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Naming Ionic Compounds - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
When naming ionic compounds, we always name the cation first with its full scientific name. Then, we name the anion, cutting off the last syllable and adding the suffix -ide . Sodium chloride, for instance, is an ionic compound containing sodium and chlorine.
Alright so we're going to talk about how to name ionic compounds. Ionic compounds we know when a metal and a non-metal come together to form a neutral compound, metal being out cations and non-metals being our anions or positively charged, negatively charged particles. So how do we name these guys? So we have our metals and our metals are just going to give us our name, we're just going to name the metals the same name as they have on the periodic table. The non-metals we're going to change their last name, we're going to drop their last syllable and just add ide to show that they've been changed to an anion.
Alright so what I'm I talking about? First take table salt for example, this Na we know it's sodium, we're going to keep it as sodium this guy Cl we know it's chlorine we're going to drop that to make chloride. So together the whole thing is going to be sodium chloride pretty easy. We have Ag2S we're going to do the same thing here, we're going to call this guy silver same thing as in the periodic table and then we have sulphur and you think it would sulphuride, it's not sulphuride it's a common mistake I see amongst a lot of students. It's sulfide, another common mistake it's with phosphorus, a lot of students say phosphoride it's just phosphide and I'll that out for you too, phosphide those are the ones that you typically get tripped up on, but otherwise it's the same thing. Just drop the last name and add ide.
But if you're given the name how do you figure out the formula for that? So we have magnesium we know magnesium is a positive 2 charge and we have chloride and chloride is in group 7 making it a negative 1 charge, so when they come together we're going to do a little cross the charges to make it our subscripts and we're going to say Magnesium sorry MgCl2 and this gives our neutral compound MgCl2 magnesium chloride. But what happens when we come across transition metals? Transitions metals are the guys in the middle of the periodic table, the reason they're called transition metals is because their oxidation number or their charges are constantly changing. They don't have a definite charge within them. So how are we going to distinguish which charge we're going to use?
We're going to use Roman numerals to describe what are the charges of that particular metal. You're going to only use the Roman numerals when you're writing out the actual name, when you're actually writing out the name, you're never ever going to use the Roman numeral when you're actually writing out the formula. So let's do a practice one, so if Fe2O3 well we have to figure out the charge of Fe the charge is not defined for us. So we have to say okay well we know using our cross idea, we know this is a minus 2, so this guy must be a plus 3. Okay great so we have to indicate that in our name, so iron III oxide, oxide is the same a it was before. So now it's iron III oxide, the III indicates that this iron is a plus 3 charge.
Okay let's go backwards, let's go from the names to the actual formula. So we have iron 2 that's telling me this iron is not the plus 3 as we thought earlier, this iron is a plus 2 charge. Oxygen as we know it's always going to be minus 2 so that's easy enough, so we just cross off the numbers to give us our formula and we get Fe2O2 does this work? This is not okay, as we know we want to make sure this is the most reduced form as possible so we're going to reduce that to FeO, fair enough? Okay, there are exceptions in the transition metals, there are some transition metals that actually do have a definite charge and those are Zinc and Silver.
Zinc is going to always be a plus 2 charge, Silver is going to always be a plus 1 charge. And there's a small trick you can remember to figure out, to remember these guys otherwise you have to memorize them. If you look at your periodic table, we know aluminum is in group 3 so it's going to be a plus 3 charge we know that already. That's defined for us, but if you go down diagonally this is going to be a plus 2, this is going to be a plus 1 just an easy way to remember that Zinc is plus 2 and silver is plus 1. So we don't have to use Roman numerals when describing those guys because they're defined for us.
Lastly when making sure we name ionic compounds properly, we have to talk about polyatomic ions. Polyatomic ions are exactly as it sounds, poly meaning multiple and atom. So these guys have multiple atoms put together. So when you come across a compound that has more than 2 elements in it, this guy has 3 main one lithium, sulphur and oxygen. We know that we're dealing with a polyatomic ion, in this case your teacher should have given you a list of maybe like 10 maybe more polyatomic ions. Those guys you have to actually memorize, unfortunately there isn't like a way we can keep those. It's just pretty important to memorize and the charges too, so make sure you, easy way I tell my students sometimes is if you laminated it and put in the shower and when you're showering and you have them in front of you, that's a great way and easy way to like just start memorizing those guys. So that's just a trick I tell my kids, so anyway if we're dealing with this, we know this is lithium as always and what is this guy? SO4, is sulfate so together this is lithium sulfate pretty easy.
But if we're going backwards, calcium we know is a plus 2 charge, hydroxide we know is a minus 1 charge if you look at the list your teacher gave you. So okay, so we need 1 calcium and 2 hydroxides. Okay would this work? No, this isn't working because this 2 is telling me I only have 2 hydrogens not 2 hydroxides, this is hydroxide. So I need to make sure I have 2 of these guys, these 2 just tells me I have 2 hydrogens. So I'm going to re-write this making it Ca I'm going to put in parenthesis just like Math Ca(OH)2 so that 2 now tells me it's going to the whole thing, the whole polyatomic ion. So that's how I'm going to write that and make sure you use parenthesis. So let's practice this real quick, this guy we're going to just do our name Magnesium and here again there's more than 1 atom we don't have to go to a polyatomic ion list nitrate.
This guy now, this is where it get tricky, this guy is 3 but you can't just write copper sulfate because copper don't forget it's a transition metal, so we have to make sure we have to indicate what charge it is. So sulfate is a minus 2 so in this case, there's 1 sulfate and that means, and there's one copper so this must be a plus 2 to make it equal. So it's copper II sulfate there's a lor of stuff to remember for naming ionic bonds but hopefully this helps you out.
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