Nomenclature – Interactive Study Tool

Naming and Writing Formulas for Chemical Compounds

Read a formula → decide the type → apply that type's rules

Four kinds of substance you name in Unit 1a

Elemental molecules

Two (or more) atoms of the same non-metal bonded together. The seven diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ — plus their friends P₄ (phosphorus) and S₈ (sulfur). The name is just the element — "hydrogen", "chlorine", "phosphorus". No prefixes, no "-ide".

Covalent (molecular) compounds

Two different non-metals sharing electrons. Greek prefixes say how many of each atom: CO₂ = carbon dioxide, OF₂ = oxygen difluoride. Second element ends in "-ide".

Ionic compounds

A metal (or NH₄⁺) transfers electrons to a non-metal or polyatomic ion. Name = metal/cation name, then non-metal/anion + "-ide"

Hydrates

An ionic compound with water locked into its crystal lattice. Name the salt, then a prefix + "hydrate" for the water: CuSO₄·5H₂O = copper(II) sulfate pentahydrate.

Which set of rules do I use? — decision flow

Start: look at the very first part of the formula.

One element, written as a molecule (e.g. O₂, also P₄ / S₈)
→ elemental molecule → name = the element.

Two different non-metals (e.g. N₂O₄)
→ covalent compound → prefix each element, "-ide" ending.

Metal + non-metal / polyatomic (e.g. CaCl₂)
→ ionic compound → metal name + non-metal "-ide". No prefixes.

…·xH₂O on the end (e.g. MgSO₄·7H₂O)
→ hydrate → name the salt, then "(prefix)hydrate".

Common-name exceptions you just memorise: H₂O = water · NH₃ = ammonia · CH₄ = methane.

Greek prefixes — used for covalent compounds & hydrates

#	Prefix	#	Prefix
1	mono‑	6	hexa‑
2	di‑	7	hepta‑
3	tri‑	8	octa‑
4	tetra‑	9	nona‑
5	penta‑	10	deca‑

In a covalent compound the prefix "mono-" is dropped on the first element only — CO₂ is "carbon dioxide", not "monocarbon dioxide". In a hydrate name, every prefix including "mono-" is kept ("…monohydrate").

Naming and Formula Writing Rules — by compound type

Open a category for its rules, worked examples, and practice

Elemental molecules — H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ (also P₄, S₈)

Rule. A few elements never exist as single atoms under normal conditions — the seven diatomic elements pair up (subscript 2), while phosphorus and sulfur travel as the larger molecules P₄ and S₈. Either way the name is simply the element name — no prefix, no "-ide".

Formula	Name	Everyday encounter
H₂	hydrogen	rocket fuel, hydrogen fuel cells
N₂	nitrogen	78% of the air; flush gas in food packaging
O₂	oxygen	21% of the air; what you breathe in
F₂	fluorine	pale-yellow gas, most reactive element
Cl₂	chlorine	pool & drinking-water disinfectant
P₄	phosphorus	white phosphorus — match heads, tracer rounds
S₈	sulfur	yellow ring molecule — gunpowder, vulcanised rubber

Memory aid — the "H O F Br I N Cl" seven: H, O, F, Br, I, N, Cl are all diatomic as free elements. HOFBrINCl's friends P₄ and S₈ are the other elements written as molecules — still named just "phosphorus" and "sulfur", no prefixes.

Practice

1. Write the formula for a molecule of chlorine gas.

Cl₂ — two chlorine atoms sharing one pair of electrons.

2. A cylinder is labelled "N₂". Is the contents an element or a compound?

An element — only one kind of atom. It is just written as a diatomic molecule. Name: nitrogen.

3. True or false: O₂ should be named "dioxygen".

False. Greek prefixes are only for covalent compounds (two different non-metals). An elemental molecule is just "oxygen".

Covalent (molecular) compounds — HCl, CO₂, H₂O … OF₂

Two different non-metals. Steps:
1. Write the less-electronegative element first (further left / lower on the table), with its prefix (drop "mono-" on the first element).
2. Write the second element's root + "-ide", with its prefix (keep "mono-" here → "monoxide").
3. Elide the double vowel only where "mono-" meets "oxide": mono+oxide → monoxide. Every other prefix stays complete — e.g. N₂O₄ is dinitrogen tetraoxide.

Formula	Name
HCl	hydrogen chloride (the gas; "hydrochloric acid" once dissolved in water)
CO₂	carbon dioxide
CO	carbon monoxide
N₂O₄	dinitrogen tetraoxide
OF₂	oxygen difluoride

Memorised common names (do not build these from prefixes): H₂O = water · NH₃ = ammonia · CH₄ = methane.

Practice

1. Name PCl₃.

Phosphorus trichloride. P first (no "mono-"), then tri- + chlor + "-ide".

2. Write the formula for dinitrogen monoxide.

N₂O. "di" → 2 N, "mon(o)oxide" → 1 O.

3. Why is CH₄ not called "carbon tetrahydride"?

It has a retained common name, methane. A few molecular compounds (water, ammonia, methane) are always named this way.

Ionic compounds — metal + non-metal (or polyatomic ion)

A metal transfers electrons to a non-metal. Steps:
1. Cation first — the metal keeps its element name (Na⁺ = "sodium").
2. Anion second — a single non-metal takes its root + "-ide" (Cl⁻ = "chloride", O²⁻ = "oxide"); a polyatomic ion keeps its own name (SO₄²⁻ = "sulfate").
3. Multivalent metal (Fe, Cu, Pb, Sn…) — show its charge with a Roman numeral: FeCl₃ = iron(III) chloride.
4. No prefixes. Find the subscripts by balancing the total charge: take the smallest whole-number count of each ion so the positive and negative charges add to zero. Mg²⁺ with Cl⁻ → two Cl⁻ (2 × 1− cancels 1 × 2+) → MgCl₂.

Ions	Ratio	Formula → name
Na⁺, Cl⁻	1 : 1	NaCl → sodium chloride
Mg²⁺, Cl⁻	1 : 2	MgCl₂ → magnesium chloride
Al³⁺, O²⁻	2 : 3	Al₂O₃ → aluminium oxide
Ca²⁺, NO₃⁻	1 : 2	Ca(NO₃)₂ → calcium nitrate
Fe³⁺, O²⁻	2 : 3	Fe₂O₃ → iron(III) oxide

Practice

1. Name K₂S.

Potassium sulfide. K⁺ and S²⁻; two K⁺ balance one S²⁻.

2. Write the formula for aluminium fluoride.

AlF₃. Al³⁺ needs three F⁻ to reach a net charge of zero.

3. CuO and Cu₂O are both "copper oxide". How do their names differ?

Copper is multivalent, so the charge is stated: CuO = copper(II) oxide (Cu²⁺), Cu₂O = copper(I) oxide (Cu⁺).

Hydrates — an ionic salt with water in the lattice

Formula: write the anhydrous salt, a raised dot "·", then the number of waters and H₂O — CuSO₄·5H₂O.
Name: name the salt exactly as an ionic compound, then add a Greek prefix + "hydrate" for the water. Keep "mono-". The dot is not spoken.

Formula	Name
CuSO₄·5H₂O	copper(II) sulfate pentahydrate
MgSO₄·7H₂O	magnesium sulfate heptahydrate (Epsom salts)
CaSO₄·2H₂O	calcium sulfate dihydrate (gypsum)
Na₂CO₃·10H₂O	sodium carbonate decahydrate (washing soda)

Heating a hydrate drives off the water ("heating to constant mass"), leaving the anhydrous salt — the blue → white change in CuSO₄ is a classic test for water. See the Hydrate Structures tab.

Practice

1. Name FeCl₃·6H₂O.

Iron(III) chloride hexahydrate. Name the salt (multivalent metal → Roman numeral), then "hexa" + "hydrate".

2. Write the formula for barium hydroxide octahydrate.

Ba(OH)₂·8H₂O. Ba²⁺ + 2 OH⁻, then "·8H₂O".

3. A 2.50 g hydrate sample is heated to 1.60 g of anhydrous salt. What left, and what is its mass?

Water — mass 2.50 − 1.60 = 0.90 g. Moles of water ÷ moles of anhydrous salt gives the coefficient "x" in ·xH₂O.

Hydrate Structures

What "·xH₂O" actually looks like inside the crystal

Water held in the lattice

FormulaCuSO₄·5H₂O

Waters in lattice5

Coordination4 on Cu²⁺, 1 on sulfate

StateHydrated

In copper(II) sulfate pentahydrate four water molecules point their δ− oxygen atoms straight at the central Cu²⁺ ion (a square around it); the fifth water is hydrogen-bonded to the SO₄²⁻ ion. In magnesium sulfate heptahydrate (Epsom salts) six waters surround Mg²⁺ in an octahedron and a seventh sits on the sulfate. Heating breaks these weak attractions and the water leaves as vapour, collapsing the hydrate to the anhydrous salt.

Bonding Simulator

Electrons shared (covalent) or transferred (ionic) — pick a compound

Choose Covalent or Ionic, then click a compound. Use bond → / transfer → to run the animation and ↺ reset to return the atoms to their separated state.

Real-World Applications

Where these names turn up outside the classroom

Elemental & covalent

The air you breathe

You inhale O₂ — an elemental molecule (oxygen). Your cells hand back CO₂ — a covalent compound (carbon dioxide, O=C=O). The surrounding N₂ (nitrogen, 78% of air) rides along unreacted.

Ionic

Sea salt from seawater

Evaporating seawater leaves an ionic solid — mostly NaCl (sodium chloride) with some MgCl₂ (magnesium chloride). Alternating Na⁺ and Cl⁻ ions stack into a cubic lattice — the grains you can see.

Hydrate

Epsom salts in the bath

MgSO₄·7H₂O — magnesium sulfate heptahydrate. Drop the crystals into warm water and the lattice (with its seven waters per formula unit) breaks apart into Mg²⁺ and SO₄²⁻ ions.

Periodic Table

Class-standard table · based on mass of carbon-12 = 12.00

Use this table to read each element's group — the number of valence electrons an atom starts with is what decides how many bonds it forms and what charge its ion carries.

Ion Chart

Class-standard chart · common cations, anions & polyatomic ions with their charges

Names, formulae, and charges of common ions — positive ions (cations) and negative ions (anions)

Every formula on the Naming and Formula Writing Rules tab traces back to this chart: read each ion's charge, then pick the subscripts that make the positive and negative charges cancel.

Summary — Nomenclature

One-screen reference

Decide the type, then name it

Type	What you see	How to name it
Elemental	one element, subscript 2	just the element name (H₂ = hydrogen)
Covalent	two different non-metals	prefix + element, prefix + root + "-ide" (drop the first "mono-")
Ionic	metal + non-metal / polyatomic	metal name + anion "-ide"; Roman numeral if multivalent; no prefixes
Hydrate	salt·xH₂O	salt name + "(prefix)hydrate"; keep "mono-"

Must-memorise

Diatomic seven: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ — plus P₄, S₈.
Common names: H₂O water · NH₃ ammonia · CH₄ methane.
Prefixes: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, nona 9, deca 10.

Common slips

● Prefixes on an ionic compound — never. CaCl₂ is "calcium chloride", not "calcium dichloride".
● Forgetting the Roman numeral on Fe / Cu / Pb / Sn compounds.
● Naming O₂ "dioxygen" — it is an element, just "oxygen".
● Speaking the "·" in a hydrate — it is silent.

Nomenclature — Interactive Study Tool