The outer planets are mostly formed of lighter elements because these three condense are a temperature

The outer planets are mostly formed of lighter elements because these three condense are a lower temperature.

Explanation:

The planets based on the location of the sun is divided into two, that is outer planets and inner planets.

The Outer planet which is far from the sun consists of larger quantity of lighter substances such helium, hydrogen, methane which would condense at the lower temperature. And remains gaseous inside the frost line that would exit far away for the sun so the temperature is low and the lighter materials are easier to condense in lower temperature to form terrestrial  rocks.