Why are the outer planets mostly gas while the inner planets are rocky?

1. The young solar system was much hotter near the Sun than far from it. 2. Close in, only materials with very high melting points stayed solid, such as metals. 3. Those materials are scarce, so the inner planets grew small and rocky. 4. Far out it was cold enough for water and methane to freeze into ices. 5. The large icy and rocky cores there could then hold on to thick layers of gas. 6. That is why the outer planets are large gas giants. 7. The outer region was never empty, and gravity acts everywhere. _Source: Siyavula Natural Sciences Grade 8 (CC BY 4.0), Comparing terrestrial planets and gas giants_