Chemical elements will react to reach the lowest energy (most stable) state. Some elements are stable by themselves, but many of them are more stable in compounds. Hydrogen and oxygen atoms are not stable alone, so they form compounds. Hydrogen atoms will react with other hydrogen atoms to form a diatomic hydrogen molecule, H₂. Oxygen will behave similarly and form O₂. When H₂ and O₂ are mixed, they can form an even more stable compound, water (H₂O), by reacting together:

Hydrogen and oxygen can form compounds other than diatomic gasses and water also. The compound H₂O₂, called hydrogen peroxide, is less stable than water, but can be synthesized nonetheless. Over time, it will decompose with the following reaction: 2H₂O₂ → 2H₂ O₂ + O₂

Additional compounds with hydrogen and oxygen exist, but are even less stable and rarer than hydrogen peroxide.

Hydrogen and oxygen can also form molecules that are ions (electrically charged molecules). For example, the ions in acids and bases are:

But why are hydrogen and oxygen more stable in compounds than by themselves? This has to do with their electron configurations. Atoms want full valence electron configurations, and one way to achieve this is by bonding with other atoms. Hydrogen atoms only have one electron, but they need two electrons to have a full 1s orbital. By forming H₂, the atoms can share electrons to achieve full 1s orbitals. Oxygen has more electrons than hydrogen, but oxygen atoms are two electrons short of having a filled 2p orbital. Bonding to form O₂ allows them to share electrons and fill the 2p orbitals. To form water, an oxygen atom bonds with two hydrogens. The oxygen shares an electron with each hydrogen, and both hydrogens share an electron with the oxygen, giving all three elements full electron valence orbitals.

To better understand molecular bonding you will need to study valence bond theory, molecular orbital theory, and even quantum chemistry. To understand reactions and how elements form compounds, you may also want to study chemical kinetics and thermodynamics. Chemical compounds are fascinating and complex and a study of many different areas of chemistry is required to truly understand the nuances of how and why they form.