Difficulty: Introductory. Prerequisites: periodic table familiarity, understanding of ionic vs covalent bonding, naming ions (see companion notes).
Molecular (covalent) compounds are named using Greek prefixes to indicate how many of each atom are present. Acids follow a separate naming system that depends on whether the acid contains oxygen. Seven elements naturally exist as diatomic molecules, and you need to know all seven by heart.
Molecular compound (covalent compound)
A compound formed by two or more non-metals sharing electrons. Unlike ionic compounds, these are named using Greek prefixes rather than charge-balancing. Think of it as: atoms holding hands instead of one stealing from the other.
Greek prefixes (naming prefixes)
The numbering system used for molecular compounds: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10). In simple terms, these just tell you how many of each atom are in one molecule.
Binary acid
An acid that contains hydrogen and one other element (no oxygen). Named with the pattern "hydro-[root]-ic acid." For example, HCl = hydrochloric acid.
Oxyacid
An acid that contains hydrogen, oxygen, and one other element. The name depends on the polyatomic ion it is derived from. If the ion ends in "-ate," the acid ends in "-ic." If the ion ends in "-ite," the acid ends in "-ous."
Diatomic element
An element that naturally exists as a molecule of two atoms bonded together. There are seven of them and they must be written with a subscript 2 in equations and formulas.
Molecular compounds are made of two non-metals. The naming rules differ from ionic compounds.
Name the first element using a Greek prefix (except: drop "mono-" on the first element).
Name the second element with the appropriate prefix, change the ending to "-ide."
When a prefix ends in "a" or "o" and the element name begins with "a" or "o," the trailing vowel on the prefix is dropped for easier pronunciation (e.g. "mono-oxide" becomes "monoxide," "tetra-oxide" becomes "tetroxide").
Prefix reference:
1 = mono-
2 = di-
3 = tri-
4 = tetra-
5 = penta-
6 = hexa-
7 = hepta-
8 = octa-
9 = nona-
10 = deca-
Worked examples from the source material:
CO = carbon monoxide (not "monocarbon monoxide," because mono- is dropped on the first element)
N₂O₄ = dinitrogen tetraoxide
P₄O₁₀ = tetraphosphorus decaoxide
PCl₅ = phosphorus pentachloride
BCl₃ = boron trichloride
SO₂ = sulfur dioxide
Binary acids contain hydrogen and one other non-metal, dissolved in water. The naming pattern is: hydro- + [element root] + -ic acid.
HCl = hydrochloric acid
HBr = hydrobromic acid
HF = hydrofluoric acid
Oxyacids contain hydrogen, oxygen, and another element. Their names are derived from the polyatomic ion they contain.
The rule is straightforward: if the polyatomic ion ends in "-ate," change it to "-ic acid." If it ends in "-ite," change it to "-ous acid." The "hydro-" prefix is never used for oxyacids.
H₂CO₃ = carbonic acid (from carbonate)
HNO₃ = nitric acid (from nitrate)
HNO₂ = nitrous acid (from nitrite)
H₃PO₄ = phosphoric acid (from phosphate)
HClO₃ = chloric acid (from chlorate)
H₂SO₄ = sulfuric acid (from sulfate)
HC₂H₃O₂ = acetic acid (a common organic acid, worth memorising on its own)
Seven elements exist naturally as diatomic molecules. When writing their formulas in any chemical context, they must carry the subscript 2.
A useful mnemonic: "Have No Fear Of Ice Cold Beer" (or "BrINClHOF").
H₂ (hydrogen)
N₂ (nitrogen)
F₂ (fluorine)
O₂ (oxygen)
I₂ (iodine)
Cl₂ (chlorine)
Br₂ (bromine)
Notice these are all non-metals. Five of them are halogens (F, Cl, Br, I) plus hydrogen, nitrogen, and oxygen.
Students often use Greek prefixes for ionic compounds, or use ionic naming rules for molecular compounds. Prefixes are only for molecular (covalent) compounds. If a metal is involved, it is ionic and does not use prefixes.
Students forget to drop "mono-" from the first element. "Monocarbon monoxide" is wrong. It is simply "carbon monoxide."
Students mix up binary acid and oxyacid naming. "Hydro-" is only used for binary acids (no oxygen). If oxygen is present, use the "-ic" / "-ous" system, never "hydro-."
Students forget the diatomic elements when writing equations. Oxygen gas is O₂, not O. This is one of the most common early errors in balancing equations.
⚠️ Distinguishing between ionic and molecular naming is a core exam skill. Expect questions where you must first identify the compound type (metal + non-metal = ionic; non-metal + non-metal = molecular) and then apply the correct naming rules.
⚠️ Acid naming ("-ate" to "-ic," "-ite" to "-ous," "hydro-" for binary acids) is heavily tested. The pattern is consistent, but students who have not practised it find it slippery under time pressure.
⚠️ Listing all seven diatomic elements from memory is a common short-answer or bonus question.
⚠️ Greek prefix questions are among the most straightforward on the exam. Free marks if you memorise the prefixes and remember to drop mono- on the first element.
Fill in the blank: The compound N₂O₄ is called dinitrogen ____. (Tetraoxide.)
True or false: Hydrosulfuric acid is the correct name for H₂SO₄. (False. H₂SO₄ is sulfuric acid. The "hydro-" prefix is only for binary acids with no oxygen.)
Fill in the blank: Name the seven diatomic elements: H₂, ____, ____, ____, ____, ____, ____. (N₂, F₂, O₂, I₂, Cl₂, Br₂.)
True or false: CO should be named "monocarbon monoxide." (False. Mono- is dropped from the first element. It is carbon monoxide.)
Fill in the blank: An acid derived from a polyatomic ion ending in "-ate" has the acid name ending in ____. ("-ic.")
Q: Write the formula for tetraphosphorus decaoxide.
A: P₄O₁₀. Tetra- = 4 phosphorus atoms, deca- = 10 oxygen atoms.
Q: Name the compound SO₂.
A: Sulfur dioxide. One sulfur (mono- dropped), two oxygens (di-).
Q: What is the name of HNO₃ when dissolved in water?
A: Nitric acid. It is derived from nitrate (NO₃⁻): the "-ate" ending becomes "-ic acid."
Q: What is the name of HNO₂ when dissolved in water?
A: Nitrous acid. It is derived from nitrite (NO₂⁻): the "-ite" ending becomes "-ous acid."
Q: Write the formula for hydrochloric acid.
A: HCl. It is a binary acid (hydrogen + chlorine, no oxygen), hence the "hydro-" prefix and "-ic" ending.
Q: A student writes the formula for boron trichloride as BrCl₃. What is wrong?
A: Br is the symbol for bromine, not boron. Boron is B. The correct formula is BCl₃.
Q: List all seven diatomic elements.
A: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
Molecular naming conventions become essential when writing and balancing chemical equations, since you need to recognise compounds from their formulas. Acid naming feeds directly into the study of acid-base chemistry (Arrhenius, Brønsted-Lowry, and Lewis definitions). The diatomic elements come back every time you write a balanced equation involving any of those seven elements in their elemental form.
Related Terms / Search Tags
Molecular compounds, covalent compounds, Greek prefixes, prefix naming system, mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca, binary acid, oxyacid, hydro- prefix, -ic acid, -ous acid, -ate to -ic, -ite to -ous, diatomic elements, BrINClHOF, H2 N2 O2 F2 Cl2 Br2 I2, carbon monoxide, sulfur dioxide, phosphorus pentachloride, hydrochloric acid, sulfuric acid, nitric acid, nitrous acid, carbonic acid, phosphoric acid, acetic acid, general chemistry, Purdue, Module 3, sections 2.5 to 2.7