Answer:
The number of electrons in the outermost shell of an atom determines its reactivity. Noble gases have low reactivity because they have full electron shells. Halogens are highly reactive because they readily gain an electron to fill their outermost shell.
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A replacement reaction which is where the positively charged portion of one of the reactants takes on the negatively charge portion of the other reactant without giving any portion of itself to the other to form products.
They are about 4.5 billion years old. Hope this helps.
Weather is the temperature or climate over a short period of time.
