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I'm studying year 11 and am learning about valence electrons and how these electrons drive intermolecular bonds. What I have not been able to find an explanation to is why all the elements on the periodic table have the number of valence electrons that they do. To me, it seems intuitive that if the halides are so eager to gain an electron, why would their natural state not be to have the 8th electron in their shell?

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    $\begingroup$ Well, but their natural state is to have the 8th electron (mostly). You might be talking about the elemental state, but elemental halogens are not commonly found in nature. $\endgroup$ Aug 7 '20 at 5:06
  • $\begingroup$ I see, I was confusing the elemental and natural states. If their natural state is to mostly have the 8 valence electrons, why do we mention the elemental state at all? Why is this what is displayed on the periodic table? If the elements natural state is to be stable as such, how do chemical equations work? Should we not assume that the atoms of these elements in the world would then be stable by default and therefor less reactive? $\endgroup$
    – Corey King
    Aug 8 '20 at 1:08
  • $\begingroup$ We mention elemental state because it does exist, albeit sometimes only in the lab. This is what characterizes an element by itself, when not in a compound with other elements; that's why it is displayed on the periodic table for that element. The chemical equations work because they are true. The chemical reactions occur because... well, look around you, and you'll see a plenty of examples. There are no such things as "more reactive" and "less reactive". Reactive towards what, that is the question? $\endgroup$ Aug 8 '20 at 1:20
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Atoms have protons (and neutrons) in the nucleus that define the element. For example, Fluorine always nine protons. The number of electrons must balance the number of protons in the atomic state. If fluorine had ten electrons, i.e. a full inner and outer shell, it would be a negative ion with net charge -1, so the electrostatic charge would repel the last electron.

It is only with the help of other atoms nearby that charges can be (almost) balanced. In $\ce{LiF}$, the fairly positively charged $\ce{Li+}$ ion attracts the comparatively negatively charged $\ce{F-}$ ion, so there is no net unbalance in a crystal of $\ce{LiF}$, though in each pair, the valence electron spends most time in the outer shell of fluorine.

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