Course: CHEM 101, Michigan State University | Source: Compounds and Stoichiometry lecture notes
Difficulty: Introductory | Prerequisites: Periodic table familiarity, atomic structure basics
Knowing what ions are and how they are named is essential for writing chemical formulas, predicting reaction products, and understanding how substances behave in solution. This material connects directly to acid-base chemistry, electrochemistry, and solution chemistry. If you can name ions and classify electrolytes, you can read a chemical equation the way a musician reads sheet music.
Cations are positive ions, anions are negative. Monatomic anions end in "-ide"; polyatomic anions with oxygen (oxyanions) use "-ate" and "-ite" endings, with "per-" and "hypo-" prefixes for extended series. Metals with multiple possible charges use Roman numerals. Strong electrolytes dissociate fully in water; weak electrolytes only partly; nonelectrolytes not at all.
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. Nonmetals become anions.
Monatomic ion
An ion made of a single atom (e.g. Na⁺, Cl⁻).
Polyatomic ion
An ion made of two or more atoms bonded together that carries a net charge (e.g. NO₃⁻, SO₄²⁻).
Oxyanion
A polyatomic anion that contains oxygen. Named using "-ate" (more oxygen) and "-ite" (fewer oxygen) suffixes.
Strong electrolyte
A substance that dissociates completely into ions when dissolved in water. Think of it as breaking apart 100%.
Weak electrolyte
A substance that ionises only partially in aqueous solution. Most of the molecules remain intact.
Nonelectrolyte
A substance that does not produce ions in solution at all. Nonpolar gases and most organic compounds fall here.
Drop the ending of the element name and add "-ide."
H⁻ = hydride, F⁻ = fluoride, O²⁻ = oxide, S²⁻ = sulfide, N³⁻ = nitride, P³⁻ = phosphide
When an element forms two oxyanions, the one with more oxygen atoms ends in "-ate" and the one with fewer ends in "-ite."
NO₃⁻ = nitrate, NO₂⁻ = nitrite
SO₄²⁻ = sulfate, SO₃²⁻ = sulfite
When there are four oxyanions in a series, prefixes extend the pattern:
"per-" = one more oxygen than the "-ate" form
"hypo-" = one fewer oxygen than the "-ite" form
ClO⁻ = hypochlorite, ClO₂⁻ = chlorite, ClO₃⁻ = chlorate, ClO₄⁻ = perchlorate
When H⁺ is added to a polyatomic anion (lowering its charge), the name gains "hydrogen," "dihydrogen," or the prefix "bi-."
HCO₃⁻ = hydrogen carbonate (or bicarbonate)
HSO₄⁻ = hydrogen sulfate (or bisulfate)
H₂PO₄⁻ = dihydrogen phosphate
NH₄⁺ = ammonium
C₂H₃O₂⁻ = acetate
CN⁻ = cyanide
MnO₄⁻ = permanganate
SCN⁻ = thiocyanate
CrO₄²⁻ = chromate
Cr₂O₇²⁻ = dichromate
BO₃³⁻ = borate
Metals that can form more than one cation use Roman numerals in parentheses to indicate the charge.
Fe²⁺ = iron(II), Fe³⁺ = iron(III)
Cu⁺ = copper(I), Cu²⁺ = copper(II)
Older naming uses "-ous" for the lower charge and "-ic" for the higher.
Fe²⁺ = ferrous, Fe³⁺ = ferric
Cu⁺ = cuprous, Cu²⁺ = cupric
Alkali metals (Group 1) always form +1 ions.
Alkaline earth metals (Group 2) always form +2 ions.
Transition metals can form multiple charges; you must memorise common ones or determine them from context.
Solid ionic compounds are poor conductors because ions are locked in a lattice. Dissolved in water, the ions separate and conduct electricity.
Strong electrolytes dissociate completely: NaCl, KI, HCl in water.
Weak electrolytes ionise only partially: acetic acid, ammonia, weak acids and bases.
Nonelectrolytes produce no ions: O₂, CO₂, glucose, most organic compounds.
Ion | Name |
|---|---|
H⁻ | Hydride |
F⁻ | Fluoride |
O²⁻ | Oxide |
S²⁻ | Sulfide |
N³⁻ | Nitride |
P³⁻ | Phosphide |
Ion | Name | Rule |
|---|---|---|
ClO⁻ | Hypochlorite | hypo- + -ite |
ClO₂⁻ | Chlorite | -ite |
ClO₃⁻ | Chlorate | -ate |
ClO₄⁻ | Perchlorate | per- + -ate |
NO₂⁻ | Nitrite | -ite |
NO₃⁻ | Nitrate | -ate |
SO₃²⁻ | Sulfite | -ite |
SO₄²⁻ | Sulfate | -ate |
Ion | IUPAC Name | Older Name |
|---|---|---|
Fe²⁺ | Iron(II) | Ferrous |
Fe³⁺ | Iron(III) | Ferric |
Cu⁺ | Copper(I) | Cuprous |
Cu²⁺ | Copper(II) | Cupric |
Electrolyte balance is fundamental in medicine: IV fluids are formulated with precise concentrations of Na⁺, K⁺, and Cl⁻ to match blood plasma. Ion naming conventions carry directly into clinical and pharmaceutical terminology, so a nurse reading "potassium chloride" knows exactly which ions are involved and at what ratio.
Students often think "more oxygens = per-" directly. The prefix "per-" means one more oxygen than the "-ate" form, not simply "the most oxygens possible." The baseline is "-ate," and everything else is relative to it.
Hydrogen is classified as a nonmetal, despite sitting in Group 1 on many periodic tables. It does not behave like the alkali metals.
Students sometimes assume all ionic compounds are strong electrolytes in the solid state. Solid ionic compounds are poor conductors; they become strong electrolytes only when dissolved or melted.
The older "-ous / -ic" naming system is still encountered in textbooks and clinical settings, but IUPAC Roman numeral naming is the standard. Know both.
⚠️ Naming ions is tested in nearly every form: multiple choice, fill-in-the-blank, and matching. Memorise the common polyatomic ions.
⚠️ The oxyanion naming series (hypo- / -ite / -ate / per-) is a high-frequency exam topic, especially with chlorine.
⚠️ Classifying substances as strong electrolytes, weak electrolytes, or nonelectrolytes is a common conceptual question. Know the examples.
⚠️ Expect to see Roman numeral naming for transition metals. If given Fe₂O₃, you should be able to determine that iron is in the +3 state and name the compound iron(III) oxide.
True or False: NO₃⁻ is called nitrite. (False, it is nitrate; NO₂⁻ is nitrite)
Fill in the blank: Monatomic anions end in ______. (-ide)
True or False: Cu²⁺ is called cuprous. (False, cuprous is Cu⁺; Cu²⁺ is cupric or copper(II))
Fill in the blank: A strong electrolyte dissociates ______ in water. (completely)
True or False: Glucose is a nonelectrolyte. (True)
Q: Name the compound FeCl₃ using IUPAC conventions.
A: Chloride is Cl⁻ (charge -1). Three chlorides give a total charge of -3, so iron must be +3. The name is iron(III) chloride.
Q: What is the name of ClO₂⁻?
A: Chlorite. It has one fewer oxygen than chlorate (ClO₃⁻) and one more than hypochlorite (ClO⁻).
Q: Classify the following as strong electrolyte, weak electrolyte, or nonelectrolyte: NaCl, acetic acid, CO₂.
A: NaCl = strong electrolyte (ionic, dissociates completely). Acetic acid = weak electrolyte (ionises partially). CO₂ = nonelectrolyte (nonpolar, no ions in solution).
Q: What is the older name for Fe²⁺?
A: Ferrous.
Q: Write the formula for ammonium sulfate.
A: NH₄⁺ has charge +1, SO₄²⁻ has charge -2. Two ammonium ions balance one sulfate: (NH₄)₂SO₄.
Ion naming is the vocabulary for every subsequent chemistry topic. In acid-base chemistry, recognising that HSO₄⁻ is bisulfate tells you the acid is only partially neutralised. In electrochemistry, knowing that Fe²⁺ and Fe³⁺ are different oxidation states of the same metal is essential for writing half-reactions. Electrolyte classification connects to colligative properties: a strong electrolyte produces more particles in solution and has a larger effect on boiling point elevation and freezing point depression.
cation, anion, monatomic ion, polyatomic ion, oxyanion, -ide, -ate, -ite, per-, hypo-, hydride, fluoride, oxide, sulfide, nitride, phosphide, nitrate, nitrite, sulfate, sulfite, hypochlorite, chlorite, chlorate, perchlorate, bicarbonate, hydrogen carbonate, bisulfate, hydrogen sulfate, dihydrogen phosphate, ammonium, acetate, cyanide, permanganate, thiocyanate, chromate, dichromate, borate, Roman numeral naming, IUPAC, ferrous, ferric, cuprous, cupric, iron(II), iron(III), copper(I), copper(II), alkali metals, alkaline earth metals, electrolyte, strong electrolyte, weak electrolyte, nonelectrolyte, dissociation, ionisation, lattice, conductor