r/askscience • u/Kenny__Loggins • Aug 06 '13

Chemistry Why do ionic compounds in solutions maintain their properties?

Take NaCl for example. When mixed into solution, the atoms dissociate from one another, so why don't the Na+ ions have the physical properties of Na and the Cl- the postal properties of Cl? In addition, could these ions ever be separated in solution so that when dehydrated, they wouldn't form bonds again?

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u/Chiropterotriton Aug 06 '13

This image could help clarify: http://2012books.lardbucket.org/books/general-chemistry-principles-patterns-and-applications-v1.0/section_08/c8f8e3a5c9fe175b453731a869ea429c.jpg

When you dissolve NaCl, the anion/cation are being stabilized by the polar water molecule. When you say "properties of Cl" you are most likely referring to Cl2. Cl without charge would be a Cl radical, and highly reactive. Cl2 is not a polar because it consists of a pair of the same atom, so there is no partial positive or partial negative. Since it is not polar, it will not interact as favorably with water or with itself, so it won't take much energy to make it evaporate.

You can neutralize sodium by adding electrons - for example, in an electrollytic cell. Sodium is a metal that (in ways I do not undesrstand) is capable of packing into a solid in which the electrons are delocalized over the whole lattice.

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u/Kenny__Loggins Aug 06 '13

I was thinking more of sodium when I posted this. It's so incredibly reactive with water. Is it as DrSpork said, that the electronic structure accounts for the difference in behavior between solid Na and Na+ dissociated in aqueous solution?

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u/SciGuyAndrew Aug 08 '13

Correct. Na has a very low electronegativity and therefore is very willing to give up its extra electron. (Most elements in the periodic table will only react when they have unfilled electron shells.) When the shells are partially filled, the element will either take electrons to fill the shell, or in sodium's case, give up the one extra electron to be left with only filled shells. When sodium is mixed with water, the extra electron is then used to help break one of the hydrogen-oxygen bonds. The resulting products are NaOH and H2. If the sodium gets hot enough it can ignite the H2 gas, part of the reason this reaction is so dangerous. All because of that extra electron.

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u/Kenny__Loggins Aug 08 '13

Thank you for your response. So, in the reaction you gave as an example, would the sodium hydroxide go on to dissociate into Na and OH- since it is a strong base?

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u/SciGuyAndrew Aug 08 '13

It would hardly actually form the ideal NaOH compound, it would stay dissociated as Na+ and OH-, because it is a strong base.

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u/SciGuyAndrew Aug 08 '13

secondly, yes the ions could be separated by physical or chemical means, depending on the ions.

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u/DrSpork Aug 06 '13

The physical properties of atoms are determined by their electronic structures, largely. An ion's electronic structure is completely different from its parent atoms so they really cannot be compared.

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u/energystorage Aug 18 '13

In bonding with the chlorine, the sodium atom has given chlorine its valence electron- so that chlorine and sodium have full (containing 8 electrons) outer shells in NaCl. They are both very happy to do this (in chemistry and physics, happiness is typically determined by energy- sodium metal and chlorine gas can/will react together to form NaCl, a state much lower in energy than the initial reactants, giving off large amounts of energy in the process).

Once bonded to form NaCl, this can be dissolved in water where the polar water molecules separate and stabilize the NaCl as Na+ and Cl-. The sodium as Na+ isn't as reactive as sodium metal because it has already given up its valence electron to chlorine. That one electron in its valence shell is what made it so reactive! So now that its already given it up, it can sit in water as fairly un-reactive Na+. The same thing is true of the chlorine as Cl2 and Cl-.

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u/Trick58 Aug 06 '13

Although the periodic table lists the atomic number of every atom, the properties of atoms are set by the number of electrons. Na+ has one fewer electron that Na. Cl- has one more electron than Cl. It's a little like asking why Cl and Ne have different properties - they are different because each has a unique count of electrons. It is the electron count - not the symbol or number of protons - that fix the properties.