Explanation:
1 and 17 are highly reactive bc they are almost full or not full at all. they need there last shell to be full in order to be happy, or not reactive. 18 has the noble gases which are not reactive at all bc they have a full valence shell and dont need any other electrons.
Answer: From their food.
Explanation: They convert the food they eat into energy which helps them generate heat.
Answer:
The Earth is essentially a closed system; it obtains lots of energy from the Sun but the exchange of matter with the outside is almost zero. Isolated systems can exchange neither energy nor matter with an outside system.
The Precambrian was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth began by the early Archean and that fossilized organisms became more and more abundant throughout Precambrian time.
