Number of atoms in O₂.

**The reasoning** When you see O₂, you're looking at a **molecular formula** that tells you the exact composition of one oxygen molecule. The small subscript number (₂) directly tells you how many atoms of that element are bonded together in that molecule. O₂ = 1 oxygen molecule containing **2 oxygen atoms** bonded together Think of it like this: if you write H₂O (water), that's 2 hydrogen atoms + 1 oxygen atom = 3 atoms total. For O₂, it's simply 2 oxygen atoms bonded together. **Why the wrong options tempt you** - **Option A (1)**: You might think "it's just one oxygen" if you confuse the molecule with a single atom (which would be written as just O, not O₂) - **Option C (3) or D (4)**: You might accidentally add or multiply numbers if you're rushing, or confuse this with counting something else like moles or valence electrons **Quick takeaway** The subscript number in a chemical formula **always** tells you exactly how many atoms of that element are present — O₂ means 2 atoms, no tricks!