I am a novice in chemistry and i have a question: I know that water is not a good conductive material (since there is a perfect bond in water I think) but I am thinking that if we add chlorine which has 7 valence electrons this means that there is a hole and this increases conductivity... am I right?

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    $\begingroup$ No, it doesn't work like that. There are ways to make water conductive, but they don't have anything to do with holes. $\endgroup$ – Ivan Neretin Jun 9 '17 at 8:10
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    $\begingroup$ but chlorine does increase conductivity (or adding impurities in general) $\endgroup$ – overclock351 Jun 9 '17 at 9:21
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    $\begingroup$ Some impurities (known as electrolytes) do indeed increase conductivity, some don't. $\endgroup$ – Ivan Neretin Jun 9 '17 at 9:28
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    $\begingroup$ Yes, it indeed does. Impurities help in dissociating the ions in water, but as Ivan Neretin said, 'they don't have anything to do with holes', here you are not doping the Cl (or, other impurities) in water. $\endgroup$ – chail10 Jun 9 '17 at 9:29
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    $\begingroup$ Oh i see (my background is in elettronics so i thought it was like semiconductors); so are the impurities that are conducting $\endgroup$ – overclock351 Jun 9 '17 at 9:31

In a nutshell, water is very much not like semiconductors. Its conductivity is due to altogether different mechanism, and follows different trends.

Typical semiconductor is a rigid network of atoms held together with bonds. Nearly any "wrong" atom would provide either extra electrons or holes (that is, missing electrons). Holes and electrons are the charge carriers here. Being very small, they move fast, so you only need a tiny amount of those impurity atoms (say, a few ppm) to create significant conductivity.

Water is quite different. You can't have free electrons in water, to begin with; they will instantly react with something and become ions. Ditto for holes. Ions are the only allowed charge carriers in water. Ions are as big as atoms, and each carries a thick coat of water molecules stuck to it, so effectively they are even bigger than atoms. An ion would move in an electric field, but with great difficulty, much like a man pushing his way through a busy crowd. (Compared to it, the electrons would be like little sparrows darting overhead.) Therefore you need a lot of ions to create some real conductivity.

Things that produce ions in water (dissociate, as we say) are called electrolytes, and there is plenty of them around. For example, table salt dissociates into $\ce{Na+}$ and $\ce{Cl-}$; sulphuric acid dissociates into $\ce{H+}$ and $\ce{SO4^2-}$. Elemental chlorine ($\ce{Cl2}$), being a neutral molecule, does not dissociate; also, you can't add much of it, for it is a gas, and its solubility is not that great. Over time, it would react with water to produce certain compounds which do dissociate, so you'll have some conductivity after all, but that's a weird and inefficient way of achieving it. Using any soluble salt is much more straightforward.

The choice of particular electrolyte depends on your ultimate purpose.

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  • $\begingroup$ Thanks a lot man and sorry for saying dumb things like the holes one :D $\endgroup$ – overclock351 Jun 9 '17 at 10:43

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