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Halogens have 7 valence electrons. Halogens are the group (vertical column) of elements on the Periodic Table that lie 2nd from the from right side. They sit next to the noble gases, which have 8 valence electrons (except helium, which only has an S shell, and therefore 2 total electrons.) The rule of thumb (which doesn't apply to the transition metals in the middle) is such that every time you move left of the noble gases, you lose 1 valence electron. Halogens, being 1 group to the left, have 7. If you move left again, you reach Oxygen (group 16), which only has 6. Move left again, you have Nitrogen (group 15), which have 5. Left again, you have Carbon (group 14) which has 4, Which is why Carbon almost always forms 4 bonds. You can't generalize about the transition metals, in the middle, but if you go all the way to the Left side of the table, as in group 1, (Na or sodium, K or potassium) you have 1 valence electron. The second column (calcium, Ca, Magnesium, Mg) has 2 valence electrons.

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14y ago

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