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.
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
The exosphere's job is to hold satellites
what is the thickness of the exosphere
Exosphere
This process is known as nuclear fusion. It occurs in stars like the sun when lighter elements such as hydrogen are combined to form heavier elements like helium, releasing a large amount of energy in the process.
Nuclear fusion, of lighter elements onto heavier elements.