Naming Ionic and Simple Molecular Compounds
Chemical names are built from rules, not memorised one by one: ion charges fix the formula of an ionic compound, and Greek prefixes count atoms in a molecular one. Reading a name backwards to a formula is the skill that makes every later equation legible.
What a learner can do afterwards
- Write the formula of an ionic compound from the charges of its ions, including common polyatomic ions
- Name a binary molecular compound using counting prefixes and explain why ionic compounds do not use them
- Use a Roman numeral correctly for a metal with more than one possible charge
1 · Read
Name an ionic compound metal first, then the nonmetal with its ending changed to ide, as in sodium chloride. Charges must balance to zero, so two potassium ions meet one oxygen ion in potassium oxide, K2O. Use the shortcut: each charge tells you how many of the other ion you need, then simplify, as in BaS.
Some metals form more than one ion, and a Roman numeral says which one. Iron(II) differs from iron(III), and copper(II) chloride, CuCl2, holds copper with a 2+ charge. Read the numeral as the charge before you balance.
Molecular compounds share atoms instead of charging them, so the name counts atoms with prefixes: mono, di, tri, tetra. N2O4 is dinitrogen tetroxide. Ionic compounds never use these prefixes, because the charges already fix the ratio.
Polyatomic ions are charged groups that stay together, like hydroxide OH-, sulfate SO4 with 2-, and nitrate NO3-. Balance them like single ions: K2SO4 needs two potassium ions per sulfate. With more than one group, wrap it in parentheses, as in Mg(OH)2 and Sr(NO3)2.
Charges fix ionic formulas, prefixes count molecular atoms, and numerals name the metal charge.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.