So every atom has electrons that encompass the nucleus like a shell. And this shell has layers, like an onion. The first layer holds two electrons, the next two layers hold eight, and every layer after that holds eighteen each.
Atoms want to have a clean outermost layer to be complete, either by gaining some electrons, or losing some electrons, or sharing them. The closer they are on either side to gaining that complete layer, the more reactive they tend to be. That’s why Group 1 metals and Hydrogen are so highly reactive, they have only one electron floating around in their outermost layer and REALLY want to ditch that electron. Meanwhile Group 17 elements like Chlorine or Fluorine are also highly reactive, because they only need to gain one more electron for a complete layer. And Group 18, the noble gases, have a complete outer layer so they almost never react with any other element under normal circumstances.
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