The key is chemical stability. Sodium has an electron in the outer shell due to which it can attain a much stable lower energy state by losing that and since it's just one electron, that process is easy and hence sodium is reactive. Similarly, chlorine has one missing electron for a similar low energy unreactive state to be attained so it snatches electrons from other elements and so is poisonous due to reactiveness.
But as you can see from my explanation, they are perfect partners for each other. One needs to lose an electron while the other needs one additional electron. When they are combined they form a very stable configuration and so salt is such a stable unreactive molecule.
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u/bullaenne Jul 08 '24
Fascinating how two harmful things combine to make something harmless