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The outermost electrons in an atom. They determine reactivity and how they can bond. It's kind of like the goal of the atom to get eight valence electrons: once they have eight, they are noble gases and will no longer react or bond. Atoms with one valence electron, alkali metals, such as sodium, and atoms with seven valence electrons, halogens, such as chlorine, are the most reactive and both very dangerous. When alkali metals and halogens react with each other, the after the violent explosions you get a salt, such as sodium chloride, which is safe to eat despite being made from a metal that explodes on contact with everything except special types of oil and a poisonous gas. The two exceptions to this are hydrogen and helium: hydrogen only has one electron, and it is very reactive, and helium is a noble gas even though it only has two electrons. You can tell how many valence electrons an atom has by counting down the columns from left to right.