In my Oct 20\u00a0<\/a>post about the periodic table and ionic compounds, I mentioned the issue of metal vs nonmetal.\u00a0 As I described in that article, it’s easy to tell for individual atoms.\u00a0 However, what about something like NO3<\/sub>–<\/span><\/sup>?<\/p>\n The answer is that if the species has a negative charge, it is a nonmetal; if its charge is positive, it’s a metal.<\/em>\u00a0 Therefore, Cr2<\/sub>O7<\/sub>2-<\/span><\/sup> is a nonmetal, even though Cr itself is a metal.\u00a0 By that same reasoning, NH4<\/sub>+<\/span><\/sup> is a metal, even though N itself is a nonmetal.<\/p>\n Species like nitrate (NO3<\/sub>–<\/span><\/sup>) and ammonium (NH4<\/sub>+<\/span><\/sup>) are called radicals or polyatomic ions. There are many of them; type “table of polyatomic ions” into your browser for lists.<\/p>\n To use a radical in a formula, you remember that the negative charges have to add up to the same as the positives.\u00a0 If you need a multiple of the radical, you use brackets.\u00a0 Here are some examples:<\/p>\n calcium hydroxide:\u00a0 Look up “ion table” in your browser to find a list of common ions.\u00a0 You’ll find calcium is Ca2+<\/span><\/sup>. Then, find (on the same table, or from a table of polyatomic ions) hydroxide. You’ll find it’s\u00a0OH–<\/span><\/sup>. The\u00a0negative sign with no number means a charge of negative 1.<\/p>\n So, we have Ca2+<\/span><\/sup> and OH–<\/span><\/sup>.\u00a0 Realizing positives must equal the negatives, we need two hydroxides (2x-1=-2).\u00a0 So we have<\/p>\n calcium hydroxide:\u00a0 Ca(OH)2<\/sub><\/p>\n By a similar process, you can assemble the following:<\/p>\n sodium sulphate:\u00a0 Na2<\/sub>SO4<\/sub><\/p>\n magnesium nitrate:\u00a0 Mg(NO3<\/sub>)2<\/sub><\/p>\n One important point to remember is that with a polyatomic ion such as NO3<\/sub>–<\/span><\/sup>, the 3 doesn’t mean there are three nitrates; it means there are three oxygens in one nitrate.\u00a0 Therefore, if you need three nitrates – such as in the case of aluminum nitrate – you would put a three outside the brackets, as follows:<\/p>\n aluminum nitrate: Al(NO3<\/sub>)3<\/span><\/sub><\/p>\n We have more to discuss about formulas of ionic compounds. Good luck with this installment.<\/p>\n