Reasons why oxygen is found up to 120km in the atmosphere but water only up to 90km:

1. Molecular mass:
- Oxygen has a molecular mass of 32 g/mol, while water has a molecular mass of 18 g/mol. Despite water having a lower molecular mass, it is found only up to 90km in the atmosphere compared to oxygen's presence up to 120km.

2. Stability:
- Oxygen molecules are more stable in the upper atmosphere due to their higher molecular weight, allowing them to be present at higher altitudes. Water molecules, on the other hand, are less stable and tend to dissociate into hydrogen and oxygen atoms at higher altitudes due to the lower temperature and pressure.

3. Bonding:
- The strong double bond between the oxygen atoms in an oxygen molecule contributes to its stability and ability to exist at higher altitudes. In contrast, water molecules are held together by weaker hydrogen bonds, making them more prone to dissociation at higher altitudes.

4. Temperature and pressure:
- The temperature and pressure conditions in the upper atmosphere play a crucial role in determining the presence of gases. Oxygen can withstand the lower temperatures and pressures found at higher altitudes, allowing it to be present up to 120km. Water, on the other hand, is more likely to vaporize and dissociate into its constituent atoms under these conditions, limiting its presence to 90km.

In conclusion, the combination of molecular stability, bonding strength, and environmental conditions determines the presence of oxygen up to 120km in the atmosphere, while water is limited to 90km despite its lower molecular mass.