Ozone is an allotrope of ______.

The correct answer is oxygen.

Ozone is an allotrope of oxygen. To understand why, let's break this down step by step.

What is an Allotrope?
An allotrope is one of two or more distinct structural forms of the same element, existing in the same physical state. Allotropes are made up of atoms of the same element, but those atoms are arranged or bonded differently, giving each allotrope unique physical and chemical properties.

Oxygen and Its Allotropes:
The element oxygen (atomic symbol: O, atomic number: 8) has two well-known naturally occurring allotropes:

1. Dioxygen (O₂) — This is the common form of oxygen we breathe. It consists of two oxygen atoms bonded together. It makes up approximately 21% of Earth's atmosphere.

2. Ozone (O₃) — This is the second allotrope of oxygen. It consists of three oxygen atoms bonded together in a bent molecular shape. Ozone is found naturally in the stratosphere, where it forms the "ozone layer" that absorbs harmful ultraviolet (UV) radiation from the sun.

Key Structural Difference:
Both O₂ and O₃ are made exclusively of oxygen atoms — no other element is involved. The only difference is in the number and arrangement of atoms:

- O₂: Linear molecule, 2 oxygen atoms, bond angle = 180°
- O₃: Bent/angular molecule, 3 oxygen atoms, bond angle ≈ 117°

Why the Other Options are Wrong:
- Carbon dioxide is a compound (CO₂), not an element — ozone cannot be its allotrope.
- Hydrogen is a different element entirely — ozone contains no hydrogen atoms.
- Nitrogen is also a different element — ozone contains no nitrogen atoms.

Conclusion:
Since ozone (O₃) is composed entirely of oxygen atoms, just like regular oxygen (O₂), but with a different molecular structure, it is definitively classified as an allotrope of oxygen.