Ions

Chemical reactions happen at the level of electrons, not nuclei — the number of protons and neutrons in an atom’s nucleus stays fixed during ordinary chemistry, while electrons are gained, lost, or shared. When an atom’s electron count changes enough that it no longer matches its proton count, that atom becomes an ion. Recognizing ions, and knowing how to name the compounds they form, is essential groundwork for the rest of general chemistry.

Key Takeaways

• An ion forms when an atom’s proton count no longer matches its electron count. A cation (positive) loses electrons; an anion (negative) gains electrons.

• A binary compound is molecular if both elements are nonmetals, or ionic if one is a metal and one a nonmetal.

• Type I ionic compounds (fixed-charge metal): cation named first (element name), anion second (root + "-ide").

• Type II ionic compounds (variable-charge metal): cation charge specified with a Roman numeral (e.g., iron(III) chloride), or with the older -ic/-ous system (ferric/ferrous). Group 1A, Group 2A, Al³⁺, Ag⁺, and Zn²⁺ never need a Roman numeral.

• Polyatomic ions are named as memorized units; oxyanion series follow the -ite (fewer O)/-ate (more O) pattern, extended with hypo-/per- for series of more than two.

• Binary covalent compounds use numerical prefixes (mono- through deca-) to show atom counts, except "mono-" is never used on the first element; water and ammonia keep their common names.

• Acids: anion without oxygen → hydro-...-ic (HCl → hydrochloric acid); anion ending in -ate → -ic acid; anion ending in -ite → -ous acid.

• Electrolytes produce ions in water and conduct electricity. A strong electrolyte dissociates completely (NaCl, KI, HCl); a weak electrolyte dissociates only partially.

What Is an Ion?

An ion is a charged particle formed when the number of protons in an atom no longer equals the number of electrons. This is distinct from a nuclear reaction, where the number of protons or neutrons in the nucleus itself changes — in ordinary chemical reactions, the nucleus is untouched, and it’s electron movement that drives bond formation and breaking.

There are two types of ions:

• A cation is a positively charged ion. It forms when an atom loses electrons, leaving it with more protons than electrons.

• An anion is a negatively charged ion. It forms when an atom gains electrons, leaving it with more electrons than protons.

An ion’s chemical behavior differs significantly from that of the neutral atom or molecule it came from, so tracking charge carefully is essential.

Molecular vs. Ionic Compounds

Once you know the elements in a binary (two-element) compound, you can predict whether it’s molecular or ionic just from where those elements sit on the periodic table:

• Molecular compounds form when both elements are nonmetals. Examples: H₂O (water), SO₂ (sulfur dioxide), HCl (hydrogen chloride).

• Ionic compounds form when one element is a metal and the other is a nonmetal. Examples: NaCl (sodium chloride), KBr (potassium bromide), CuS (copper(II) sulfide).

Before naming ionic compounds, it helps to know the common monatomic ions that appear in them repeatedly:

Common monatomic cations: hydrogen, lithium, sodium, potassium, cesium, beryllium, magnesium, calcium, barium, aluminum, silver.

Common monatomic anions: hydride, fluoride, chloride, bromide, iodide, oxide, sulfide, nitride, phosphide.

Naming Binary Ionic Compounds (Type I)

Type I binary ionic compounds involve a metal that forms only one type of cation — its charge never varies. The formula is always written cation first, anion second, and the name follows the same order:

• The cation is named first, taking its name directly from the element (e.g., Na is "sodium" in a compound name).

• The anion is named second, using the root of the element’s name plus the suffix "-ide" (e.g., Cl⁻ is "chloride").

Practice examples:

• CsF is cesium fluoride.

• AlCl₃ is aluminum chloride.

• LiH is lithium hydride.

Naming also works in reverse — from name to formula. Calcium hydroxide contains Ca²⁺ (calcium only ever forms this ion) and OH⁻ (hydroxide). Since two hydroxide anions are needed to balance one Ca²⁺, the formula is Ca(OH)₂.

• Potassium iodide: KI, containing K⁺ and I⁻.

• Calcium oxide: CaO, containing Ca²⁺ and O²⁻.

• Gallium bromide: GaBr₃, containing Ga³⁺ and Br⁻ — three bromide ions are needed to balance the charge of one Ga³⁺.

Naming Binary Ionic Compounds (Type II)

Type II binary ionic compounds involve a metal that can form more than one type of cation, so its charge must be specified in the name — unlike Type I metals, which always carry the same charge.

For example, FeCl₂ contains Fe²⁺, while FeCl₃ contains Fe³⁺. Both are legitimate iron chloride compounds, so the name must distinguish them using a Roman numeral indicating the cation’s charge:

• FeCl₂ = iron(II) chloride

• FeCl₃ = iron(III) chloride

Metals that never need a Roman numeral, because they only ever form one ion:

• Group 1A elements (always form +1 ions)

• Group 2A elements (always form +2 ions)

• Aluminum (always Al³⁺)

• Silver (virtually always Ag⁺ — so "silver chloride," not "silver(I) chloride," even though the latter is technically correct)

• Zinc (always Zn²⁺)

An older naming system (still seen in older literature) applies only to metals that form exactly two ions: the ion with the higher charge takes the suffix "-ic," and the ion with the lower charge takes the suffix "-ous."

• Fe³⁺ = ferric; Fe²⁺ = ferrous → FeCl₃ = ferric chloride; FeCl₂ = ferrous chloride.

• Cu²⁺ = cupric; Cu⁺ = cuprous.

Polyatomic Ions and Oxyanion Naming

A polyatomic ion is a charged group of multiple atoms that behaves as a single unit in a compound. Their names must simply be memorized — for example, ammonium nitrate, NH₄NO₃, contains the polyatomic ions ammonium (NH₄⁺) and nitrate (NO₃⁻).

The table below covers commonly tested polyatomic ions:

Some elements form a whole series of oxyanions — anions built from that element plus different numbers of oxygen atoms. These series follow a consistent naming pattern:

• With two members in the series: fewer oxygens gets "-ite," more oxygens gets "-ate." Example: sulfite (SO₃²⁻) vs. sulfate (SO₄²⁻).

• With more than two members: "hypo-" (fewest oxygens) and "per-" (most oxygens) are added as prefixes to the -ite/-ate stem. Example, the chlorine oxyanion series: hypochlorite → chlorite → chlorate → perchlorate, in order of increasing oxygen content.

Naming Binary Covalent Compounds

Binary covalent compounds (also called Type III compounds) form between two nonmetals. They don’t contain ions, but they’re named using a similar pattern to binary ionic compounds:

• The first element is named first, using its full element name.

• The second element is named as if it were an anion (root + "-ide").

• Numerical prefixes indicate how many atoms of each element are present:

The prefix "mono-" is never used on the first element — CO is "carbon monoxide," never "monocarbon monoxide."

Two compounds are always called by their common names instead of a systematic name: water (H₂O) and ammonia (NH₃).

Naming Acids

An acid can be thought of as a molecule with one or more H⁺ ions attached to an anion. How you name it depends on whether that anion contains oxygen:

Anion without oxygen → prefix "hydro-" + anion root + suffix "-ic":

• HCl → hydrochloric acid

• HCN → hydrocyanic acid

• H₂S → hydrosulfuric acid

Anion with oxygen → the acid’s suffix depends on the anion’s own ending:

• Anion ends in "-ate" → acid ends in "-ic": H₂SO₄ (sulfate) → sulfuric acid; H₃PO₄ (phosphate) → phosphoric acid; HC₂H₃O₂ (acetate) → acetic acid.

• Anion ends in "-ite" → acid ends in "-ous": H₂SO₃ (sulfite) → sulfurous acid; HNO₂ (nitrite) → nitrous acid.

Electrolytes

Electrolytes are substances that produce ions when dissolved in water, and those ions let the resulting solution conduct electricity. They’re biologically important — sodium, potassium, calcium, and chloride ions play key roles in nerve and muscle function and fluid balance, and are found naturally in the body as well as in foods and beverages like sports drinks.

Ionic compounds tend to make good electrolytes because they dissolve readily into their constituent ions; nonpolar covalent compounds are the weakest electrolytes, since they don’t form current-carrying ions at all.

• A strong electrolyte dissociates completely into its constituent ions in water. Examples include ionic compounds like NaCl and KI, along with molecular compounds with highly polar covalent bonds — like HCl — that fully ionize when dissolved.

• A weak electrolyte ionizes or hydrolyzes only incompletely in aqueous solution, so only some of the dissolved solute converts into ions.

Common MCAT Mistakes

• Naming Type II metals without a Roman numeral or -ic/-ous suffix. Variable-charge metals like iron and copper need their charge specified in the name (iron(III) chloride, not "iron chloride") — omitting it leaves the compound ambiguous between FeCl₂ and FeCl₃.

• Confusing -ite and -ate endings, or forgetting hypo-/per- for longer oxyanion series. Fewer oxygens takes -ite, more takes -ate; a series longer than two members extends outward with hypo- (fewest O) and per- (most O), as in the hypochlorite → chlorite → chlorate → perchlorate series.

• Using "mono-" on the first element of a binary covalent compound. CO is "carbon monoxide," never "monocarbon monoxide" — the mono- prefix is only ever applied to the second element.

• Assuming every dissolved ionic compound is a strong electrolyte. Most ionic compounds dissociate completely and are strong electrolytes, but electrolyte strength is ultimately about how completely a substance ionizes in water — some molecular compounds with highly polar bonds, like HCl, are also strong electrolytes, while many ionic compounds with low solubility contribute few ions in solution.

MCAT-Style Concept Check

Question: A compound with the formula Cu₂O is copper(I) oxide. Which type of naming rule does this compound follow, and why is the Roman numeral necessary?

• A) Type I, because copper only ever forms one type of cation

• B) Type II, because copper is a metal that can form more than one type of cation

• C) Type III, because the compound forms between two nonmetals

• D) It’s an acid name, because oxide is an oxyanion

Answer: B

Explanation: Copper is a variable-charge (Type II) metal — it can form Cu⁺ or Cu²⁺. Since O²⁻ requires two Cu⁺ ions to balance its charge in Cu₂O, the cation here is Cu⁺, so the Roman numeral (I) is required to distinguish this compound from copper(II) oxide (CuO), which contains Cu²⁺. Type I naming (A) only applies to fixed-charge metals; Type III (C) is for compounds between two nonmetals; oxide is a monatomic anion, not an oxyanion, so (D) is incorrect.

FAQ

How do you know if a compound is ionic or molecular just from its formula?

Check whether the elements are metals or nonmetals. If both elements in a binary compound are nonmetals, it’s molecular (e.g., SO₂). If one is a metal and one is a nonmetal, it’s ionic (e.g., NaCl).

When does an ionic compound’s name need a Roman numeral?

Only when the metal is a Type II (variable-charge) metal — one that can form more than one type of cation, like iron or copper. Type I metals, including all Group 1A and 2A elements, aluminum, silver, and zinc, always form the same charge, so their names never need a Roman numeral.

What’s the difference between naming an oxyanion series with two members versus more than two?

A two-member series uses -ite for fewer oxygens and -ate for more oxygens (sulfite vs. sulfate). A series with more than two members adds hypo- (fewest oxygens) and per- (most oxygens) as prefixes on top of the -ite/-ate stem, as in the four-member chlorine oxyanion series.

What makes an electrolyte "strong" versus "weak"?

A strong electrolyte dissociates completely into ions in water — most ionic compounds and highly polar covalent compounds like HCl behave this way. A weak electrolyte only partially ionizes or hydrolyzes, so only some of the dissolved solute becomes ions in solution.