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According to quite a few books I have read, helium belongs to the p-block with reasoning along the lines of "it's an inert gas, so let's clump it together with other noble gases which are in the p-block."

However, on Wikipedia, helium is classified as an s-block element which, to me, makes sense because the last (or so-called "differentiating") electron that is added to helium will occupy an s sub-shell.

But my teachers are adamant that helium is a p-block element, and the fact that our textbook covers helium under the chapter "The p-block Elements" just gives them more reinforcement of that idea.

So: is helium an s- or a p-block element? Could someone provide concrete reasons for and against the block selection?

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    $\begingroup$ It is a noble gas with no p electrons. Another example of a dumb arbitrary classification for little gain in understanding. $\endgroup$ – Jon Custer Aug 27 '16 at 13:50
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    $\begingroup$ What Jon says. You've got reason behind you, your teachers only have their faith in a stupid text book. Stand up to them! $\endgroup$ – Karl Aug 27 '16 at 14:54
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    $\begingroup$ chemistry.stackexchange.com/questions/13958/… $\endgroup$ – Mithoron Aug 27 '16 at 20:34
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    $\begingroup$ Technically it's an s-block, but follow what your teacher wants if you wanna pass. $\endgroup$ – user34234 Aug 28 '16 at 16:02
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Blocks in the periodic table should not be mixed up with groups (like noble gases). The reason why Helium is considered a noble gas is because its outermost (and at the same time only) shell is fully occupied by its 2 electrons. When you look at the electron configurations in the PTE you can see that the first element which happens to have an occupied p-orbital is boron. Due to this fact boron also has to be the first p-block element, since the block definition is as follows:

An element belongs to the block x, if the electron, which is added in comparison to the previous element (by atomic number), is in an orbital of type x in the ground-state electron configuration of that element.

Or the fast explanation: Why should helium be considered a p-block element, if it doesn't even have any electron in a p-orbital or any shell including a p-orbital?

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    $\begingroup$ Jup. One electron in the 1s shell also doesn't make hydrogen an alkali metal. $\endgroup$ – Karl Aug 27 '16 at 14:57
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Helium is an s-block element with properties like those of the p-block noble gases.

In a periodic table more concerned with chemical behaviour you're likely to see He in the p-block.

In a table more concerned with showing the filling of electron shells you're likely to see He in the s-block.

The optimal solution is to show He in the s-block but to colour it as a noble gas or a p-block element.

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