# Writing Formulas for Ionic Compounds

## Homework Statement

Of the following, which correctly shows an ion pair and the ionic compound the two ions form?

## Homework Equations

a. Fe3+, O3+ = Fe2O3
b. Cr3+, I- = CrI
c. Cu2+, O2- = Cu2O2
d. Sn4+, N3- = Sn4N3

## The Attempt at a Solution

a. Fe3+, O3+ = Fe2O3 Incorrect
b. Cr3+, I- = CrI Incorrect
c. Cu2+, O2- = Cu2O2 Correct
d. Sn4+, N3- = Sn4N3 Incorrect

I am just wondering if that is correct? Thanks everyone!

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symbolipoint
Homework Helper
Gold Member
The key to understand #2 is to evaluate how the charges of the ions and the number of 'species' relate in the compounds. You seem to have that. I'm just not too certain about your choice #C because my best guess is that you have 1 to 1 formula ratio for cuprous oxide in the symbolically stated compound (meaning, say CuO ). My guess could very well be wrong, but I have never seen a copper compound of oxide reported as Cu2O2

Ah ok, so then I guess A would be correct, correct?

symbolipoint
Homework Helper
Gold Member
Ah ok, so then I guess A would be correct, correct?
How do you justify this? What is O+3?

Borek
Mentor
Looks like.

Note - cuprous oxide (or copper(I) oxide) would be Cu2O, CuO is cupric (or copper (II)) oxide.

symbolipoint
Homework Helper
Gold Member
Looks like.

Note - cuprous oxide (or copper(I) oxide) would be Cu2O, CuO is cupric (or copper (II)) oxide.
I was slightly confused not thinking carefully enough.

All look incorrect to me.

a.) O3+ should be O2-; in addition, two positive ions do not form an ionic bond.
b.) Ionic compound should be CrI3 (one Cr3+ per three I-).
c.) Ionic compound should be CuO (one Cu2+ per one O2-).
d.) Ionic compound should be Sn3N4 (three Sn4+ per four N3-).

Given the trend of b-d giving typical ions but the incorrect ionic compound, I would guess that (a) is a typo. If (a) is changed to "Fe3+, O2- = Fe2O3," then that would be correct.