Here's the breakdown:

* Hydrogen (H) has one electron in its outer shell.

* Chlorine (Cl) has seven electrons in its outer shell.

To become stable, hydrogen needs one more electron, and chlorine needs one more electron. This is why they form a covalent bond, where they share an electron pair.

The resulting molecule, HCl, is a gas at room temperature and is highly acidic. It is commonly used in many industrial processes and is a component of stomach acid.