Are the outer planets are much less dense than the inner planets why?

The outer planets.

The four inner planets (Mercury, Venus, Earth, Mars) are similar to Earth - relatively small, and with a rocky surface.

The four outer planets (Jupiter, Saturn, Uranos, Neptune) are gas giants - between 5 and 11 times the diameter of Earth, and made up mainly of gas. They don't have a solid surface you can step on - just gas that gets denser as you go deeper inside.