Here's why:

* Oxygen's Valency: Oxygen typically forms two bonds. In other words, it needs to share two electrons to achieve a stable configuration.

* Hydrogen's Valency: Hydrogen can only form one bond. It has only one electron to share.

Therefore, a combination of one oxygen and three hydrogen atoms would result in an unstable structure.

The correct formula for the compound you might be thinking of is H2O, which is water.