Source: General Chemistry, Purdue University – Bonding lecture notes Difficulty: Introductory | Prerequisites: Bonding Types and Physical Properties notes; familiarity with the periodic table layout, including group numbers
Once you know the difference between ionic and covalent compounds, the next step is learning how to name them and write their formulas correctly. This topic is pure vocabulary and pattern recognition, and it comes up constantly for the rest of general chemistry. If you cannot name a compound or write its formula from a name, every subsequent topic (stoichiometry, reactions, equilibria) becomes harder than it needs to be.
You should already be comfortable identifying metals vs non-metals and knowing which groups carry which charges. If that is not solid yet, review the bonding types notes first.
Ionic compounds are named by combining the cation and anion names. Charges must balance to zero. Polyatomic ions are groups of covalently bonded atoms that carry a net charge, and you need to memorise the common ones. Transition metals can have multiple charges, so you use Roman numerals to specify which one you mean.
Formula unit
The simplest combination of positive and negative ions that represents an ionic compound with a net charge of zero. Think of it as the "recipe" for one repeating unit of the ionic crystal.
Polyatomic ion
Two or more atoms joined by covalent bonds that together carry a net electrical charge. In simple terms, it is a molecule-sized chunk that behaves as a single ion.
Cation
A positively charged ion, formed when an atom loses one or more electrons. Metals form cations.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. Non-metals form anions.
Binary compound
A compound made of exactly two different elements. When two non-metals form more than one binary compound, Greek prefixes (di-, tri-, tetra-, etc.) are used to distinguish them.
Greek prefixes (for covalent binary naming)
The naming system used when two non-metals form multiple compounds: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), and so on. These are used to specify how many of each atom are present.
Knowing the typical charge for each main-group column saves you from memorising individual elements:
Group 1 (alkali metals): +1
Group 2 (alkaline earth metals): +2
Group 13: +3
Group 14: neutral (these elements typically form covalent bonds rather than ions)
Group 15: −3
Group 16: −2
Group 17 (halogens): −1
Skip the transition metals for now; they follow their own rules (see below).
The formula unit must have a net charge of zero. You balance the charges of the cation and anion until they cancel out.
Example: sodium chloride. Na is +1, Cl is −1. One of each: NaCl.
Example: calcium chloride. Ca is +2, Cl is −1. You need two Cl⁻ to balance one Ca²⁺: CaCl₂.
Example: aluminium oxide. Al is +3, O is −2. You need 2 Al and 3 O to balance: Al₂O₃.
If more than one polyatomic ion is needed, wrap the polyatomic ion in parentheses before adding the subscript: Ca(NO₃)₂, not CaNO₃₂.
This naming pattern recurs across many anions and is a favourite exam question:
-ide suffix = no oxygen (e.g., sulfide, S²⁻)
-ite suffix = fewer oxygens (e.g., sulfite, SO₃²⁻)
-ate suffix = more oxygens (e.g., sulfate, SO₄²⁻)
The same logic applies to other families: nitrite (NO₂⁻) vs nitrate (NO₃⁻), and carbonate (CO₃²⁻).
NH₄⁺ – ammonium (the most common polyatomic cation)
NO₂⁻ – nitrite
NO₃⁻ – nitrate
SO₃²⁻ – sulfite
SO₄²⁻ – sulfate
CO₃²⁻ – carbonate
Polyatomic anions are far more common than polyatomic cations. Ammonium (NH₄⁺) is the main polyatomic cation you will encounter at this level.
Many transition metals can form ions with more than one charge. When naming their compounds, you must specify the charge using a Roman numeral in parentheses.
Common transition metals and their possible charges:
Cr: +2, +3
Mn: +2, +3
Fe: +2, +3
Co: +2, +3
Cu: +1, +2
Ag: +1 (only one common charge)
Zn: +2 (only one common charge)
Cd: +2 (only one common charge)
Hg: +2
Sn: +2, +4
Pb: +2, +4
Example: FeCl₂ is iron(II) chloride (Fe is +2). FeCl₃ is iron(III) chloride (Fe is +3). The Roman numeral removes ambiguity.
Silver, zinc, and cadmium each have only one common charge, so in practice you will sometimes see their names written without Roman numerals. Exams vary on whether they expect the numeral for these; follow your instructor's preference.
When two non-metals form more than one possible compound, use Greek prefixes on both elements (dropping "mono-" on the first element):
CO – carbon monoxide
CO₂ – carbon dioxide
N₂O₄ – dinitrogen tetroxide
Charge-balancing shortcut: if the cation has charge +a and the anion has charge −b, the subscript on the cation is b and the subscript on the anion is a (then simplify if possible). For Al³⁺ and O²⁻, you get Al₂O₃.
Knowing how to read and write chemical formulas is the basis for understanding everything from the active ingredients on a medicine label to the composition of fertilisers (ammonium nitrate, NH₄NO₃, is one of the most widely used). The iron(II)/iron(III) distinction matters in real chemistry too: iron(II) sulfate is used as an iron supplement, while iron(III) chloride is used in water treatment. Same metal, different charge, completely different applications.
Students often confuse the -ite and -ate suffixes. Remember: -ate has more oxygen. A mnemonic some students use: "-ate" has an extra letter compared to "-ite," so it has extra oxygen.
Students sometimes forget to use parentheses around polyatomic ions when there is more than one. Ca(OH)₂ is correct; CaOH₂ is wrong and implies something different.
Students mix up when to use Greek prefixes vs Roman numerals. Greek prefixes are for covalent compounds (non-metal + non-metal). Roman numerals are for ionic compounds with transition metals.
Writing the formula unit with a net charge that is not zero. Always check: do the total positive and negative charges cancel?
⚠️ The -ide / -ite / -ate suffix pattern is tested frequently. Be able to go in both directions: name to formula and formula to name.
⚠️ Formula units must net to zero charge. If your formula does not balance, it is wrong, full stop.
⚠️ Expect at least one question involving a transition metal with variable charge. You will need to deduce the charge from the formula, then name it with the correct Roman numeral.
⚠️ Parentheses around polyatomic ions are a common marking point. Omitting them when needed will cost you marks.
True or false: Sulfate has more oxygen atoms than sulfite. (True – sulfate SO₄²⁻ has 4, sulfite SO₃²⁻ has 3)
Fill in the blank: The formula unit for calcium carbonate is _______. (CaCO₃)
True or false: Greek prefixes are used to name ionic compounds with transition metals. (False – Roman numerals are used for those)
Fill in the blank: NH₄⁺ is called the _______ ion. (Ammonium)
True or false: The net charge of a formula unit must be zero. (True)
Q: Write the formula for iron(III) oxide.
A: Fe₂O₃. Iron(III) means Fe³⁺. Oxide is O²⁻. Cross the charges: 2 iron, 3 oxygen.
Q: Name the compound Cu₂O.
A: Copper(I) oxide. Each Cu must be +1 to balance the single O²⁻, so the Roman numeral is (I).
Q: What is the difference between a sulfide and a sulfate?
A: Sulfide (S²⁻) contains no oxygen. Sulfate (SO₄²⁻) contains four oxygen atoms bonded to sulfur.
Q: Write the formula for ammonium sulfate.
A: (NH₄)₂SO₄. Ammonium is NH₄⁺ (+1 each). Sulfate is SO₄²⁻ (−2). You need two ammonium ions, so parentheses: (NH₄)₂SO₄.
Q: When do you use Greek prefixes in naming?
A: When naming binary covalent compounds (non-metal + non-metal) that can form more than one compound. They indicate how many atoms of each element are present.
This connects to stoichiometry, where you need correct formulas to balance equations and calculate molar masses. It also connects to acid–base chemistry: many acids are simply hydrogen combined with a polyatomic anion (sulfuric acid = H₂SO₄, nitric acid = HNO₃). Understanding polyatomic ions now saves significant time when you reach acids, bases, and salts later in the course.
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