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I am having trouble understanding how valence electrons are counted in Helium.

My references are:

  1. Here where it appears that Helium has 0 valence electrons because the shells are finished
  2. Here where it appears that Helium has two valence electrons (which is what I thought)

How many valence electrons does helium have?

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    $\begingroup$ You're comparing Wikipedia on the same grounds as a comment in a funny picture? I'm sure you can answer your own question... $\endgroup$ – Nicolau Saker Neto Jun 1 '13 at 23:04
  • $\begingroup$ @NicolauSakerNeto Appears to be from a test question. $\endgroup$ – Mikhail Jun 2 '13 at 6:54
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Helium has two electrons in total, and according to the aufbau principle, it adopts the electronic configuration $\mathrm{1s^2}$. This means it has two electrons in s orbitals with a principal quantum number of 1.

The last (and only) level of helium's electronic configuration is $\mathrm{1s^2}$, and therefore $\ce{He}$ has 2 valence electrons.

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    $\begingroup$ I would like to add that the "valency" is 0 although it has two "valence" electrons. $\endgroup$ – Eric Brown Jun 1 '13 at 23:25
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Valence electrons

Valence electrons are electrons in the outermost shell of an atom

Valency

Valency is the number of electrons involved in the formation of a chemical bond

Coming to your question

The number of valence electrons of helium is 2 because of its configuration which is $1s^2$

Hovewer, the valency is 0 because it is already stable and none of the outermost electrons are involved in the formation of chemical bonds.

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The outermost shell of an atom can accomodate a max of 8 electrons. It was observed that the atoms of elements,having a completely filled outermost shell show little chemical activity. In other words their combining capacity is 0. Of these inert elements, the helium atom has 2 electrons in its outermost shell and all other elements have atoms with 8 electrons in the outermost shell. Their combining capacity of the atoms of other elements,that is their tendency to react & form molecules with same or different elements,was thus explained as an attempt to attain a fully-filled outermost shell.

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