Sciencemadness Discussion Board - Newbie stuff- Ionic Compounds

Newbie stuff- Ionic Compounds

bradleyrose - 24-8-2019 at 16:48

ok, I'm totally new to this chemistry stuff and Darkfire has been kind enoguh to teach me chemistry over IM message conversations and email. I understood most of the stuff (it took a while

) I just have one question that I dont get and Darkfire hasn;t been able to explain it to me fully because I always have to get off the computer whenever my brother tells me to...

anyways, to the question:

can someone explain to me what chemical formula is made when Ca and S come together? and also, how is this made? like, I know stuff like when two atoms with the same charge come together (-2 and +2) there are no numbers in the formula. Darkfire, please reply to this topic and you can explain it here and maybe I'll finally get it.

and if I dont, maybe someone else can tell me?

Basically, the question is: how are the numbers arranged and how do you know what numbers to use when you are trying to make a new compound with atoms where one has a charge of - or + 2 and one has a charge of - or + 3. Obvioulsy both have to have different signs or else they can't be attractedd to eachother to make an ionic compund.

thank you for your help.

DraconicAcid - 24-8-2019 at 19:05

Atoms near the sides of the periodic table will gain or lose electrons in order to have the same number of electrons as a noble gas.

The alkali metals (Group IA) all have one more electron than a noble gas. They will all lose one electron to for a cation with a +1 charge.
The alkali earth metals (Group IIA) all have two more electrons than a noble gas, and will lose two electrons to form a cation with a charge of +2.
The halogens (Group VIIA) all have one fewer electron than a noble gas, so they will gain one electron to form an anion with a charge of -1.
Group VIA elements (O, S, etc.) are short two electrons, so they will gain two to become anions with a charge of -2.
The transition metals are variable- they can usually lose at least two electrons, possibly more, forming ions with charges from +2 to +(Group Number). So titanium (Group IVB) can be +2, +3, or +4. Transition metals on the right hand-side of the block tend to stick to +2 or +3. Main group metals tend to be their group number, or that group number -2. (So lead, in Group IVA, can be +2 or +4.)

The important metals that only form one kind of ion are in a nice diagonal line- silver, zinc, and aluminum. Silver is in Group IB, it's always +1. Zinc is in Group IIB, it's always +2. Aluminum is in group IIIA, it's always +3.

Once you know the charges, the total negative charge has to balance the total positive charge. If the cation and anion have the same charge (+1 and -1, or +3 and -3), then one of each will balance fine (e.g., NaCl, CaS, AlN). If they don't have the same charge, you need find multiples of one or the other (sometimes both) in order to make them balance.

For example, if you have calcium chloride. Calcium is +2, chloride is -1. Calcium has twice the charge of chloride, so you need twice as many anions. CaCl2

Arcaeca - 24-8-2019 at 21:41

Draconic is correct, but I feel that this part should be answered more explicitly:

Quote:

Basically, the question is: how are the numbers arranged and how do you know what numbers to use when you are trying to make a new compound

The first thing you'd want to do is determine what kindof compound will be created - in particular, whether the things you're trying to combine are ionically-bonded or covalently-bonded. The typical rule of thumb for what's probably an ionic compound - though not actually what makes the compound ionic - is that it's a metal bonded to a nonmetal. In your case, Ca is the metal (an alkaline earth metal, specifically) and S is the nonmetal.

(Again, this is just a rule of thumb; a notable exception is the ammonium cation, NH₄⁺, which isn't a metal and yet can still form ionic compounds with non-metal anions, like NH₄Cl, where Cl is also a nonmetal. So you have two nonmetals bonded together... and yet it's still ionic.)

Let's stick with ionically-bonded compounds for now, which is the premise of the thread. How do you determine the formula?

Essentially, everyone memorizes a big long list of ions (generally in high school chemistry class) and what charges they have. Everyone just sort of knows of the top of their head that potassium forms +1 ions, copper can be +1 or +2, oxygen is -2, hydroxide is -1, sulfate is -2, etc.

You can determine these from scratch by - for monoatomic atoms, at least - consulting the periodic table, as DraconicAcid describes; you can predict the charge that an ion of any given element will have based on the column that the element is in, in the same way that, sure, you could solve 4 + 6 by getting out 4 pennies and 6 pennies and combining them into one pile and counting up the new amount. I don't mean to scare you with this talk of memorization, but I'm trying to make the point that chemists don't consult that list of ions every time an ionic compound is mentioned. At some point they just instinctively know which charge should accompany each ion, just as at some point you just know that 4+6=10.

You can figure the charges out yourself using the periodic, in other words - just don't reinvent the wheel if you don't have to. If you forget which charge an ion will have, you can figure it out using what Draconic said, but don't be misled into thinking that every time you want to figure out the formula for an ionic compound that that has to involve a trip to the periodic table. It doesn't.

So figure out the charges of the ions you're working with, and then the next part is context-dependent, but we'll go with the simplest case. Usually people are talking about neutral compounds (i.e., compounds with no overall charge, or an overall charge of 0) - or at least, it's typically implied. So then it becomes a math problem: how many of each ion do you need for all the charges to cancel out and leave you with an overall charge of 0?

If the charges of your ions are the same, just one positive and one negative, e.g. Ca⁺² and S^-2, as in your example, then it's easy: + 2 - 2 = 0 already, so 1 of each creates a neutral compound. So your formula is CaS.

But what if the charges aren't the same? Take iron(III) oxide for example, made of Fe⁺³ and O^-2 ions. Let's see what happens if we try sticking just 1 of each together:

Fe⁺³O^-2: overall charge of +3 - 2 = +1 (oops, too much positive charge; add another oxide ion?)
--> Fe⁺³O^-2₂: overall charge of +3 + 2(-2) = 3 - 4 = -1 (too much negative charge; add more iron?)
--> Fe⁺³₂O^-2₂: overall charge of 2(+3) + 2(-2) = 6 - 4 = +2 (too much positive charge again, but hey, wouldn't one more oxide ion neutralize it?)
--> Fe⁺³₂O^-2₃ = 2(+3) + 3(-2) = 6 - 6 = 0 (perfectly balanced, as all things should be)

And so iron(III) oxide's formula is Fe₂O₃, which is determined by figuring out some whole number to multiply each ion by (2 for iron, 3 for oxide) so that the total charge comes out to 0. Which is what DraconicAcid meant by:

Quote:

If they don't have the same charge, you need find multiples of one or the other (sometimes both) in order to make them balance.