The Nomenclature Decision Tree
Before writing a name or formula, you must answer one fundamental question: Is there a metal present?
Chemical naming is split into two completely separate systems. Mixing them up is the most common error in chemistry! Greek prefixes like di- and tri- belong only to covalent compounds. Ionic compounds never use prefixes because their element ratio is mathematically fixed by electrical charge balance.
Branch A: Ionic Compounds
Condition: Contains a metal cation (or NH4+) + nonmetal anion (or polyatomic ion).
- Rule: Cation name + Anion name (ending in -ide or polyatomic name).
- No Prefixes: Never write "disodium monoxide". Na2O is simply sodium oxide because Na is +1 and O is −2; 2:1 is the only valid ratio.
- Roman Numerals: Used if the metal is a transition metal with multiple charges (e.g. Fe2+ vs Fe3+).
Branch B: Covalent Compounds
Condition: Contains only nonmetals sharing valence electrons.
- Rule: [Prefix]-FirstElement + [Prefix]-SecondElement-ide.
- Use Prefixes: Carbon and oxygen can combine as CO (carbon monoxide) or CO2 (carbon dioxide). Prefixes tell us 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 & Charge Balance
Ionic formulas represent neutral formula units where total positive charge equals total negative charge.
Because an ionic crystal lattice is electrically neutral overall, the ratio of cations to anions must balance to net charge zero. You can determine main-group ion charges directly from periodic table column positions:
| 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− |
Transition Metals & Roman Numerals
Transition metals (like iron, copper, cobalt) can form multiple stable ions with different positive charges (oxidation states). We specify the exact charge of the metal cation using a Roman numeral in parentheses immediately 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
Crucial Rule: The Roman numeral reports the cation's charge, NOT the subscript count in the formula!
Interactive Practice: Ionic Charge Balancer
Adjust the counts of cations and anions until the net charge reaches zero, then verify the resulting chemical formula and name.
Charge-balance ledger
Fill the ledger one row at a time. The last row is the case that 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.
Working backwards: where does the Roman numeral come from?
Every question so far has handed you the cation charge. Here you are given only a formula, and you have to recover the charge from it.
Checkpoint: charge balance and the Roman numeral
Work out the ratio on paper first, then commit.
Polyatomic Ions & Formula Writing
Polyatomic ions are covalently bonded atomic clusters that act as a single unit with a net charge.
Polyatomic ions are outside of externally assessable content for this unit, but are essential foundational knowledge for lab work, chemical hygiene, and future chemistry topics.
Polyatomic ions travel together as unbreakable groups during ionic compound formation. When balancing charges, if you need more than one polyatomic ion in a formula unit, enclose the entire ion in parentheses before adding the subscript!
Common Cation (+1)
- NH4+ : Ammonium
Common Anions (−1)
- OH− : Hydroxide
- NO3− : Nitrate
- HCO3− : Bicarbonate
Common Anions (−2 / −3)
- SO42− : Sulfate
- CO32− : Carbonate
- PO43− : Phosphate
Combine Ca2+ and NO3−:
To balance the +2 charge from Calcium, we need two Nitrate ions (−1 × 2 = −2). We write: Ca(NO3)2 : Calcium nitrate.
Writing CaNO32 would mean 32 oxygen atoms! Always use parentheses when multiplying a polyatomic ion.
From name to formula
The lesson so far has run formula → name. Target PS1-2.2 asks for both directions, and this is the harder one: nothing on the page tells you the subscripts, so you have to build them from the charges.
Checkpoint: parentheses and lowest terms
Two formulas, one answer. Both halves have to be right.
Binary Covalent Naming
Using Greek prefixes to specify exact nonmetal atom counts in molecular formulas.
Covalent compounds form between nonmetals. Because nonmetals can combine in multiple discrete ratios (e.g. CO vs CO2, NO vs NO2 vs N2O4), charge balance does not apply. Instead, we use Greek prefixes to state the number of atoms of each element directly:
Three Rules for Covalent Prefixes
- First Element: Keep its element name. Add a prefix if there are 2 or more atoms (e.g. N₂O₄ → dinitrogen...). If there is only 1 atom, drop mono- (e.g. CO₂ → carbon..., not monocarbon...).
- Second Element: Always attach a prefix (even mono-!) and change the ending to -ide (e.g. 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.
Drill: prefix or no prefix?
Four compounds, one decision each. The wrong names in this drill are the four most 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
Connecting systematic chemical names to actual hazard safety profiles on our classroom chemical tray.
The Classroom Chemical Tray Connection
On Block 1.1, you observed real chemical bottles on our front bench. Now you can decode exactly what is inside those bottles from their names alone before opening any lid!
Ionic compound containing Na+ and polyatomic OH−. Dissolves in water to release corrosive hydroxide ions.
Covalent compound. Prefix mono- indicates 1 oxygen atom. Lethal odourless gas from incomplete combustion.
Ionic compound. Roman numeral (II) reports Cu2+ charge balanced by SO42− polyatomic anion.
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 respective structures.
- Classifies correctly (1 mark): Iron(III) oxide is ionic (metal Fe + nonmetal O); Sulfur trioxide is covalent (nonmetals S + O only).
- Formulas correct (1 mark): Iron(III) oxide = Fe2O3; Sulfur trioxide = SO3.
- Explains Roman numeral (1 mark): Roman numeral (III) specifies the cation charge (Fe3+), which requires charge balancing with O2− to form a 2:3 ratio (Fe2O3).
- Explains Greek prefix (1 mark): Prefix tri- directly states the molecular atom count (3 oxygen atoms covalently bonded to 1 sulfur atom in an SO3 molecule).
Self-score: 4 = all four points clearly explained · 3 = correct formulas but mixed up Roman numeral vs prefix explanation · 2 = correct formulas only · ≤1 = incomplete classification.
In industry and emergency response, reading chemical names instantly tells responders whether a spilled substance will dissociate into ions in waterways (like copper sulfate) or evaporate as molecular toxic gases (like carbon monoxide), dictating immediate containment and safety protocols.