How did Asteroid belt created?

Sorry, everyone. I became a middle school student. I was so busy and I can't posted about cosmos. Today, I have a enough time. I will post about Asteroid belt's creation process. 

These are planet's semi-major axis.

Mercury:0.4AU

Venus:0.7AU

Earth:1.0AU

Mars:1.6AU

Jupiter:5.2AU

Saturn:10AU

Uranus:19.6AU

Let's multiply 10
Mercury:4
Venus:7
Earth:10
Mars:16
Jupiter:52
Saturn:100
Uranus:196                                                                    

Let's subtract 4
Mercury:0
Venus:3
Earth:6
Mars:12
Jupiter:48
Saturn:96
Uranus:192

You can find a rule. But there is a problem. There is no planet that have 24.
Celestial body that have 24 is not a planet, but asteroid belt. You can have a doubt. Why there is no planet at 2.8AU? Why most of asteroids are in the belt? Astronomers deduce that there was a planet. But the planet is split by Jupiter's gravity. So many pieces of the planet became Asteroid belt. But this planet was very small. It is smaller and ligter than moon