Decoding the chemical language: from formula to name.
Sodium hydroxide. Calcium nitrate. Carbon monoxide. A chemical bottle's name is not an arbitrary label: it is a precise instruction manual for what atoms are inside and how they are bonded. By the end, you can name ionic and covalent compounds, write formulas from names, and read the names on safety labels.
Core rules of nomenclature
- Step 1, check for a metal: If a metal (or ammonium NH4+) is present, the compound is ionic. If only nonmetals are present, it is covalent.
- Ionic Rules: Name the cation first, then the anion. Use no Greek prefixes. Charge balance fixes the ratio.
- Polyatomic Ions: Polyatomics stay intact as groups (e.g., SO42− is sulfate; use parentheses if you need more than one: Ca(NO3)2).
- Covalent Rules: Use Greek prefixes (mono-, di-, tri-, tetra-) to report atom counts. End the second element in -ide. Drop mono- on the first element.
Nomenclature Decision Flowchart
The nomenclature decision tree
Before you write a name or a formula, answer one question: is there a metal present?
Chemical naming has two separate systems. Mixing them up is the most common error here. Greek prefixes like di- and tri- belong only to covalent compounds. Ionic compounds never use prefixes, because charge balance fixes their ratio.
Branch A: Ionic compounds
Condition: Contains a metal cation (or NH4+) plus a nonmetal anion (or polyatomic ion).
- Rule: Cation name + anion name (ending in -ide, or a polyatomic name).
- No prefixes: Never write “disodium monoxide”. Na2O is sodium oxide, because Na is +1 and O is −2, and 2:1 is the only valid ratio.
- Ratio: Charge balance fixes it, so the name never counts atoms.
Branch B: Covalent compounds
Condition: Contains only nonmetals sharing valence electrons.
- Rule: [Prefix]-first element + [prefix]-second element-ide.
- Use prefixes: Carbon and oxygen can combine as CO (carbon monoxide) or CO2 (carbon dioxide). The prefixes tell you the exact molecular count.
- Exception: Omit mono- on the first element. Write carbon dioxide, not monocarbon dioxide.
Sort: which branch does each formula take?
One question decides everything downstream. Scan each formula for a metal, and watch for the one cation that behaves like a metal without containing one.
Checkpoint: naming across the branches
Commit to an answer before checking. The wrong options here are all things students actually write.
Ionic compounds and charge balance
An ionic formula shows a neutral formula unit: the total positive charge equals the total negative charge.
An ionic crystal is electrically neutral overall, so the cations and anions must balance to a net charge of zero. You can read main-group ion charges from the column positions on the periodic table.
| Group 1 | Group 2 | Group 13 | Group 15 | Group 16 | Group 17 |
|---|---|---|---|---|---|
| +1 | +2 | +3 | −3 | −2 | −1 |
| Na+, K+ | Mg2+, Ca2+ | Al3+ | N3−, P3− | O2−, S2− | F−, Cl−, Br− |
Forgetting that the charges must cancel. Na+ and O2− need two sodium ions, so the formula is Na2O, not NaO.
How many of each ion make the charges cancel?
Adjust the counts of cations and anions until the net charge reaches zero. Then check the formula and the name.
- Cation (positive ion)
- Anion (negative ion)
Charge-balance ledger
Fill the ledger one row at a time. The last row needs parentheses: decide how many nitrates you need before you decide how to write them. Type formulas without subscript formatting. Na2O and Ca(NO3)2 are both accepted.
EnrichmentRoman numerals for transition metals
Extra depth. The rest of the lesson works without it.
Transition metals (like iron, copper, and cobalt) can form several stable ions with different positive charges, called oxidation states. The name states the exact charge with a Roman numeral in parentheses right after the metal's name.
- Fe2+ + 2 Cl− → FeCl2: iron(II) chloride
- Fe3+ + 3 Cl− → FeCl3: iron(III) chloride
- Cu+ + O2− → Cu2O: copper(I) oxide
- Cu2+ + O2− → CuO: copper(II) oxide
The Roman numeral reports the cation's charge. It does not report the subscript in the formula.
Working backwards: where does the Roman numeral come from?
Every question so far has handed you the cation charge. Here you get only a formula, and you recover the charge from it.
Checkpoint: charge balance and the Roman numeral
Work out the ratio on paper first, then commit.
On a label. Copper(II) sulfate (CuSO4) is an ionic compound with an environmental hazard label. The Roman numeral (II) reports the Cu2+ charge, which the SO42− ion balances.
A stockroom bottle is labeled “Iron(III) oxide” and another is labeled “Sulfur trioxide”.
1. Classify each compound as ionic or covalent and state your reasoning.
2. Write the correct chemical formula for both compounds.
3. Explain what the Roman numeral (III) in iron(III) oxide and the prefix tri- in sulfur trioxide tell you about their structures.
- Classification (1 mark): You say iron(III) oxide is ionic (metal plus nonmetal) and sulfur trioxide is covalent (nonmetals only).
- Formulas (1 mark): You write Fe2O3 and SO3.
- Roman numeral (1 mark): You say (III) gives the Fe3+ charge, and balancing with O2− needs a 2:3 ratio.
- Greek prefix (1 mark): You say tri- counts atoms: three oxygen atoms bond to one sulfur atom in SO3.
Polyatomic ions and formula writing
Polyatomic ions are covalently bonded atomic clusters that act as a single unit with a net charge.
Polyatomic ions are not on the unit's external assessment. You still need them for lab work, chemical safety, and later chemistry.
Polyatomic ions travel together as unbreakable groups. If a formula needs more than one of them, put the whole ion in parentheses before you add the subscript.
| Ion | Name | Charge |
|---|---|---|
| NH4+ | Ammonium | +1 |
| OH− | Hydroxide | −1 |
| NO3− | Nitrate | −1 |
| HCO3− | Bicarbonate | −1 |
| SO42− | Sulfate | −2 |
| CO32− | Carbonate | −2 |
| PO43− | Phosphate | −3 |
Combine Ca2+ and NO3−.
To balance the +2 charge from calcium, you need two nitrate ions (−1 × 2 = −2). Write Ca(NO3)2: calcium nitrate.
Writing CaNO32 would mean 32 oxygen atoms. Always use parentheses when you multiply a polyatomic ion.
From name to formula
So far you have run formula to name. Writing a formula from a name is harder, because the name has no subscripts. You build them by balancing ionic charges.
Checkpoint: parentheses and lowest terms
Two formulas, one answer. Both halves have to be right.
Writing a formula that balances but is not in lowest terms. Mg2(SO4)2 is neutral, but the formula is MgSO4.
Binary covalent naming
Greek prefixes state the exact number of nonmetal atoms in a molecular formula.
Covalent compounds form between nonmetals. Nonmetals can combine in several ratios, such as CO and CO2, so charge balance does not fix the formula. Instead, Greek prefixes state the number of atoms of each element.
Three rules for covalent prefixes
- First element: Keep its element name. Add a prefix if there are 2 or more atoms (N2O4 → dinitrogen...). If there is only 1 atom, drop mono- (CO2 → carbon..., not monocarbon...).
- Second element: Always add a prefix, even mono-, and change the ending to -ide (CO → carbon monoxide, N2O4 → dinitrogen tetroxide).
- Vowel dropping: Avoid double vowels like mono-oxide or tetra-oxide. Drop the ‘a’ or ‘o’ to write monoxide and tetroxide.
Writing mono- on the first element. CO is carbon monoxide, not monocarbon monoxide.
Drill: prefix or no prefix?
Four compounds, one decision each. The wrong names in this drill are common ways students mix the two systems.
Checkpoint: reading prefixes in both directions
One formula to name, one name to write. Prefixes have to survive the round trip.
Storyline bridge: decoding GHS chemical labels
Connect systematic chemical names to the hazard labels on the classroom chemical tray.
In Block 1.1, you saw real chemical bottles on the front bench. Now you can decode what is inside from the names alone, before you open any lid.
| Hazard label | Name and formula | What the name tells you |
|---|---|---|
| Corrosive | Sodium hydroxide (NaOH) | An ionic compound of Na+ and the polyatomic ion OH−. It dissolves in water and releases corrosive hydroxide ions. |
| Toxic gas | Carbon monoxide (CO) | A covalent compound. The prefix mono- shows 1 oxygen atom. It is a lethal, odorless gas from incomplete combustion. |
A stockroom bottle is labeled “Aluminum oxide” and another is labeled “Sulfur trioxide”.
1. Classify each compound as ionic or covalent and state your reasoning.
2. Write the correct chemical formula for both compounds.
3. Explain what the 2:3 ratio in aluminum oxide and the prefix tri- in sulfur trioxide each tell you.
- Classification (1 mark): You say aluminum oxide is ionic (metal plus nonmetal) and sulfur trioxide is covalent (nonmetals only).
- Formulas (1 mark): You write Al2O3 and SO3.
- Ratio (1 mark): You say two Al3+ give +6 and three O2− give −6, so the charges cancel.
- Greek prefix (1 mark): You say tri- counts atoms: three oxygen atoms bond to one sulfur atom in SO3.
Chemical names tell emergency responders what they are facing. They can work out whether a spilled substance breaks into ions or evaporates as a toxic molecular gas, and that sets the safety plan.
Adaptive Competency Practice: HS-PS1-2
Test your mastery of chemical nomenclature, ionic charges, and covalent prefixes one question at a time. The adaptive engine identifies skill gaps and awards an authenticated completion receipt.