Naming Compounds: Ionic & Covalent Nomenclature

A compound's naming rules depend on what kind of bond holds it together. Ionic compounds form between a metal and a nonmetal (or a metal and a polyatomic ion) — atoms transfer electrons to become charged ions that attract each other. Covalent (molecular) compounds form between two nonmetals — atoms share electrons instead of transferring them. The first question to ask about any formula or name is: metal + nonmetal (ionic), or nonmetal + nonmetal (covalent)? Everything else follows from that answer.

A monatomic ion is a single atom that has gained or lost electrons to become charged. Metals lose electrons to form positive ions (cations); nonmetals gain electrons to form negative ions (anions). For many main-group elements, the charge is predictable from the element's group number on the periodic table.

Naming a monatomic anion: drop the ending of the element name and add -ide. Chlorine → chloride. Oxygen → oxide. Nitrogen → nitride. Cations from main-group metals just keep the element's name: sodium ion, magnesium ion, aluminum ion.

A polyatomic ion is a group of covalently bonded atoms that carries an overall charge and moves as a single unit in reactions and formulas. These names must be memorized — they don't follow the group-number pattern. The ones below cover the large majority of formulas you'll see in an intro chemistry course.

An ionic compound's name is simply cation name + anion name (drop the word "ion"). Ionic compounds are always electrically neutral overall — the positive and negative charges must cancel out. Use that fact to write the formula: find the smallest whole-number ratio of cations to anions that makes the total charge zero.

The criss-cross method for writing a formula from a name: write the cation's charge as a number, write the anion's charge as a number, then swap them down as subscripts (drop signs, drop any 1s, reduce the ratio if it shares a common factor).

Most transition metals (and a few main-group metals like tin, lead, and bismuth) can form more than one possible charge. A name like "iron chloride" is ambiguous — it could be FeCl₂ or FeCl₃. To resolve this, the metal's charge for that specific compound is written as a Roman numeral in parentheses right after the metal's name. This system is called the Stock system.

Finding the Roman numeral from a formula: work backward from the known anion charge. In FeCl₃, each chloride is 1−, and there are 3 of them, so the total negative charge is 3−. For the compound to be neutral, iron must be 3+, so this is iron(III) chloride. In FeCl₂, the same logic gives iron a 2+ charge, so it's iron(II) chloride.

Going the other direction (name → formula): the Roman numeral IS the cation's charge, so criss-cross it against the anion's charge exactly as before. Copper(II) oxide: Cu²⁺ and O²⁻ → CuO. Copper(I) oxide: Cu⁺ and O²⁻ → Cu₂O.

Covalent compounds form between two nonmetals sharing electrons, so there's no ion charge to balance — instead, the name spells out the exact number of each atom using Greek prefixes. The first element keeps its full name; the second element gets the -ide ending, same as an anion.

An acid is a compound that releases H⁺ ions when dissolved in water, and acids get their own naming pattern based on the anion that pairs with the hydrogen. Binary acids (H + one nonmetal, no oxygen) use hydro- ... -ic acid. Oxyacids (H + a polyatomic ion containing oxygen) follow the -ate/-ite pattern: -ate → -ic acid, -ite → -ous acid, with no "hydro-" prefix.

1. Write the formula for iron(III) sulfate. (Fe³⁺ and SO₄²⁻ → criss-cross and reduce: Fe₂(SO₄)₃) 2. Name the compound N₂O₅. (Two nitrogens, five oxygens → dinitrogen pentoxide) 3. Name the acid H₂SO₄(aq). (Sulfate ends in -ate → drop "-ate," add "-ic acid," no hydro- prefix since it contains oxygen → sulfuric acid)

Using nomenclature-practice.csv: 4. For each item where given_type is "formula," write the compound's name. 5. For each item where given_type is "name," write the compound's formula. 6. For every ionic item involving a transition metal or a metal with more than one possible charge (N2, N3, N4, N5, N11), show the charge-balancing work you used to find the Roman numeral or the ion's charge.