Chemical formula for dinitrogen oxide
Cations from Groups IA, IIA, IIIA (except for T1) will form only one ion. If a cation is capable of forming more than one ion, it will have a Roman numeral in parenthesis (stock number) with it. Monoatomic ions have a charge equal to their group number. Also, it is common practice to write the cation (+) first when writing chemical formulas.
Let's work through some examples.
Write the chemical formula for magnesium oxide.
Let's break it down by ions. Magnesium is a Group IIA ion so it will have a +2 charge. Oxide is one of the ions you should have memorized and you know it has a 2 charge. Also, oxygen is a Group VIA element which carries a 2 charge. We want the compound to have a net zero charge so if we do a little math, we find that plus 2 minus 2 equals zero. This means we need one of each to balance the compound (A 1:1 ratio). Placing the cation first, we have our formula for magnesium oxide, MgO.
Compounds where the charges don't come out even
Write the formula for aluminum chloride.
Again, let's break it down by ions. Aluminum is a Group IIIA element and carries a +3 charge. Chloride is a Group VIIA element and carries a 1 charge. If we do the math, we see that +3 1 does not equal zero. It equals +2. We need two more negative ions to zero out the compound. Add two more chloride ions and add them up. We now have 3 for the chloride ions. Back to our math, +3 3 does equal zero. So we need one aluminum ion and 3 chloride ions to zero out the compound. As before, place the cation first in the formula and we have our formula for aluminum chloride, AlCl3.
Polyatomic anions
Most, not all, polyatomic anions end with the suffix ite or ate. Make sure you memorize the polyatomic ions so you can recognize them.
Write the formula for sodium sulfate.
As before, we'll examine the ions. Sodium is a Group IA element and carries a +1 charge. Sulfate is a polyatomic ion (SO42) which carries a 2 charge. If we do our math, we'll see that the two do not zero out. + 1  2 equals 1. To make it zero out, we need to add another +1 charge (Sodium ion). When we add another sodium ion, we end up with a balanced formula of Na2SO4.
We use the subscript of 2, NaSO4, rather than 2NaSO4 because a leading number, (in this case the 2), indicates that there are two of the entire element, which is incorrect. Our formula, Na2SO4 is correctly written.
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