Do you mean elements or molecules? Gravity is acting more weakly on them (inverse square law).
The exosphere is the upper layer of the atmosphere. The exosphere has a very low density, due to the lighter gases that are present in the exosphere. Hydrogen, with some helium, carbon dioxide, and atomic oxygen is found near the exobase. he exosphere is the last layer before outer space.
The Exosphere
You think to prometium and technetium; but today these chemical elements are considered as being natural.
Lighter elements are composed of fewer protons and neutrons compared to heavier elements. They tend to have fewer total nucleons and lower atomic numbers. Lighter elements are typically found at the beginning of the periodic table, while heavier elements are found towards the end.
The prefix "exo-" means external or outside, meaning that the exosphere is outside what we normally think of as the atmosphere, and is an extension of Earth's molecules into space.
The exosphere, which is the outermost layer of Earth's atmosphere, contains a very low density of particles, including hydrogen. Hydrogen makes up about 0.5% to 1% of the exosphere's composition. However, its concentration can vary based on solar activity and other factors. Overall, the exosphere primarily consists of lighter gases, with hydrogen being one of the most abundant, albeit still in minimal amounts compared to other layers of the atmosphere.
Lighter elements tend to have neutrons equal to protonms or only a slight excess. Heavier elements have an excess of neutrons over protons
The lighter elements turn to gas at a lower temperature and because of their lack of atmosphere are released from the planets out into space
what is the thickness of the exosphere
Exosphere
The exosphere's job is to hold satellites
I think you mean the: Exosphere. The Exosphere is the uppermost layer of the atmosphere and contains mainly Hydrogen with a bit of Helium as well as Carbon Dioxide with some Atomic Oxygen near the Exobase.