When hydrogen (H) and chlorine (Cl) combine, they form the ionic compound hydrogen chloride, which is also known as hydrochloric acid (HCl) when dissolved in water.

Here's why:

* Hydrogen has one electron in its outer shell and tends to lose it to achieve a stable configuration.

* Chlorine has seven electrons in its outer shell and tends to gain one electron to achieve a stable configuration.

When they combine, hydrogen loses its electron to chlorine, forming a positive hydrogen ion (H+) and a negative chloride ion (Cl-). These ions are then attracted to each other due to their opposite charges, forming the ionic compound hydrogen chloride (HCl).