes per- and hypo- stay: HClO₄ is perchloric acid, HClO is hypochlorous acid.

Key idea: -ate goes with -ic, -ite goes with -ous, and hydro- appears only for acids without oxygen.

Worked Example 1: Formula of an Ionic Compound

Problem: Write the formula of aluminum oxide.

Step 1. Write the ions and their charges. Aluminum forms only Al³⁺; oxide is O²⁻.

Step 2. Balance the charges with the smallest whole numbers. 2 Al³⁺ carry +6 and 3 O²⁻ carry −6:

Step 3. Write the formula. The numbers become subscripts.

Final Answer: Al₂O₃

1. Aluminum oxideionic: metal + nonmetal
2. Step 1ions: Al³⁺ and O²⁻
3. 2 Al³⁺ and 3 O²⁻+6 and −6 cancel
4. Step 3numbers become subscripts
5. Al₂O₃Final answer

A quick way to find the numbers is to cross the charges over (the 3 of Al³⁺ becomes the subscript of O, the 2 of O²⁻ the subscript of Al), then reduce if both numbers share a factor.

Your turn: formula of an ionic compound

Same three steps as Worked Example 1: the ions and charges, the smallest numbers that give zero total charge, then the formula.

Worked Example 2: A Formula with a Polyatomic Ion

Problem: Write the formula of calcium phosphate.

Step 1. Write the ions and their charges. Calcium forms only Ca²⁺; phosphate is PO₄³⁻.

Step 2. Balance the charges with the smallest whole numbers. 3 Ca²⁺ carry +6 and 2 PO₄³⁻ carry −6:

Step 3. Write the formula. Two phosphate ions are needed, so PO₄ goes in parentheses with a 2 outside.

Final Answer: Ca₃(PO₄)₂

Without the parentheses, Ca₃PO₄₂ would mean 42 oxygen atoms. The subscript 4 belongs to the ion and never changes.

Your turn: formulas with polyatomic ions

Same steps as Worked Example 2: watch for a polyatomic ion that is needed more than once.

Worked Example 3: Naming a Compound of a Metal with Several Ions

Problem: Name the compound Fe₂O₃.

Step 1. Identify the anion and its charge. Oxide is O²⁻. Iron forms more than one ion (Fe²⁺ and Fe³⁺), so its charge must be worked out.

Step 2. Find the charge of the metal ion. 3 O²⁻ carry a total charge of −6, so the 2 iron ions carry +6 together:

Step 3. Name the compound. Metal name, its charge as a Roman numeral in parentheses, then the anion name.

Final Answer: iron(III) oxide

The subscript 2 of iron is not the charge. Rust is mostly iron(III) oxide; FeO, with Fe²⁺, is iron(II) oxide.

Your turn: naming ionic compounds

Same steps as Worked Example 3. When the metal has only one common ion, skip the Roman numeral.

Worked Example 4: A Molecular Compound and an Acid

Problem: Name (a) N₂O₅ and (b) the acid HNO₂.

(a) Step 1. Identify the compound type. Nitrogen and oxygen are both nonmetals, so N₂O₅ is a binary molecular compound: name it with prefixes, not charges.

(a) Step 2. Name each element with its prefix. 2 N: dinitrogen. 5 O: penta- + ox + -ide, and the a of penta is dropped before oxide: pentoxide.

Final Answer: dinitrogen pentoxide

(b) Step 1. Identify the anion. Without its H⁺ ion, HNO₂ leaves NO₂⁻, the nitrite ion.

(b) Step 2. Apply the acid naming rule. -ite becomes -ous.

Final Answer: nitrous acid

Compare HNO₃: its anion is nitrate, so it is nitric acid. One more oxygen atom changes -ous to -ic.

Your turn: molecular compounds and acids

Each problem is like part (a) or part (b) of Worked Example 4, in either direction: from formula to name or from name to formula.

Common Mistakes

Common mistake: Using the charges as the subscripts of their own ions. For aluminum oxide the 3 of Al³⁺ goes to oxygen, not to aluminum: Al₂O₃, not Al₃O₂.

Common mistake: Forgetting parentheses around a polyatomic ion. Ca(NO₃)₂ has two nitrate ions; CaNO₃₂ is not a formula.

Common mistake: Reading the Roman numeral as a count. Iron(III) means Fe³⁺. Fe₂O₃ is iron(III) oxide, not iron(II) oxide.

Common mistake: Using prefixes for ionic compounds. CaCl₂ is calcium chloride, not calcium dichloride. Prefixes are only for compounds of two nonmetals.

Common mistake: Using charges or Roman numerals for molecular compounds. SO₃ is sulfur trioxide, not sulfur(VI) oxide.

Common mistake: Mixing up -ic and -ous, or adding hydro- to an oxyacid. HNO₃ is nitric acid (nitrate), HNO₂ is nitrous acid (nitrite); hydro- is only for acids without oxygen, such as hydrochloric acid.

Key Takeaway

First decide the kind of compound. Ionic: balance the ion charges to get the formula, and name cation then anion, with a Roman numeral only for metals that form more than one ion. Molecular (two nonmetals): count atoms with prefixes. Acids: one H per negative charge; -ide becomes hydro- + stem + -ic, -ate becomes -ic, -ite becomes -ous.

The relations to keep at hand:

Practice: randomized problems

Each problem below uses fresh numbers every time: work it out, check your answer, then open the worked solution to compare your steps.

Your results

This summary collects every practice problem you try on this page and suggests what to review next.

Figures: original diagrams by Dr. Ray’s Learning Center, licensed CC BY 4.0. Ion names and charges: K. Whitten, R. Davis, M. Peck and G. Stanley, General Chemistry, 9th ed., Table 6-6 and sections 6-3 and 6-4; naming rules: IUPAC, Nomenclature of Inorganic Chemistry (Recommendations 2005). The practice problems are generated and checked by an audited engine; see the lesson audit log.