Which of the following formulas represents a molecule of ozone?

An element can exist in more than one structural form called allotropes, which differ in how their atoms are bonded together. For oxygen, two common allotropes are dioxygen, with two atoms bonded, and ozone, with three atoms bonded; the number of atoms of an element in a molecule is shown by the subscript following its symbol in the chemical formula.

The question asks for the formula that represents a molecule of ozone. Ozone forms when three oxygen atoms bond together, which gives the formula O₃.

• O₂ contains only two oxygen atoms, so it represents ordinary oxygen gas, not ozone.

• H₂O₂ contains two hydrogen atoms in addition to two oxygen atoms, making it hydrogen peroxide, a completely different compound rather than an allotrope of oxygen.

Since only the three-oxygen-atom formula matches the structure of ozone, O₃ is the correct representation.