Testing for Bond Type left you with two families of substance that behave nothing like each other. Chemistry names them differently too, and that is not decoration — the naming system you reach for depends on which family you are holding, so identifying the compound is the first step of naming it, not something you do afterwards.
This page is the reasoning. Naming Rules at a Glance is the same material as a lookup sheet for when you are mid-question and just need the rule.
Two families, and you can tell them apart before you name them
Everything you measured lines up on one side or the other.
| Property | Ionic, such as | Molecular, such as |
|---|---|---|
| Made from | a metal and a non-metal | non-metals only |
| Held together by | attraction between ions, throughout a lattice | shared pairs within each molecule |
| Melting point | high — sodium chloride melts at 801 °C | low — methane boils at −161 °C |
| What melting breaks | attractions running through the whole crystal | only the weak attractions between molecules |
| Conducts as a solid | no — the ions cannot move | no — there are no ions |
| Conducts molten or dissolved | yes — the ions are now free to move | no |
| Physical character | hard, brittle, cleaves along planes when struck | soft, or a liquid or gas at room temperature |
| The formula means | the simplest whole-number ratio of ions | the actual number of atoms in one molecule |
The last row is the one that gets skipped and matters most. There is no molecule of sodium chloride, so is a formula unit — a statement that the crystal contains one sodium ion for every chloride ion. is a different kind of claim entirely: it says one real molecule contains one carbon atom and four hydrogen atoms.
The decisive laboratory test is the molten-or-dissolved conductivity row, and the reason is worth saying out loud: conducting requires charged particles that are free to move. Ionic compounds have the charges all along and only lack the freedom; melting or dissolving supplies it. Molecular compounds never have the charges at all.
Ionic compounds: charges decide the formula
Write the cation first, then the anion. The metal keeps its element name; the non-metal takes its stem and gains -ide. Sodium plus chlorine gives sodium chloride; magnesium plus oxygen gives magnesium oxide.
The formula comes from the charges, because a compound is electrically neutral overall. Magnesium is and chlorine is , so it takes two chloride ions to balance one magnesium ion: . Crossing the charges over as subscripts is a shortcut for that reasoning, and it works as long as you remember to reduce to the lowest ratio afterwards — with crosses over to , which is wrong. It is .
Multivalent metals need a Roman numeral, because the name has to identify which ion you mean. Iron forms both and , so “iron chloride” is ambiguous and unusable. Iron(II) chloride is ; iron(III) chloride is . The numeral gives the charge on the metal ion, not the number of anything — a mistake so common it is worth reading twice. Going backwards from a formula, work the charge out from the anions: has two copper ions balancing one , so each copper carries and the name is copper(I) oxide.
Polyatomic ions are groups of atoms that travel together carrying a single charge, and they keep their own names — sulfate stays sulfate, it does not become sulfide. When you need more than one of them the whole group goes in brackets: calcium nitrate is , never . Learn the common ones from Polyatomic Ions; there are about twenty that carry almost all the work, and they will save you more time than any other thing you memorise this year.
Molecular compounds: prefixes count the atoms
Two non-metals means there are no charges to balance, so nothing tells you the ratio — you have to state it. That is what the Greek prefixes are for.
Carbon and oxygen make both and , and both are real, common, and completely different in what they do to you. Calling them carbon monoxide and carbon dioxide is not fussiness; it is the only thing keeping them apart. Dinitrogen tetroxide is .
One convention: mono- is dropped from the first element and kept on the second. It is carbon dioxide, not monocarbon dioxide; but it is carbon monoxide, not carbon oxide.
Some molecular compounds keep older names that ignore all of this — water, ammonia, methane. Those are not exceptions to learn a rule for. They are simply names that were in use before the system existed and were too entrenched to change.
Acids look like ionic compounds and are named as their own thing
A formula starting with hydrogen, in aqueous solution, is an acid, and acids have a third naming convention layered on top of the other two. The short version:
- No oxygen in it: hydro- + stem + -ic acid. is hydrochloric acid.
- Contains an -ate polyatomic ion: stem + -ic acid. Sulfate gives sulfuric acid, .
- Contains an -ite polyatomic ion: stem + -ous acid. Sulfite gives sulfurous acid, .
The full pattern, including hydrates and the multivalent metals worth memorising, is set out in Naming Rules at a Glance. Why an acid behaves as an acid at all is Acids and Bases.
Naming is one of the few things in this course that is pure practice. Understanding the system takes twenty minutes; becoming fluent takes repetition, and that is what Naming Practice is for. Do it until you stop needing the reference sheet, because every quantitative page after this one assumes you can write a correct formula without thinking about it.
Curriculum connection
B2.7
write chemical formulae of binary and polyatomic compounds, including those with multiple valences, and name the compounds using the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system [AI, C]
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B3.5
compare and contrast the physical properties of ionic and molecular compounds (e.g., and ; and )
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