we've learned about how atoms of par elements tend to gain or lose a certain amount of electrons in order to gain noble gas electron configuration in the process they will become ions and these ions come together in particular ratios with one another to form ionic compounds an ionic compound is any compound consisting of two or more ions that are bound to one another due to the electrostatic interaction of positively charged and negatively charged ions these ions can either be monoatomic consisting of only one atom or they can be polyatomic consisting of multiple atoms but still bearing one formal charge but whatever they're form these ions are what make up ionic compounds so we want to be able to predict the ways that they will combine the basic rule is that cations and anion come together in a particular ratio so as to form a neutral compound this generally occurs between a metal cation and a non-metal annion and the easiest way to predict the ratio is to know the charges of the common ions for example if a sodium atom encounters a chlorine atom the sodium will readily transfer its lone veence electron to chlorine in order for both of them to attain noble gas electron configuration once this has occurred there will be a sodium 1 plus ion and a chlorine 1 minus ion since the have a charge of 1 plus and one minus they can simply combine in a one: one ratio to form a molecule of NAC this is an ionic compound because it involves two formally charged ions participating in an ionic bond with one another let's say sodium ions were instead to interact with oxygen we know from its position on the periodic table that oxygen will tend to form 2 minus ions an O2 minus ion would have to combine with two sodium ions in order to form a neutral compound because it takes two 1+ ions to cancel out one 2 minus ion so here is na2o what about aluminum and oxygen from the table we know aluminum forms a 3 plus ion so we would need two aluminum ions and three oxygen ions to get a total of plus and minus 6 so here is al23 this works with polyatomic ions as well take the sulfate ion s so42 minus how will this combine with potassium we would need two k+ ions to combine with one s04 2 minus to give a neutral compound so we get k2so4 we do not list charges when writing the molecular formula of an ionic compound because an ionic compound is neutral but we must be able to look at the molecular formula of an ionic compound and know that it is comprised of formally charged ions then we will be one step closer to understanding the composition of molecules