Diagram of a silicon atom
Silicon is heavier than oxygen, because oxygen has only eight protons in its center and silicon has fourteen. When the star runs out of oxygen, it will begin to turn the silicon into heavier elements like iron.
When the red giant star eventually becomes a supernova and explodes, the remaining silicon inside the star shoots out into space and forms part of a new nebula. Silicon is unstable, and does not usually exist on its own outside of stars - silicon generally fastens itself to some other kind of atom, or to other silicon atoms, to make a molecule.
Sometimes the dust in one of these nebulae pulls together to form a planet, and some of the silicon atoms become part of that new planet. The crust of the planet Earth, for example, is about one quarter silicon atoms. Silica is what rocks like quartz and feldspar and granite are made of. Most beach sand is also made of silica. All plants and animals need small amounts of silica in order to live, though silica is especially important to plants.
Because silica is pretty common, people also use silica to make glass, and in making computers. It's also what Silly Putty is made of!
To find out more about atoms, check out these books from Amazon.com or from your library: