Naming Ionic and Simple Molecular Compounds

Chemists do not memorize a separate name for every compound. They use nomenclature, a shared set of naming rules, so that a name like sodium chloride or dinitrogen tetroxide tells you exactly what is inside. The simplest compounds are binary compounds, which contain only two elements. For an ionic compound, you name the metal ion first, then the nonmetal ion with its ending changed to -ide, like NaCl becoming sodium chloride. The charges of the ions decide the formula: the positives and negatives must balance out to zero. Some metals, like iron, can have more than one charge, so the name includes a Roman numeral, as in iron(II) or iron(III), to say which one. Molecular compounds follow different rules. Since the atoms are shared, not charged, the name uses counting prefixes like mono, di, tri, and tetra to show how many of each atom there are, so N2O4 is dinitrogen tetroxide. Once you know these rules, you can read a name and write the formula, or read a formula and write the name.

When atoms join together, they can form two main kinds of compounds. In an ionic compound, electrons move from one atom to another. The atom that loses electrons becomes a positive ion, and the atom that gains them becomes a negative ion. The opposite charges pull the ions together into a solid, like table salt. Metals on the left side of the periodic table usually team up with nonmetals on the right side to make ionic compounds. In a molecular compound, atoms share electrons instead of trading them, which usually happens when two nonmetals combine, like in water or carbon dioxide. Knowing which kind of compound you have matters because the naming rules are different for each. For ionic compounds, the ion charges tell you the formula, because the total charge must add up to zero. For molecular compounds, you simply count the atoms. You can often predict which type will form just by looking at where the elements sit on the periodic table.

A normal atom has no overall charge because its positive protons and negative electrons balance each other. When an atom gains or loses electrons, it becomes a charged particle called an ion. Ionic compounds form when positive and negative ions stick together, and writing their formulas is easier than it sounds. Start with a binary compound, one made of exactly two elements: a metal and a nonmetal. Its name is the metal name followed by the nonmetal name with the ending swapped to -ide, so chlorine becomes chloride. To get the formula, look up the charge of each ion. Then pick the smallest numbers of each ion that make the total charge zero. If calcium is 2+ and chloride is 1-, you need two chlorides for every calcium, so the formula is CaCl2. Watch out for polyatomic ions, which are groups of atoms that carry one shared charge, like sulfate. If you need more than one of a polyatomic ion, wrap it in parentheses before adding the number, as in Ca(OH)2.

Some ions are just a single atom with a charge, like Na+ or Cl-. These are called monatomic ions. But other ions are whole groups of atoms that stay together and carry one shared charge. These are called polyatomic ions, and they show up in chemistry all the time. A few common ones worth knowing are hydroxide (OH, charge 1-), nitrate (NO3, charge 1-), carbonate (CO3, charge 2-), sulfate (SO4, charge 2-), and ammonium (NH4, charge 1+), which is a rare positive polyatomic ion. Polyatomic ions form ionic compounds just like single-atom ions do: the total positive charge must balance the total negative charge. So sodium (1+) pairs with one nitrate (1-) to make NaNO3, while calcium (2+) needs two hydroxides, written Ca(OH)2. The parentheses matter: they show that the 2 applies to the whole OH group, not just the hydrogen. When you name these compounds, the polyatomic ion keeps its own special name, so NaNO3 is sodium nitrate.