Question

why are inner planets rocks and outer planets gas

Answer 1

Well this is because n the Solar System, we have two groups of planets - the outer gas giants and the inner rocky ones. The rocky ones are the first four, which are much smaller and denser than the outer four, containing a much higher percentage of metal elements, and less gases (Mercury and Mars do not in fact have any atmosphere to speak of). Even Venus' thick, scorching atmosphere only accounts for a small fraction of its overall composition. The other four are instead a lot larger and have very dense atmospheres with (relatively) small cores.
The reason for this state of affairs, put simply, is that when the Solar System was born, most of the gases near the Sun were soon either vapourised or blown away by the Solar wind so that only heavy metal & rock materials were left to form the inner planets. Further away though there was plenty of gas and this accumulated together to form behemoths the size of Saturn and Jupiter.

Answer 2

Hope this helps :))