Source: Chemistry by Overby, Chapter 2.6
Difficulty: Introductory | Prerequisites: Periodic table basics, understanding of atoms and ionic charge.
Naming compounds is the language layer of chemistry: before you can write equations, predict products, or calculate moles, you need to read and write chemical names and formulas fluently. This chapter covers two major families of compounds (ionic and molecular) and the systematic rules for naming each. If you missed the earlier material on the periodic table, ion charges, and electronegativity, revisit that first, because naming rules lean heavily on knowing which elements are metals, which are nonmetals, and what charges they typically carry. By the end of this section you should be able to move in both directions, from name to formula and from formula to name, for any compound you will see on Exam 2.
Ionic compounds (metal + nonmetal, or metal + polyatomic ion) are named cation-first, anion-second, with Roman numerals for metals that have variable charges. Molecular compounds (nonmetal + nonmetal) use Greek prefixes to show how many of each atom are present. Learn the common polyatomic ions by heart, know the three trivial names (water, ammonia, hydrogen peroxide), and practise moving between name and formula in both directions.
Ionic compound
A compound formed by the electrostatic attraction between a cation (positive ion) and an anion (negative ion), typically a metal bonded to a nonmetal or polyatomic ion. Think of it as: the metal gives up electrons, the nonmetal takes them, and opposite charges hold the pair together.
Cation
A positively charged ion, formed when an atom loses one or more electrons. In simple terms, metals become cations.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. In simple terms, nonmetals become anions.
Monatomic ion
An ion consisting of a single atom that has gained or lost electrons (e.g. Na+, Cl-). Think of it as: one atom, one charge.
Polyatomic ion
An ion made of two or more atoms covalently bonded together that carry an overall charge (e.g. SO4 2-, NH4+). In simple terms, a cluster of atoms that acts as a single charged unit.
Molecular compound
A compound formed by covalent bonding between two or more nonmetals. Think of it as: atoms sharing electrons rather than transferring them.
Greek prefixes (for molecular naming)
The counting system used in molecular compound names: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10). In simple terms, these prefixes tell you how many of each atom are in the molecule.
Systematic (Roman numeral) naming
The naming convention used for ionic compounds where the metal can form more than one charge. The Roman numeral in parentheses after the metal name indicates the charge of the cation. For example, Fe2O3 is iron(III) oxide because each iron carries a 3+ charge.
Trivial (nonsystematic) name
A commonly used, non-IUPAC name for certain well-known compounds. The three you must know: H2O = water, NH3 = ammonia, H2O2 = hydrogen peroxide.
Identifying ionic vs. molecular: If the formula starts with a metal (or NH4+), it is ionic. If it contains only nonmetals (and is not NH4+), it is molecular.
Basic ionic naming: Write the cation name first, then the anion name. For monatomic anions, change the element ending to -ide (chlorine becomes chloride, oxygen becomes oxide).
Metals with one common charge: Group 1A metals are always +1, Group 2A are always +2, aluminium is always +3. No Roman numeral is needed for these.
Metals with variable charge (Roman numeral system): Transition metals and a few main-group metals (Sn, Pb) can form more than one cation. Use a Roman numeral in parentheses to show the charge: FeCl2 = iron(II) chloride, FeCl3 = iron(III) chloride.
To find the charge: use the anion charges to work backwards. In FeCl3, each Cl is -1, so three chlorides give -3 total, meaning Fe must be +3.
Compounds with polyatomic ions: Name the polyatomic ion as a unit. NaNO3 = sodium nitrate. Ca(OH)2 = calcium hydroxide. The parentheses around the polyatomic ion in the formula mean there is more than one of that ion.
You must know the formulas, charges, and names of the polyatomic ions covered in lecture. Key ones include: NO3- (nitrate), SO4 2- (sulfate), PO4 3- (phosphate), CO3 2- (carbonate), OH- (hydroxide), NH4+ (ammonium), ClO3- (chlorate).
The -ate / -ite pattern: -ate is the "common" form, -ite has one fewer oxygen. For example, sulfate (SO4 2-) vs. sulfite (SO3 2-).
Determine the charge on each ion.
Find the lowest ratio that makes the total charge zero.
Example: aluminium oxide. Al is +3, O is -2. You need two Al (+6 total) and three O (-6 total). Formula: Al2O3.
Polyatomic ions in multiples go in parentheses: calcium nitrate = Ca(NO3)2.
Greek prefixes required: Both elements get a prefix indicating the number of atoms, except mono- is dropped from the first element.
N2O4 = dinitrogen tetroxide
CO = carbon monoxide (mono- only on the second element)
CO2 = carbon dioxide
The more metallic element comes first (further left or lower on the periodic table).
The second element ends in -ide.
H2O = water
NH3 = ammonia
H2O2 = hydrogen peroxide
These three do not follow the systematic rules. Know them by heart.
Greek prefixes table
Number | Prefix |
|---|---|
1 | mono- |
2 | di- |
3 | tri- |
4 | tetra- |
5 | penta- |
6 | hexa- |
7 | hepta- |
8 | octa- |
9 | nona- |
10 | deca- |
Balancing charges shortcut: Cross-multiply the absolute values of the charges to get the subscripts, then reduce to the lowest whole-number ratio.
Example: Mg2+ and PO4 3-. Cross: Mg3(PO4)2. Check: 3(+2) + 2(-3) = 0. Balanced.
Every chemical on a pharmacy shelf, a food label, or a safety data sheet has a systematic name. When a nurse reads "sodium chloride" on an IV bag, they know the formula (NaCl) and what it is (table salt in solution). When an environmental scientist reports "dinitrogen monoxide" emissions, anyone trained in naming recognises N2O (laughing gas, also a greenhouse gas). Getting names and formulas right is the entry point for every applied chemistry task.
Using Greek prefixes for ionic compounds. Students sometimes write "disodium oxide" for Na2O. Greek prefixes are only for molecular (nonmetal + nonmetal) compounds. Ionic compounds rely on charge balance to imply the subscripts.
Forgetting the Roman numeral for variable-charge metals. Writing "iron chloride" is ambiguous. You must specify iron(II) chloride or iron(III) chloride.
Confusing -ide with -ate/-ite. The -ide ending is for monatomic anions and simple molecular compounds. The -ate and -ite endings are for polyatomic ions containing oxygen.
Dropping the parentheses around polyatomic ions. Ca(OH)2 is correct. CaOH2 implies a different (and incorrect) formula. The parentheses mean the entire polyatomic ion is multiplied by the subscript.
⚠️ You will almost certainly be asked to name a compound from its formula and to write a formula from a name. Practise both directions.
⚠️ Expect at least one question requiring the Roman numeral system for a transition metal compound.
⚠️ Polyatomic ion formulas and charges are fair game. If you have not memorised them, that is the single highest-return use of your study time for this section.
⚠️ Know the three trivial names (water, ammonia, hydrogen peroxide). These are easy marks if you remember them, lost marks if you do not.
⚠️ Be ready to distinguish ionic from molecular compounds based on the formula alone (metal present = ionic, all nonmetals = molecular).
True or false: The compound PCl5 should be named using Greek prefixes. (True, it is molecular.)
Fill in the blank: In iron(III) oxide, the Roman numeral III tells you the charge on the ______ ion. (iron / Fe cation)
True or false: The name for NaBr is "sodium bromide," with no Greek prefix. (True, it is ionic.)
Fill in the blank: The trivial name for NH3 is ______. (ammonia)
True or false: Ca(NO3)2 contains two nitrogen atoms and six oxygen atoms per formula unit. (True.))
Q: Name the compound MgBr2.
A: Magnesium bromide. (Mg is always +2, Br is always -1, no Roman numeral needed.)
Q: Write the formula for copper(II) sulfate.
A: CuSO4. Copper(II) means Cu2+, sulfate is SO4 2-. Charges balance one-to-one.
Q: Name the molecular compound N2O3.
A: Dinitrogen trioxide. (Two nitrogen atoms, three oxygen atoms, both get Greek prefixes.)
Q: Write the formula for diphosphorus pentoxide.
A: P2O5. Di- = 2 phosphorus atoms, penta- = 5 oxygen atoms.
Q: Is the compound KNO3 ionic or molecular? Name it.
A: Ionic (K is a metal). Potassium nitrate.
Q: What is the systematic name for Fe2O3?
A: Iron(III) oxide. Total negative charge from three O2- ions is -6, shared by two Fe ions, so each Fe is +3.
Naming compounds connects directly to Chapter 9 (Chemical Bonding): once you identify a compound as ionic or molecular, the bonding model changes (ionic = electron transfer, molecular = electron sharing via Lewis structures). It also connects to Chapter 3 (Moles and Molar Mass): you cannot calculate the molar mass of a compound unless you can write its correct formula from its name. In Chapter 4, solution chemistry requires you to identify the ions that dissolve when an ionic compound enters water, which depends on correctly reading the formula.
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