Naming Binary Compounds

Introduction

Every compound needs a name that any chemist in the world will recognise. Names such as "sodium chloride", "magnesium oxide" and "iron sulfide" are not random: they follow simple rules. The easiest compounds to name are binary compounds , which contain only two elements. Learning the two key rules — put the metal first, and end the non-metal with -ide — lets you name hundreds of compounds and read formulae with confidence.

Core explanation

What "binary" means. A binary compound contains exactly two elements. The number of atoms does not matter: NaCl, MgCl₂ and Al₂O₃ are all binary because each contains only two kinds of atom. A compound such as NaOH contains three elements (sodium, oxygen and hydrogen), so it is not binary.

Rule 1 — the metal comes first. In a compound of a metal and a non-metal, the metal is named first and keeps its full element name. Sodium stays "sodium", calcium stays "calcium", aluminium stays "aluminium". The formula also normally starts with the metal symbol: NaCl, CaO, Al₂O₃.

Rule 2 — the non-metal ends in -ide. The non-metal is named second. Take the start of its name (the stem) and add -ide. The common ones are worth learning by heart:

Element Stem + ending Example compound --- --- --- Chlorine chloride sodium chloride, NaCl Bromine bromide potassium bromide, KBr Iodine iodide silver iodide, AgI Fluorine fluoride calcium fluoride, CaF₂ Oxygen oxide magnesium oxide, MgO Sulfur sulfide zinc sulfide, ZnS Nitrogen nitride lithium nitride, Li₃N Hydrogen hydride sodium hydride, NaH Phosphorus phosphide calcium phosphide, Ca₃P₂

Why this order? In an ionic compound the metal forms the positive ion (the cation ) and the non-metal forms the negative ion (the anion ). Chemists always name the positive part first and the negative part second. The -ide ending signals "this is a simple negative ion made from one element".

The name does not show the numbers. For simple ionic compounds, the name does not say how many of each ion are present. Magnesium chloride is MgCl₂ and aluminium oxide is Al₂O₃, yet neither name contains a number. The ratio comes from the charges (or valencies) of the ions, which you will meet later in this unit. Molecular compounds of two non-metals, such as carbon dioxide, use prefixes instead — a topic for a later page.

Hydroxide and cyanide. A few groups of atoms also end in -ide, such as hydroxide (OH⁻) and cyanide (CN⁻). Compounds such as sodium hydroxide therefore end in -ide but are not binary. Treat these as exceptions to remember.

Step-by-step reasoning

To name a metal–non-metal binary compound from its formula:

1. Identify the two elements from their symbols. 2. Write the full name of the metal first. 3. Take the stem of the non-metal's name. 4. Add -ide to the stem and write it second. 5. Check: exactly two elements, metal first, one word ending in -ide.

Visual explanation

Imagine the name built from two coloured cards laid side by side. The first card, in red, carries the metal's full name: "calcium". The second card, in blue, has the non-metal's name with its ending snipped off and a new tag "-ide" glued on: "chlor" + "ide". Placed together, the cards read "calcium chloride".

Real-world analogy

Naming a binary compound is like writing a two-part address label: the first line is the person's full name, and the second line is a short, standard code for the town. Everyone follows the same format, so any reader instantly knows which part is which and nothing is confused.

Real-world example

Toothpaste packaging often lists "sodium fluoride" as the active ingredient. The name tells you at once that the compound contains sodium, a metal, and fluorine, a non-metal, present as fluoride ions. Those fluoride ions help strengthen tooth enamel against decay.

Why?

Why change the non-metal's ending at all? Because a fluoride ion is not the same thing as fluorine. Fluorine is a pale yellow, extremely reactive gas made of F₂ molecules; fluoride is a stable negative ion found in toothpaste. The different ending prevents chemists confusing the element with the ion it forms.

Common misconception

"Sodium chloride contains chlorine gas." It does not. Salt contains chloride ions, which have very different properties from the toxic green gas chlorine. The -ide ending tells you the element is present in combined form as an ion, not as the free element.

Worked example

Question: Name the compounds with the formulae KI, CaS and Li₂O.

Reasoning: KI contains potassium (metal) and iodine (non-metal): potassium + iod-ide. CaS contains calcium and sulfur: calcium + sulf-ide. Li₂O contains lithium and oxygen: lithium + ox-ide. The subscript 2 does not appear in the name.

Answer: Potassium iodide, calcium sulfide and lithium oxide.

Quick check

1. What is the name of the compound with the formula MgBr₂? Answer: Magnesium bromide.

Exam focus

Examiners often ask you to name a compound from a formula or to spot why a name is wrong. Common errors that lose marks are writing "chlorine" instead of "chloride", putting the non-metal first, and misspelling "sulfide". Remember that hydroxide ends in -ide but contains three atoms.

Advanced insight

The -ide ending has a longer history than modern ion theory. It came from the French naming system of Lavoisier and his colleagues in 1787, who used "-ure" (as in chlorure); English chemists adapted this to -ide. Today IUPAC rules keep the metal-first, -ide pattern for ionic binaries and extend it to compounds of two non-metals, where the less electronegative element is named first, as in hydrogen chloride.

Summary

A binary compound contains exactly two elements. In a metal–non-metal binary compound, the metal is named first using its full name, and the non-metal is named second with its ending changed to -ide: chloride, oxide, sulfide, nitride and so on. The name does not show the numbers of each ion. A few polyatomic groups, such as hydroxide, also end in -ide.

Practice questions

1. Name the compound with the formula AlCl₃. Answer: Aluminium chloride. 2. Write the -ide names formed by nitrogen, bromine and phosphorus. Answer: Nitride, bromide and phosphide. 3. Is sodium hydroxide (NaOH) a binary compound? Explain. Answer: No, because it contains three elements: sodium, oxygen and hydrogen. 4. A student writes "chloride sodium" for NaCl. Which rule has been broken? Answer: The ending is correct but the order is wrong: the metal must be named first, so the correct name is sodium chloride. 5. Explain why the name "zinc sulfur" is incorrect for ZnS. Answer: The non-metal must take the -ide ending to show it is present as an ion, so the correct name is zinc sulfide.