All ionic solids (easily recognized as being the compounds of metals - along with
The negative ion is frequently a polyatomic ion, meaning it consists of a number of atoms (bonded together covalently) but having one or more extra electrons. The negative ion is written second in the formula, and stated second in naming the compound.
The ratio of ions in the compound will be the simplest numbers that create a total charge of zero.
So sodium carbonate consists of two sodium ions