Light elements like hydrogen and helium form a large percentage of the outer planets and sun is made up primarily of hydrogen. why are these elements nearly absent from the inner planets?

Instead of Helium and Hydrogen there are heavier elements in inner planets like Nitrogen and oxygen.

Explanation:

As the inner planets like Mercury, Venus and Mars and earth have a solid surface which is separated by the Sharpe atmosphere, and their gravity is too low to hold these light gases. Uranus is primarily composed of gases and ice of which 83% is hydrogen and 15% helium and rest are trace gases. As in the early composition of the atmosphere, the inner planets like the earth had a solid atmosphere composed of solid silicates elements like magnesium, iron, calcium, and potassium.  As the earth has a highly unstable atmosphere only carbon, nitrogen and oxygen combined in lightning occur. And planet mercury has no atmosphere thus gravity is too low to hold any gas.