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Worked example: Finding the formula of an ionic compound

AP.Chem:
SAP‑2 (EU)
,
SAP‑2.B (LO)
,
SAP‑2.B.3 (EK)
To find the formula of an ionic compound, first identify the cation and write down its symbol and charge. Then, identify the anion and write down its symbol and charge. Finally, combine the two ions to form an electrically neutral compound. In this video, we'll walk through this process for the ionic compound calcium bromide. Created by Sal Khan.

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  • blobby green style avatar for user graccebird121
    I'm still a little confused on how to know what the chemical name is going to end with depending on the number of ions
    (40 votes)
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  • starky tree style avatar for user kacey
    At , this is confusing. This way of figuring out the number of atoms works on this particular equation, but I'm not sure on how to work it on equations with more atoms.
    (17 votes)
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  • mr pants teal style avatar for user Andrea Balingit
    Why did he not put a parenthesis? Ca(Br)2?
    (13 votes)
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    • orange juice squid orange style avatar for user awemond
      Parentheses are generally only used for complex ions. Since the anion here, Br, is a single atom, there is no need to include parentheses. If the anion had been, for example HSO4-, then we would have included parentheses to make it clear that here are two of these complex anions: Ca(HSO4)2.
      (15 votes)
  • starky ultimate style avatar for user Matt Broadley
    What is the rule for an ionic compound ending in ate, ite, or ide (or any other suffix)? The way I understand it right now is that "ide" is from when there is two of an anion, "ite" is for three of an anion, and "ate" is for four of an anion. Is this right?
    (6 votes)
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    • mr pants purple style avatar for user Ryan W
      -ide is just the negative anion of that element, chloride, sulfide, nitride

      -ate depends on the central atom, it's generally the most common n eg phosphate is PO4 3-, sulfate is SO4 2- but nitrate is NO3 - and chlorate is ClO3 -

      -ite is 1 less oxygen than -ate, phosphite is PO3 3-, sulfite is SO3 2-, nitrite is NO2 -

      Hypo- -ite means 1 oxygen less than -ite eg ClO2- is chlorite and ClO- is hypochlorite

      Per- -ate means 1 oxygen more than -ate eg ClO3 - is chlorite and ClO4- is perchlorate
      (23 votes)
  • blobby green style avatar for user Matthew Nodar
    How do we know that Bromine will gain an electron just based on where it is on the periodic table?
    (5 votes)
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  • starky sapling style avatar for user Ahmed Mohamed Salah Elkhodary
    At , the ionic compound was turned into a formulae. but how we can turn a transition element like iron 2 ,iron 3 (Fe2,Fe3) or copper 1,copper 2 (Cu1,Cu2)
    (3 votes)
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    • mr pants purple style avatar for user Ryan W
      You would need to know what oxidation state the metal is in first. It should always be included in the name.

      Iron(II) bromide would be FeBr2 while iron(III) bromide would be FeBr3

      Copper(I) bromide would be CuBr while copper(II) bromide would be CuBr2

      Does that help?
      (6 votes)
  • blobby green style avatar for user Jean Helen
    Its still not clear how there are 2 bromides in the end. Can you please explain?
    (1 vote)
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    • mr pants purple style avatar for user Ryan W
      Calcium commonly forms a cation with a charge of +2
      Bromine commonly forms an anion with a charge of -1

      In the formula of an ionic compound we are showing the ratio between the ions.

      The overall charge of any ionic compound is 0 so for that to happen we need 2 bromide ions for every 1 calcium ion.
      So the formula is CaBr2
      (6 votes)
  • piceratops seedling style avatar for user css6
    How do you calculate a transition element with a nonmetal element to form a formula? Example: zinc phosphide
    (2 votes)
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  • hopper cool style avatar for user 12345
    At , Sal said that you must have 2 bromides for each calcium, so that it can be neutral, why can't it be CaBr+?
    (2 votes)
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    • purple pi teal style avatar for user Joana
      Most atoms and molecules in nature tend to a state of neutrality, which guarantees stability. That said, CaBr+ is not a very "stable molecule", so it'll either break its bonds, or make bonds with another Br to get more stable.
      (3 votes)
  • spunky sam orange style avatar for user Super learner 24
    how do make formulas with elements with multiple oxidation states like FE
    (2 votes)
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    • leaf red style avatar for user Richard
      Usually how it works is that iron (Fe) will be paired with an anion which has a constant negative charge. This will give the ionic compound an overall charge which both the iron cation and paired anion have to have their charges sum up to.

      So if you're given FeO and you have to name it we know a few things. Oxygen's charge is going to be -2 since that's elements in its group display, and the overall charge of the compound is 0 (neutral). So to figure how iron's charge it would be: (1)x + (1)-2 = 0, or x = 2, the 1's in parentheses being the number of iron and oxygen atoms. So iron has a +2 charge. Naming it would look like Iron(II) oxide, with the roman numeral 2 showing iron's oxidation state.

      If we go to the other way and start with a name like Iron(III) oxide then we can figure out the formula similarly. We know the charge on iron, +3, the charge on oxygen, -2, and the overall charge is 0. Since the overall charge is 0, the positive and negative charges from the irons and oxygens have to sum up to 0. The least common multiple between 2 and 3 is 6 so we have to multiply the +3 by 2 to get +6, and the -2 by 3 to -6. This means we need 2 iron atom, and 3 oxygen atoms, or Fe2O3.

      Hope that helps.
      (2 votes)

Video transcript

- [Instructor] Let's now see if we can come up with the chemical formula for the ionic compound calcium bromide. And like always, if you are inspired, pause the video and see if you can come up with it on your own. All right, so the convention is that we write the positive ion first and so that's a pretty good clue that calcium is going to be the positive ion. Now let's look at the periodic table to confirm that it's likely that calcium would ionize as a cation. Well, calcium is right over here in Group Two, and Group Two elements, also known as alkaline earth metals, they tend to ionize by losing two electrons and that's because they have two electrons in their outermost shell and they would like to lose them. And so when calcium ionizes, it is going to be, it is going to ionize as Ca2+. Now, let's look at the bromide part. The -ide tells us that this is going to be a negative ion or it's going to be an anion. And if you look at where bromine sits in our periodic table, right over here, we see it is a halide. We see that it likes to gain an electron and so it makes sense that it's going to be our anion. And so bromine would like to gain an electron to have eight electrons in its outermost shell. So, our bromide anion is going to look like this. It's going be to 1-. It's gonna wanna gain an electron, that's what the elements in this group like to do. Now, what is the formula going to be, and remember, the key here is for an ionic compound, especially one that has, well, we don't see any net charge here, for an ionic compound, these things are going to cancel each other out. The charge of the calcium cation is going to cancel out with the bromide anion. So how is that going to happen? Well, have you 2+ here, you only have 1- here, so you're gonna have to have two bromides for every of the calcium ions. So this is going to be, for every one of the calciums, you're going to have two bromides. So it's going to be like this, Br2, and there you have it, that is the chemical formula for calcium bromide. And how did we know that we have two bromides for every calcium? Well, because when calcium ionizes, it's going to be 2+, it's a Group Two element right over here. And bromine only gets a -1 or a 1- charge, so you're gonna need two of the bromides for every one of the calciums.