Name binary and polyatomic ionic compounds, including those with variable-charge transition metals, and write correct formulas from a compound's name.
Naming a binary ionic compound (two elements) is straightforward: state the metal's name unchanged, then change the nonmetal's ending to "-ide." NaCl is sodium chloride; MgO is magnesium oxide; Al2O3 is aluminum oxide. Notice the subscripts themselves are never spoken in the name -- "aluminum oxide" says nothing about the 2-to-3 ratio, because that ratio is already forced by the charges once you know the elements involved.
Many transition metals can form more than one possible charge (iron can be Fe2+ or Fe3+, for example), so a Roman numeral in parentheses is added right after the metal's name to specify which charge is meant in that particular compound: FeCl2 is iron(II) chloride (the 2 chlorides, each -1, mean iron must be +2 to balance), while FeCl3 is iron(III) chloride (3 chlorides require iron to be +3).
Polyatomic ions are groups of atoms that carry an overall charge and stay together as a single unit through a reaction -- a short list is worth memorizing: nitrate (NO3-), sulfate (SO4 2-), carbonate (CO3 2-), phosphate (PO4 3-), hydroxide (OH-), and ammonium (NH4+, the one common positive polyatomic ion). Ca(NO3)2 is calcium nitrate (parentheses around NO3 show that the entire polyatomic ion is doubled to balance calcium's +2 charge). To write a formula from a name, use the crisscross method: the cation's charge number becomes the anion's subscript, and the anion's charge number becomes the cation's subscript, then simplify to the lowest ratio if possible.
Mastery looks like: They correctly name binary and polyatomic ionic compounds (including variable-charge metals) and correctly write formulas from names using the crisscross method.
Common stumbles: Forgetting the Roman numeral for a variable-charge transition metal; forgetting to reduce a crisscrossed formula to its lowest ratio when the numbers share a common factor.