Nomenclature & Chemical Formulas

1 Types of Compounds

All chemical compounds fall into a small number of categories, each with its own naming rules. Identifying the type first is the essential first step before naming anything.

Type	Composition	Examples	Naming rule
Binary Ionic	Metal + nonmetal	NaCl, MgO, FeCl₂	Metal name + nonmetal stem + "-ide"
Ionic w/ polyatomic ion	Metal or NH₄⁺ + polyatomic ion	Na₂SO₄, Ca(OH)₂	Cation name + anion name
Molecular / Covalent	Nonmetal + nonmetal	CO₂, N₂O₄, PCl₅	Greek prefixes for each element
Acids	H⁺ released in water	HCl, H₂SO₄, HNO₃	Depends on binary vs oxyacid

🔑Key question: Is it ionic (metal + nonmetal or polyatomic ion) or molecular (two nonmetals)? That single distinction determines the entire naming approach.

2 Naming Binary Ionic Compounds

A binary ionic compound contains exactly two elements: a metal cation and a nonmetal anion. The naming rule is straightforward, with one critical exception for transition metals.

🔑Rule: [metal name] + [nonmetal stem] + "-ide"

Fixed-Charge Metals

Main-group metals (Groups 1, 2, and Al) always have one possible charge. No Roman numeral is needed.

NaCl

sodium chloride

MgO

magnesium oxide

Al₂O₃

aluminum oxide

Variable-Charge Metals (Transition Metals)

Transition metals like Fe, Cu, Pb, Sn can have multiple charges. You must use a Roman numeral in parentheses to specify which charge is present in a given compound.

💡To determine the charge on the metal: use the known charge of the anion and the subscript to calculate what the metal's charge must be to make the total charge zero.

FeCl₂

iron(II) chloride — each Cl is −1, two Cl → Fe must be +2

FeCl₃

iron(III) chloride — three Cl → Fe must be +3

CuO

copper(II) oxide — O is −2, one O → Cu must be +2

Common Nonmetal Stems for "-ide" Names

Nonmetal	Stem	Ion name
Chlorine (Cl)	chlor-	chloride
Bromine (Br)	brom-	bromide
Iodine (I)	iod-	iodide
Oxygen (O)	ox-	oxide
Sulfur (S)	sulf-	sulfide
Nitrogen (N)	nitr-	nitride
Phosphorus (P)	phosph-	phosphide
Fluorine (F)	fluor-	fluoride

3 Naming Ionic Compounds with Polyatomic Ions

A polyatomic ion is a charged group of two or more atoms bonded together that acts as a single unit. When a polyatomic ion is part of an ionic compound, its name is used unchanged — no "-ide" suffix is added.

🔑Rule: [cation name] + [polyatomic anion name]. The polyatomic ion's name is already complete — do not modify it.

Na₂SO₄

sodium sulfate — Na⁺ (sodium) + SO₄²⁻ (sulfate)

Ca(OH)₂

calcium hydroxide — Ca²⁺ (calcium) + OH⁻ (hydroxide)

NH₄Cl

ammonium chloride — NH₄⁺ (ammonium) + Cl⁻ (chloride)

⚠️The parentheses in Ca(OH)₂ tell you there are two complete hydroxide groups. The subscript outside the bracket applies to the entire ion. When writing the name, you still just say "calcium hydroxide" — not "calcium dihydroxide".

4 Writing Ionic Formulas

To write an ionic formula, you need to combine the ions so that the overall charge equals zero. The criss-cross method is the fastest approach.

🔑Criss-cross method: swap the magnitudes of the two ions' charges and use them as subscripts. Then simplify if possible (reduce to lowest whole number ratio).

Criss-Cross Examples

Na⁺ + SO₄²⁻

Na₂SO₄ — swap 1 and 2 as subscripts

Ca²⁺ + PO₄³⁻

Ca₃(PO₄)₂ — swap 2 and 3; brackets around PO₄

Mg²⁺ + SO₄²⁻

MgSO₄ — criss-cross gives Mg₂(SO₄)₂; simplify: 2/2 = 1

✓Always verify: the sum of all charges in the formula must equal zero. For Ca₃(PO₄)₂: 3(+2) + 2(−3) = +6 − 6 = 0 ✓

Rules for Subscripts

Always simplify to lowest whole number ratio (Ca₂S₂ → CaS)
When a polyatomic ion has a subscript greater than 1, enclose it in parentheses: Ca(NO₃)₂ not CaNO₃₂
A subscript of 1 is never written (NaCl, not Na₁Cl₁)

5 Naming Molecular (Covalent) Compounds

Molecular compounds form between two nonmetals. Unlike ionic compounds, the atoms don't have predictable charges, so you must use Greek number prefixes to specify exactly how many atoms of each element are present.

🔑Rule: [prefix + first element name] + [prefix + second element stem + "-ide"]. Exception: if the first element has only one atom, "mono-" is omitted from the first element.

Greek Number Prefixes

Number	Prefix	Number	Prefix
1	mono-	5	penta-
2	di-	6	hexa-
3	tri-	7	hepta-
4	tetra-	8	octa-

Examples

carbon monoxide — 1 C (no mono), 1 O (mono required on 2nd)

CO₂

carbon dioxide

N₂O₄

dinitrogen tetroxide

PCl₅

phosphorus pentachloride

💡When a prefix ends in "a" or "o" and the element name starts with a vowel, drop the final vowel from the prefix for pronunciation: "tetraoxide" → "tetroxide", "monooxide" → "monoxide".

6 Naming Acids

Acids are compounds that release H⁺ ions in water. Their naming follows two different patterns depending on whether the acid contains oxygen.

Binary Acids (no oxygen)

Formula: H + nonmetal only. Rule: "hydro-" + nonmetal stem + "-ic acid"

HCl (aq)

hydrochloric acid — hydro + chlor + ic acid

HBr (aq)

hydrobromic acid

H₂S (aq)

hydrosulfuric acid

Oxyacids (contain oxygen)

Formula: H + polyatomic ion containing oxygen. The name of the polyatomic ion's ending determines the acid suffix:

Ion ends in -ate → acid ends in "-ic acid"
Ion ends in -ite → acid ends in "-ous acid"

H₂SO₄

sulfuric acid — SO₄²⁻ = sulfate (-ate → -ic)

H₂SO₃

sulfurous acid — SO₃²⁻ = sulfite (-ite → -ous)

HNO₃

nitric acid — NO₃⁻ = nitrate (-ate → -ic)

HNO₂

nitrous acid — NO₂⁻ = nitrite (-ite → -ous)

7 Common Polyatomic Ions

These must be memorized — they appear throughout all of chemistry. Organized by charge:

Ion	Name	Charge
NH₄⁺	ammonium	+1
OH⁻	hydroxide	−1
NO₃⁻	nitrate	−1
NO₂⁻	nitrite	−1
HCO₃⁻	hydrogen carbonate (bicarbonate)	−1
ClO₄⁻	perchlorate	−1
ClO₃⁻	chlorate	−1
MnO₄⁻	permanganate	−1
CH₃COO⁻	acetate	−1
SO₄²⁻	sulfate	−2
SO₃²⁻	sulfite	−2
CO₃²⁻	carbonate	−2
CrO₄²⁻	chromate	−2
Cr₂O₇²⁻	dichromate	−2
PO₄³⁻	phosphate	−3

💡Memory pattern: if you know the "-ate" ion, the "-ite" version has one fewer oxygen atom and the same charge. Example: nitrate NO₃⁻ → nitrite NO₂⁻.

8 Common Mistakes to Avoid

Mistake	What to do instead
Forgetting Roman numerals for transition metals	Always check — if the metal is Fe, Cu, Pb, Sn, Cr, Mn, etc., determine its charge and add the Roman numeral.
Not simplifying subscripts	Ca₂S₂ must become CaS. Always reduce to the lowest whole-number ratio.
Confusing "-ic" and "-ous" for acids	"-ic acid" comes from an "-ate" ion; "-ous acid" comes from an "-ite" ion. Memorize the pattern.
Writing "mono" on the first element in molecular naming	Omit "mono" from the first element only (CO = carbon monoxide, not monocarbon monoxide).
No brackets around polyatomic ions with subscript > 1	Write Ca(NO₃)₂ not CaNO₃₂ — brackets show the subscript applies to the whole ion.
Using Greek prefixes for ionic compounds	Greek prefixes are only for molecular compounds. Ionic compounds use Roman numerals (if needed) instead.

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