If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: AP®︎/College Chemistry>Unit 1

Lesson 7: Valence electrons and ionic compounds

# Worked example: Finding the formula of an ionic compound

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.

## Want to join the conversation?

• 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
• 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.
• The overall structure needs to be neutral in terms of charge. Thus Ca has a charge of +2 and Br has a charge of -1. Therefore, to get a neutral compound requires two Br and one Ca - ie, CaBr2.
• Why did he not put a parenthesis? Ca(Br)2?
• 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.
• 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?
• -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
• How do we know that Bromine will gain an electron just based on where it is on the periodic table?
• It wants to fill its outer shell, and if it can do that easily by gaining an electron, that's what it will do.
• 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)
• 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?
• How do you know which elements are going to be 2+ or 1- and so on?
• For the most part it depends on the group (column) in which the element is found in on the periodic table. It corresponds to how many valence electrons those elements in that group have and therefore how many they wish to donate to other atoms to form cations. Elements in group 1, the alkali metals, have one valence electrons so they tend to lose that one electron and take on a +1 charge. Elements in group 2, the alkaline earth metals, take on a +2 charge for a similar reason. Elements in group 13 take on a +3 charge. Elements in group 14 have the possibility of taking on a +4 or a -4 charge. And the groups further to the right tend to take on negative charges to become anions because they like to accept electrons now instead of donate them.

Collectively these elements in groups 1-2 & 13-18 are known as main block elements and have (usually) constant charges. The elements in the middle of the periodic table, the transition metals, are odd in that they have the potential of taking on several possible positive charges.

Hope that helps.
• I'm pretty sure Bromine is a Halogen but Sal says that it is a Halide. Is there any difference between the two terms?
• Halides are chemical compounds that contain halogens, so this example, CaBr2, is a Halide. Bromine itself is a Halogen.
• Its still not clear how there are 2 bromides in the end. Can you please explain?
(1 vote)
• 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
• There is something I don't understand. How can I figure out the formula of an ionic metal compound using the periodic table? How do I know that Iron is going to form a more than one positive ion? My textbook has this example question where it says, "What is the chemical name for the ionic compound CuCl2?" The answer is, of course, copper (II) chloride. But why can't it be Copper (IV) chloride? Or Copper (III) chloride? Do you use electron configurations? Or does this have to do with the charge of the atoms?
Sorry for a million questions, but I DO need help! Thank you!
• Ionic compound’s empirical formulae always have a neutral net charge, or a total charge of 0. The positive charges must equal and cancel all the negative charges. So the cumulative charges of the cations (a metal often) must equal the charges of the anions (a nonmetal often).

The elements in the s and p blocks (groups 1,2, & 13-18) are collectively referred to as main group and for the most part have constant charges. Group 1 elements have 1+ charges for example, while group 17 elements have 1- charges. This is a generalization and there are exceptions to this rule; lead for example is a main group element but can take on +2 and +4 charges.

Elements in the middle of the periodic table, the transition metals, can take on several different positive charges in different compounds. We differentiate between these possible charges by using roman numerals in their names.

So CuCl2, has copper (the cation and a transition metals with variable charges) and chlorine (the anion with a constant 1- charge). We know the whole formula must be neutral to the copper’s positive charge must cancel the negative charges of the two chlorines. We can determine the copper’s charge using algebra: x + 2(-1) = 0, where x is the copper’s charge. Doing some quick math, we find that x = 2, so copper has a 2+ charge. The name should therefore be copper(II) chloride.

Copper(III) chloride and copper(IV) chloride would have the formulas CuCl3 and CuCl4 respectively using the same rules.

Hope that helps.