Step 1: Concept.
Atomicity means the number of atoms bonded together in one molecule of an element.
Step 2: Apply it to ozone.
Ozone's formula is O3, so one molecule of ozone contains three oxygen atoms bonded together, unlike ordinary oxygen gas O2, which has only two.
Step 3: Final Answer.
The atomicity of oxygen in ozone is 3.
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Approach Solution -2
Comparing ozone side by side with ordinary oxygen gas makes the atomicity obvious.
1: A single free oxygen atom is written simply as O, with no subscript, which is not how ozone is written, so this cannot be the atomicity of ozone.
2: Ordinary oxygen gas, the O2 we breathe, is written with a subscript 2, showing two bonded atoms. Ozone is a distinctly different, less stable molecule with one more oxygen atom bonded in than O2 has.
3: Ozone's formula, O3, has a subscript 3 rather than a subscript 2, showing it carries one additional oxygen atom compared to ordinary O2. That third atom bonded into the molecule is what gives ozone its atomicity of three.
4: There is no known stable form of oxygen written with a subscript 4, so this option matches nothing that ozone or ordinary oxygen gas represents.
Reading off the subscript in the formula O3 directly gives the number of oxygen atoms bonded in one ozone molecule.