Question

What is the name of the compound `Cr_2(CO_3)_3`?

Answer 1

Chromium(III) carbonate

Explanation:

In order to name an ionic compound, you must identify the cation, which is the positively charged ion, and the anion, which is the negatively charged ion.

Cations are always written first in the chemical formula of an ionic compound, followed by the anions.

`"Cr"_ color(red)(2) ("CO"_ 3)_color(blue)(3)`

It's important to remember that ionic formulas are written using the crisscross rule, which states that when a cation and an anion form an ionic compound, the charge on the cation becomes the subscript of the anion and the charge of the anion becomes the subscript of the cation in the chemical formula of the compound.

In your case, the compound contains chromium, `"Cr"`, as its cation. Since it has a subscript of `color(red)(2)`, it follows that the charge of the anion must be equal to `color(red)(2-)`.

Now, the anion is actually a polyatomic ion called the carbonate anion. Notice that it's written between parentheses, which tells you that it contains one atom of carbon and three atoms of oxygen, and that it has a subscript of `color(blue)(3)`.

This means that the charge on the cation will be equal to `color(blue)(3+)`.

This means that the ions that make up this compound are

`color(red)(2) xx "Cr"^(color(blue)(3+)" "` and `" " color(blue)(3) xx "CO"_3^(color(red)(2-)`

Because chromium is a transition metal, which means that it can have multiple oxidation states, you must use Roman numerals to express its oxidation state in the compound.

Since it carries a `color(blue)(3+)` charge, it follows that you must use the (III) Roman numeral.

The cation will thus be chromium(III) and the anion will be the carbonate anion, which means that the compound's name will be

chromium(III) carbonate `-> "Cr"_2("CO"_3)_2`