### #25 on Chapter 3

Posted:

**Thu Nov 02, 2017 2:39 pm**Could someone please explain how to do this problem? I'm confused on how to determine the compound formula when it comes to ions with 2+ and 3- charge, and vice versa.

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=29&t=22971

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Posted: **Thu Nov 02, 2017 2:39 pm**

Could someone please explain how to do this problem? I'm confused on how to determine the compound formula when it comes to ions with 2+ and 3- charge, and vice versa.

Posted: **Thu Nov 02, 2017 2:55 pm**

If you have ions of different charges, you can just switch their charges to make them coefficient. For example, in part C, where you have to write the chemical formula for indium(III) sulfide, Indium has a 3+ charge and Sulfur has a 2- charge. If you give the one element a coefficient that is the number associated with the other's charge, you will get the correct answer. In2S3 would be your answer. The charge of this overall molecule is 0 (2*+3 + 3*-2 = 0). This trick only works if you have two elements in your compound.

Posted: **Thu Nov 02, 2017 7:29 pm**

You can use the oxidation numbers (charges) on the elements to find the subscript for the element it is paired with. For part a), Magnesium has an oxidation number of +2 and Arsenic has an oxidation number of -3, so you use the "criss cross" rule. The way this works is you have Mg^+2 and As^-3, and you can "criss-cross" the oxidation numbers, having the 2 become a subscript for As and the 3 become a subscript for Mg. You can follow this for all of the parts of this question. Hope this helps!

Posted: **Sun Nov 05, 2017 1:52 pm**

Is there a reason I should know for why this rule works?

Posted: **Sun Nov 05, 2017 4:24 pm**

Jessica Lutz 1C wrote:Is there a reason I should know for why this rule works?

The reason the criss-cross method outlined above works is because it balances charges.

For example in magnesium Nitride, magnesium has a charge of 2+ and nitride 3-. Therefore, three magnesium ions are needed to balance out two nitrogen ions.

Mathematically it can be outlined like the following:

(3x 2+) = 6+ of magnesium

(2+ 3-) = 6- of nitrogen

They essentially cancel each other out to form a stable compound.

Posted: **Tue Nov 07, 2017 6:45 pm**

how do you figure out the charges on the atoms?

Posted: **Tue Nov 07, 2017 9:50 pm**

Correct me if I'm wrong but I'm inferring you determine the charge based on how many electrons it needs to take out/it needs to add in order to attain a noble gas electron configuration? So for example for Mg, since it is in the 3rd period s-block, it is easier for it to take out 2 electrons (and thus have a positive charge) in order to attain a noble gas configuration of [Ne]. Arsenic, on the other hand, is at n=4 p-block, which means it only needs to add 3 more electrons (thus a negative charge) to attain a noble gas configuration of [Ar].

I'm not sure about this reasoning so if someone out there knows the logic behind the charge, please let us know!

I'm not sure about this reasoning so if someone out there knows the logic behind the charge, please let us know!