Question

Write the formula for the following compound: Chromium (III) oxide.

Answer 1

Hint: Each constituent element is identified by its chemical symbol by a chemical formula. It also indicates the proportionate number of atoms of each element.
For example, aluminium sulfate has the chemical formula of \[{\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}\] .

Complete step by step answer:
Chromium is a chemical element having the symbol ${\text{Cr}}$ and the atomic number 24. It is a member of transition elements and most commonly exists in $ + 3$ and $ + 6$ oxidation states.
One of the important chromium compounds in $ + 3$ oxidation state is the chromium (III) oxide. It is also one of the principal oxides of chromium.
 It is an ionic compound. So the chromium cation has positive charge of 3 and the oxide anion has negative charge of 2 as oxygen is a group 16 element and so requires two more electrons to complete its octet.
Now, an ionic compound must be electrically neutral. So the total positive charge of the cation must be balanced by the total negative charge of the anion.
Thus, two chromium (III) cations and three oxide anions are required in order to have a neutral compound.
$2 \times {\left[ {{\text{Cr}}} \right]^{3 + }} + 3 \times {\left[ {\text{O}} \right]^{2 - }} \to {\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

Hence, the chemical formula of chromium (III) oxide is ${\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ .

Note:
- The simplest of chemical formulae are the empirical formulae.
- The empirical formula of a chemical compound is just a simple expression of the relative number of each type of atom in the compound or the ratio of the elements present in the compound.
- It is the standard for ionic compounds.