Why don't dissolved ionic bonds revert to the elemental properties?

[1MileCrash]

Sodium is a soft, reactive metal.
Chlorine is a poisionous, reactive gas.

Expose them to one another and they will form table salt, through ionic bonding. The compound is bonded throughout, forming a geometric matrix of bonds.

If submerged in water, it will be dissolved into free Na+ and Cl- ions.

Why then, does dissolved salt still have the properties of salt and not the individual, free ions?

 

[sjb-2812]

Sodium is a soft, reactive metal.
Chlorine is a poisionous, reactive gas.

Expose them to one another and they will form table salt, through ionic bonding. The compound is bonded throughout, forming a geometric matrix of bonds.

If submerged in water, it will be dissolved into free Na+ and Cl- ions.

Why then, does dissolved salt still have the properties of salt and not the individual, free ions?

Not 100% sure what you're asking here, but sodium metal is not Na+, just as chlorine gas is not Cl-

 

[1MileCrash]

So the fact that it is ionized changes its properties, and these altered properties just so happen to be identical to one of its various possible compounds?

 

[1MileCrash]

What I am asking, is why do unbonded Na and Cl ions still have physical properties of the bonded crystal?

If I walked into a room full of ionized chlorine gas, it would still be poisonous. Why isn't it poisonous to drink a glass full of free Cl ions?

 

[espen180]

It is poisonous. Eat enough table salt and I assure you you'll get really ill or die from it. Chemical properties are goverened by electrons. Therefore it makes perfect sense that a neutral atom and it's ions have different properties. In the case of Cl, it's chemistry is mostly based on it having 7 valence electrons, and sodium's chemistry is largely based on it having 1 valence electron. That is why elements in the same group have similar chemistry.

 

[sjb-2812]

What properties of NaCl do you perceive as being the same as those of Na, of Cl₂?

Typically, all compounds with Na⁺ ions will give a yellow flame test, for instance, and all those with Cl^- will precipitate with Ag⁺, for instance. But I sense that isn't really what you're asking..

 

[Borek]

What I am asking, is why do unbonded Na and Cl ions still have physical properties of the bonded crystal?

They don't.

 

[Ygggdrasil]

The chemical properties of an atomic or molecular species is largely determined by the configuration of electrons of that atomic or molecular species. Because atomic Na⁰ and ionic Na⁺ have different numbers of electrons, their properties are very different. In fact, the properties of Na⁺ are closer to that of Ne, which has the same electron configuration as Na⁺, than they are to Na⁰.

 

[Borek]

In fact, the properties of Na⁺ are closer to that of Ne, which has the same electron configuration as Na⁺, than they are to Na⁰.

That's interesting, care to elaborate?

 

[Ygggdrasil]

First a clarification: when I say that the properties of sodium ions (Na⁺) are similar to neon atoms (Ne), I am specifically referring to the chemical reactivity (the property most dependent on a chemical species' electron configuration).

Like Ne, Na⁺ is chemically inert and will not react with other compounds to form covalent bonds. It will not give up or take in any electrons under most circumstances. In contrast, elemental sodium (Na⁰) is extremely reactive and will very readily donate an electron to another chemical species.

The main differences between the properties of Ne and Na⁺ comes from the fact that Ne is electrically neutral while Na⁺ is positively charged. Ne atoms, lacking both significant reactivity with other chemical species and significant intermolecular forces to hold them to other atoms, are gaseous and have a very low boiling point (27 K). Na⁺ also lack significant chemical reactivity, but do have very strong intermolecular forces (electrostatic interactions) that bind them non-covalently to other molecules. These electrostatic forces keep Na⁺ bound to anions in crystals or to polar solvent molecules in solution. These are differences in the physical properties of the species, however, and not differences in their chemical properties.

 

[1MileCrash]

It is my understanding that while valence electrons determine reactivity and possible bonds, and chemical behavior, what the substance definitively is is based purely on its atomic number.

 

[Ygggdrasil]

Without getting too much into philosophy, what do you mean by the identity of a substance and "what a substance definitively is." In naming a substance, atomic number plays a crucial role. In this case, you can say that scientists somewhat artificially link the number of protons in an atom to its identity.

However, if you consider the chemical and physical properties of the element as an essential part of its identity, then the valence electrons are more important in determining an element's identity than the number of protons. Sodium is reactive because it has 11 electrons, not because it has 11 protons.

Here's a bit of an extreme example. In a paper published this week in the journal Science, researchers made helium act like hydrogen by replacing one of the electrons in helium with a negative muon. Because the negative muon is 207 times more massive than the electron, the muon "orbits" much closer to the nucleus and cancels out one of the positive charges so that the electron "feels" only the pull equivalent to one proton. Even though this species has two protons, it can substitute for hydrogen in chemical reactions.

Similarly, the researchers can create a hydrogen-like species with no protons by creating a species where an electron orbits a positive muon. This hydrogen-like species can also substitute for hydrogen in chemical reactions.

Here's the citation for this paper:

Fleming et al. (2011) Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H2. Science 331: 448. http://dx.doi.org/10.1126/science.1199421" (subscription required to read the paper).

A summary of the study is available here: http://www.newscientist.com/article/dn20049-atomic-disguise-makes-helium-look-like-hydrogen.html

 

[1MileCrash]

Then why can we easily tell the difference between elements within the same period? They vary in color, texture, etc

 

[Ygggdrasil]

Many physical properties of substances are dependent on the physical size of the atoms and molecules. For example, the polarizability of atoms increases with size. Since atoms that are more polarizable have stronger intermolecular dispersion forces, this concept helps explain why the boiling point of the Nobel gases increases as you go down the column from the smallest Nobel gas (helium boils at 4 K) to the largest Nobel gas (radon boils at 211 K).

In addition, there are also differences in chemical reactivity between elements in the same column even though they contain the same number of valence electrons. Here, it is important to note that the number of valence electrons is not the only factor influencing the chemical properties of an atom; the energies of the valence electrons matter as well. As atoms increase in size, the valence electrons are farther away from the nucleus and become less tightly held. This means that the atoms require less energy for these electrons to depart, and similarly, the atoms gain less energy when new valence electrons arrive. This principle helps explain why fluorine is much more reactive than the other halogens. Fluorine's valence shell is much closer to the nucleus than any of the other halogens and it therefore releases the most energy when it gains an electron.

The key importance of the distance of the valence shell from the nucleus in determining chemical properties is illustrated by the case of zirconium and hafnium. Because of an effect known as the Lanthanide contraction, zirconium and the element directly below it on the peroidic table, hafnium, have approximately the same atomic radius (159 pm for Zr and 156 pm for Hf). Even though hafnium has 32 more electrons than zirconium, hafnium and zirconium have extraordinarily similar properties because the two elements share the same configuration of valence electrons and have a similar atomic radius. In fact, the two elements are so similar that they are among two of the most difficult elements to purify from each other. Indeed, even though most zirconium deposits contain hafnium impurities, hafnium was not recognized as a separate element until 134 years after the discovery of zirconium, largely due to the fact that no one could separate the two elements.

 

[Opus_723]

Why does a solution of Na+ and Cl- still taste like salt? I'm pretty sure if you were to taste Na+ and Cl- separately, they would not taste like salt. Why does being in solution together make them act as if they were NaCl, though they are still independent of each other?

 

[Borek]

You mean - why does solution of NaCl taste like solid NaCl?

Are you sure you know how solid NaCl tastes? Are you sure it doesn't dissolve in saliva on your tongue so that the taste you know is that of dissolved NaCl?

 

[Opus_723]

I thought of that after I posted, but wasn't sure. Thanks for pointing that out. In that case, maybe the answer to the OP's question is that NaCl (aq) and NaCl (s) simply don't share the same properties?

 

[Borek]

https://www.physicsforums.com/showpost.php?p=3113119&postcount=7

 

[Opus_723]

Oh, sorry, I missed that post somehow. Thanks for answering anyway. That clears things up for me.

 

[1MileCrash]

Borek, your last post clears up a lot for me, thanks. But id still wonder about a solution of only free na+, only free cl-, and a solution containing both (ie dissolved salt) would taste compared to on another.

 

[Ygggdrasil]

The taste buds responsible for detecting salty flavors respond to Na⁺ and not Cl^-. The protein responsible for detecting Na⁺ is not completely specific for Na⁺ as it will also respond to other alkali metal ions (e.g. Li⁺, K⁺).

 

[cgk]

Why does a solution of Na+ and Cl- still taste like salt? I'm pretty sure if you were to taste Na+ and Cl- separately, they would not taste like salt. Why does being in solution together make them act as if they were NaCl, though they are still independent of each other?

You cannot taste solid NaCl. You only taste the ions, because they are disolved (partly) in the process of tasting them. Note also that different alkali-metal/halogen salts taste almost identically.


btw: I really liked Ygggdrasil's Hf/Zr example. I was not aware of that before.

 

[symbolipoint]

Borek, your last post clears up a lot for me, thanks. But id still wonder about a solution of only free na+, only free cl-, and a solution containing both (ie dissolved salt) would taste compared to on another.

No. Need a counter-ion. No aqueous solution containing only dissolved cation. No aqueous solution containing only dissolved anion.