Ilooked and looked in my packet all about physical and chemical changes. i can't find the answer to this question. me?

One could argue that photosynthesis is the most important chemical reaction of all, since it makes energy from solar energy. Plants are are at the bottom of most food chains, and you get O2 as a side product, which a lot of organisms rely on for respiration and for other chemical reactions. 

I hope this helps. I'm not sure if the "passage" has any additional information since you didn't attach it.

They are all full and don't need any more electrons in there outer shell.

The correct answer is "atomic weight".