This is the walkthrough for the practice test on ionic names and formulas. Which of the following would be the correct name for iron with a subscript 2, oxygen with a subscript 3? So it's going to be iron oxide.
Okay, iron oxide. Because we take the name of the first element, and we take the name of the second element, and change the ending to ide. The trick with this one is that iron is a transition element, so it's from this middle part of the periodic table here. Which means it's one of the many elements that sometimes has more than one charge. So we need to know which iron ion we're dealing with here.
Here's how we do it. Oxygen. is a minus 2, because it's from group 16 over here.
So it's always a minus 2. The formula says that there are 3 oxygens. So 2 times 3 gives us the total charge of minus 6. That means that the total positive charge has to be positive 6. So if the total positive charge is 6, the formula over here says that there are 2 irons. So 6 divided by 2... means that each of those irons is a plus 3 ion. So to indicate the plus 3, we put the Roman numeral 3 in parentheses in the middle, we call it iron 3 oxide, and the correct choice is C.
What can you say about the following group 1 metal and group 16 nonmetal? First of all, remember the group 1 metal is going to be positive, the nonmetal is going to be negative. Now we have to figure, so they will combine.
And we have to figure out what are the charges so that we can work out what the ratio might be. Well, we have this chart that I used in class that says you've got all the different groups. 1, 2, 13, 14, 15, 16, 17, 18. Groups 18 and 14 don't mostly form ions. We can kind of leave those out for now. All the elements in group 1 form a plus 1 ion.
All the elements in group 2 form plus 2. Group 13 is plus 3. And then we skip over to group 15, which is minus 3. 16 is minus 2. 17 is minus 1. You need to know this for the test. It's not going to be given to you. So we have a group 1 metal. That means it's going to have a charge here of plus 1. And the group 16 nonmetal means it's going to have a charge of minus 2. So let's just call these x and y. So x is 1 plus and y is 2 minus.
So it's not a 1 to 1 ratio because if I just put 1x and 1y together, the charges don't cancel out and they have to cancel out to make an ionic compound. They're opposite charges so they're not going to repel. If I had 2... X's and 1y then the total positive charge would be plus 2 the total minus charge negative charge would be minus 2 So that is what we're trying to get so the answer here would be C They will combine in a 2 to 1 ratio. What would the formula be from the following ions?
We're going to do a similar thing to what we just did except that we're already given the charges So a is plus 1 B is minus 3 and here's where you could just use the crisscross method You write the symbol for each element, and then you take the number from the positive and make it the subscript of the negative. You take the number from the negative and make it the subscript of the positive, and you get A3B. And just remember that you don't take the charge with you, so it's not A negative 3B or A positive 3B. It's just A3B.
The 3 in this case just tells you that there are 3A. Ions for every one B ion. It's a three to one ratio What type of elements form negative ions if you're looking at the periodic table, okay?
very simplified version everything from Roughly here to the left those are all metal all metals form positive ions Then this sort of middle line here are the metalloids. They're not going to form ions. The noble gases are on the right.
18, they do not form ions either. The elements up here in this corner are called the nonmetals, and they are the ones that form negative ions. So these are negative, all of these in here are positive, the group in the middle and the group on the end don't form ions. The choice there was B, nonmetal. In order to form a positive ion, that should say, an element must lose...
Electrons and protons are gain electrons and protons. Well remember, we're talking about the balance between the number of protons, which are positive, and the number of electrons, which are negative. When you have the same number, like let's say 10 of each, the total charge is zero, because you've got 10 positive and 10 negative, you add those together you get zero, no charge, neutral.
But if you change the number of electrons, because this is a key, The number of protons does not change, never changes, okay? But the number of electrons can either go up or down. If you take away an electron, and now you have nine electrons and still have ten protons, because that number never changes, then you've got more positive than negative, which means you're going to have a positive ion.
So when you lose... an electron, you get a positive ion. If I added an electron or gained an electron, now I've got 11 electrons, still have 10 protons because that never changes, then that would be a negative ion.
So lose an electron, you become positive. Gain an electron, you become negative. Okay, so doing some names and formulas. Given the name magnesium sulfide, what is the formula?
Well, this is where you have to go back to the... Periodic table and figure out what are the charges. Magnesium is a metal.
It's in group 2, so it has a charge of plus 2. Sulfide, which is sulfur, and we change the ending to ide, so sulfide was sulfur by itself. In a compound, we call it sulfide. Sulfide, or sulfur, is in group 16, so it's going to be a negative 2. Well, when you've got a negative 2 and a positive 2, the charges are equal. So you can just put one magnesium with one sulfur and they cancel each other out.
In this formula, we have calcium and we have fluorine. So when it's just two elements, there's just two capital letters here, just two elements, you use the name of the first element, so calcium, and then you use the name of the second element. Fluorine, but you change the ending to ide so calcium and fluorine form the compound calcium fluoride last one on this page Potassium nitride so all of these by the way are called binary compounds because there are only two elements Which means there are only two capital letters so potassium symbol K.
It's in group one has a charge of plus one Nitride is nitrogen and it's in group 15, so it has a charge of negative 3. Well, if you use the crisscross method, you get K3, I'll take the number from the negative ion to go with the positive, and then you get N1, since this was a plus 1. So if you go back and think about it, 3 potassiums, each with a charge of plus 1, gives you a total of plus 3. 1 nitrogen with a charge of negative 3, Gives you a total of negative 3. So the charges here are equal, which means you got the formula right, K3N. The next thing that happens in some of these compounds is you have a transition metal. So these are the ones in the middle of the periodic table.
And most or many of those elements have more than one oxidation state, which means sometimes they have a charge of, say, plus 1. Other times it might be plus 2. Some of them have even more possibilities. The point is you don't know just by using the name of the element which charge it has if it's a transition element. So that is why we use these Roman numerals. A Roman numeral in the name of a compound tells you the charge of the metal ion, the cation.
So the fact that this says copper, Roman numeral 2, oxide, means that I have copper with a plus 2 charge. Other times it might be copper with a plus one charge, but the Roman numeral says two, so the charge is plus two. And then I have oxygen.
Oxide is oxygen with the minus 2 charge. So the formula, since the charges are equal of the ions, is just CuO. You just put one of each together, and then you have the answer. So the Fe3P2, here's iron.
Fe is iron, which has from the middle part of the periodic table. So we don't know it's charged just by looking at where it is on the periodic table. So what I have to do is work backward.
From the negative charge, phosphorus has a charge of negative 3. It's in group 15, so it's always negative 3. According to the formula, there are two of them. That means the total negative charge is 6, which means the total positive charge is 6. So if I take the total positive charge of 6 and I divide it by the number of irons, which is 3, I get a charge for each iron of plus 2. And the way to kind of double check this is go back to the formula. Did the numbers crisscross?
Yeah, I have iron plus 2, and in the formula I have P2. The phosphorus has a minus 3, and in the formula I have iron 3. So if the numbers crisscross once you work out the charges, then you probably get it right. And then the way we write the name is iron, Roman numeral 2, because it has a charge of plus 2. And then the other one is phosphide. Phosphorus becomes phosphide.
So iron 2-phosphide. Down here, we have chromium 4-nitrate. Okay, so chromium 4-nitrate.
So now we have a transition metal, and we also have a negative ion that ends in A-T-E, which means it's a polyatomic ion. So I'm going to go over to the other side and I'm going to look for nitrate, and it's right here, NO3 with a charge of negative 1. Well the formula, or the name rather, tells me that this chromium has a charge of plus 4. So I've got a plus 4 and I've got a minus 1. That means I need 4 nitrates to go with my chromium. So the formula is going to be CR. Chromium just one chromium and then I put the whole formula for nitrate in parentheses So n o three because I can't change the little three there in parentheses And then I have to put a four out there So there's four nitrates for every chromium four nitrates with a charge of minus one each gives me a total of minus four and The chromium the Roman numeral tells me that the chromium is a charge of plus four and then the last one on this page I've got Cobalt and OH, and I've got two OHs.
So I've got chromate and two OHs. OH, again, there's three capital letters, so I've got a polyatomic ion. OH is hydroxide. It's got a charge of minus one.
So OH has a charge of minus one, but there are two of them, so the total negative charge is negative two. That means that the total positive charge has to be 2. And in the formula, there's only one cobalt, so that cobalt is plus 2. Since cobalt is in the middle of the periodic table, I use a Roman numeral to indicate the charge, so this is cobalt 2 hydroxide. Some more polyatomic ions in this group. We have barium phosphate.
Barium is an element. In group 2, it's going to have a charge of plus 2. Phosphate, find it over here, PO4, with a charge of negative 3. So we've got a plus 2 and a negative 3, which means, if I switch the numbers, I'm going to have 3 bariums and then 2 phosphates. But because it's a polyatomic ion, I have to put the whole thing in parentheses and put the 2 on the outside.
Barium phosphate, Ba3, parentheses PO4, closed parentheses, 2. Next one I've got calcium, and then I've got NO3. Remember, NO3 is nitrate. It's going to be over here.
Nitrate has a charge of minus 1. So I've got calcium, which is plus 2. It's not a transition metal, so I don't need a Roman numeral. calcium Nitrate. Don't change the ending from nitrate to nitride.
Nitride means just nitrogen by itself. Nitrate means this nitrogen with three oxygens. Potassium hydroxide.
And hydroxide is not on the periodic table because it's a polyatomic ion. And we have to look up hydroxide in the table over here. and we'll see that hydroxide is OH, charge of negative 1. Potassium is an alkali metal with a charge of plus 1. So if I just put one potassium and one hydroxide together, they cancel out.
You don't need parentheses around the OH here because there's only one OH. You only need parentheses when you have multiple polyatomic ions. And the last one here we have Mg, which is magnesium.
It's not a transition metal, so I don't need to put a Roman numeral. And then I have CO3, which if I look over here is carbonate. It has a charge of minus 2, but that doesn't really affect the name.
It's just called magnesium carbonate. Okay, and then just a couple more that have more than one thing going on. We've got transition metals with polyatomic ions, or maybe two polyatomic ions together.
So the first one is nickel to acetate. So we've got to do a couple things. The first of which is, notice that the acetate is not on this chart.
Acetate is an ion C2H3O2, and it has a charge of minus one. So nickel 2 means that the charge is plus 2. Acetate has a charge of minus 1. So I'm going to need two acetates to cancel out my one nickel. Nickel has a symbol of Ni, and then acetate is C2H3O2, and I need two of them. So I put C2H3O2 in parentheses with a 2 on the outside.
Ni parentheses C2H3O2, close parentheses, 2. is nickel acetate. Next one I have lead, and I have sulfur, which is sulfide. In the formula I have two sulfurs. Each of those sulfurs has a charge of minus two. So that means the total is negative four, which means the positive charge has to be four.
There's only one lead atom in my formula. So that lead atom has to have a charge of plus 4, and since lead is one of our metals that has more than one charge, we call it lead 4 sulfide. Okay, we change the ending from sulfur to sulfide. Fe2 OH, sorry, FeOH2.
Remember, OH is over here, it's hydroxide, it has a charge of minus 1. There's two of them in the formula, so the total charge is minus 2. I'm going to put this down here. So we've got minus 2, which means we need plus 2. That plus 2 comes from just one iron atom, so the iron has a charge of plus 2. So we're going to call that iron, Roman numeral 2, in parentheses, hydroxide. And the fact that there are two hydroxides doesn't change the name in terms of the hydroxide, but it helps us figure out the charge of the iron atom. And the last one, ammonium carbonate. Now we have two polyatomic ions.
If you look for ammonium on the periodic table, you're not going to find it because it's not an element. It's a polyatomic ion. And carbonate, the fact that it ends in A-T-E, tells you that it is a polyatomic ion.
So we're going to look on the chart, and we find ammonium, it's down here at the bottom, it's the only one of these that's positive, has a charge of plus 1. And then carbonate has a charge of minus 2. So we've got a plus 1 and a minus 2, which means we're going to need to switch the numbers. NH4, I can't change that, but if I put the whole thing in parentheses, I can put a 2 outside. Then co3 so ammonium in parentheses with a 2 carbonate Then I've got a total of plus 2 and a total of minus 2 so there you go Make sure you know the chart that shows you the groups and the charges that go with them like group 1 is plus 1 group 2 Is plus 2 and so on you will get the list of polyatomic ions given to you with the test You will get a periodic table to use you should not need a calculator for any of these problems and you need to know what is a transition metal versus what is just a normal representative element like group one group two and so on alkali metals so there you go