we have previously learned how to name ionic compounds and deduce their formula from the ionic charges for all the ions in the examples provided they have one thing in common they are made of one atom and are therefore known as monatomic ions in this lesson we will learn how to name and deduce the formula of ionic compounds made from polyatomic ions these polyatomic ions are ions made of more than one atom remember that mono means one and poly means many some common polyatomic ions include hydroxide sulfate nitrate phosphate carbonate and ammonium note that with the exception of the ammonium ion all of the listed polyatomic ions are anion you will have to remember the formula and the charges of these polyatomic ions let's grab a piece of paper and a pencil and work on the first example write the sodium ion and nitrate ion side by side remember to include their charges let's crisscross these charges since we never put one in a formula the formula for the ionic compound in question is N3 when naming ionic compounds made from polyatomic ions we just write their names as they are the name there therefore is sodium nitrate now for our second example let's write a magnesium ion and a hydroxide ion side by side let's crisscross these charges the structure of the hydroxide ion is not changed when forming an ionic compound for this example we simply need two hydroxide ions to fully balance out the plus two charge of the magnesium ion this is represented by placing the O and H in round brackets and the two as a subscript so the formula is mg in Brackets 2 and its name is magnesium hydroxide let's say we did not put the O in Brackets we would get mg2 this would mean that for each magnesium ion there is one oxygen atom and two hydrogen atoms which is incorrect are you ready to try this on your own Now find the formula and name the ionic compound formed from potassium and sulfate ions please pause the lesson to try this and resume when you are done the formula is k2so4 and the name is potassium sulfate we know that ionic compounds have an overall neutral charge so it makes sense to have two potassium ions to fully balance the negative -2 charge of the sulfate ion transition metals are multivalent this means that they can form Ion with many different charges for example these copper ions can have either A+ one or plus two charge if we form an ionic compound from copper 2+ ions and chloride ions we must specify the valency of the copper ion in its name so for cu2 the name is copper 2 chloride note that the veency is written as Roman numerals in round brackets right after the transition metal