You're right to be confused! Fluorine (F) is actually larger than oxygen (O). This is a bit counterintuitive, as fluorine is to the right of oxygen on the periodic table, and we usually expect elements to get smaller as we move rightwards.

Here's why fluorine is larger than oxygen:

* Effective Nuclear Charge: Fluorine has one more proton than oxygen, leading to a stronger attraction between the nucleus and electrons. This stronger pull might lead you to think fluorine would be *smaller*.

* Electron-Electron Repulsion: However, both fluorine and oxygen have a full outer shell of electrons (2s²2p⁵). This means that in fluorine, there's more electron-electron repulsion due to the extra electron. This repulsion pushes the electrons further apart, increasing the atomic radius.

In essence, while fluorine has a stronger pull from the nucleus, the increased electron-electron repulsion outweighs this effect, making fluorine larger than oxygen.

Let me know if you'd like to explore any other periodic trends!