How many elements are in the compound C2H8O

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.

Ido not understand. 

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.