[Ian000Bell]

I am retired and reading through a chemistry book (Principles of General Chemistry by Martin Silberberg) for self-interest.  I am struggling with a statement from this book which is, "there are no molecules in an ionic compound".

This book defines a molecule as, "an independent structure of two or more atoms bound together". 

An ionic compound is a substance composed of charged particles.  The example cited is sodium chloride - the positively charge sodium ion is bound to the negatively charged chloride ion.  The sodium and chloride ions are atoms that have lost/gained an electron.  Nevertheless, they are still atoms.  So two atoms/ions are bound together (because of opposite charges) and therefore, by definition, they constitute a molecule.  Accordingly, ionic compounds are composed of molecules, contradicting the statement in this book that "there are no molecules in an ionic compound".

It seems I have missed something important.   Can someone please explain where my understanding is wrong?

Thanks,

Ian

[Orcio_87]

https://en.wikipedia.org/wiki/Molecule

Read the definition: "Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules.".


https://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Sodium-chloride-3D-ionic.png/632px-Sodium-chloride-3D-ionic.png

There are no reason to isolate single Na-Cl molecule, since 5 other chlorine atoms bonded to Na are the same.

[Borek]

it is possible to isolate and observe NaCl molecules in highly exotic conditions (high vacuum, right temperature). But normally sodium chloride doesn't exist as molecules, but as a 1:1 mixture of separate ions. Even in crystals they are just packed side by side (see the structure posted in the previous post), but they are not bonded* together.

*this is actually a bit more complicated, but getting into details will only confuse you further

[jeffmoonchop]

What about salts. There are many molecular coformers which form crystalline products by the charge interactions between them. For example my paper:

First Comparative Study of the Three Polymorphs of Bis(isonicotinamide) Citric Acid Cocrystals and the Concomitant Salt 4-Carbamoylpyridinium Citrate Isonicotinamide

which shows a salt structure of deprotonated citric acid ionically bonded to proton acceptor isonicotinamide.

Are there no molecules in this salt? Maybe its the wording ionic compound that is confusing. Because I wouldn't call my salt an ionic compound.

[Orcio_87]

a salt structure of deprotonated citric acid ionically bonded to proton acceptor isonicotinamide.

Are there no molecules in this salt?

There are two sort of molecules - citric acid anion and protonated isonicotinamide.

[Borek]

Guys, please don't get into confusing details of systems that require completely different level of understanding chemistry. You are not helping the OP at all.

[Ian000Bell]

@Orcio_Dojek1

The Wikipedia reference you cited, in turn, cites Encyclopedia Britannica (EB).   EB also seems to use circular wording which only confuses the issue further.   For example, EB defines a molecule as "a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance."  The key point is the substance cannot be divided further if the composition and properties of the substance is to be retained.   In the example of table salt (sodium chloride) if the bond between sodium and chlorine ions is broken then we no longer have the properties common to table salt.    So here again, this example seems to be consistent with my understanding that bonded sodium and chlorine ions form a molecule and therefore again, ionic compounds consist of molecules.

EB then seems to contradict itself by stating, without any explanation or references,  "Not all substances are made up of distinct molecular units...no distinct aggregate identifiable as a molecule of sodium chloride exists."


@Borek
Thank you!   Your comment ("Guys, please don't get into confusing details...") is bang on the money!


How can such a seemingly simple concept be so difficult to explain?

[Ian000Bell]

@Borek

You wrote, "Even in crystals...but they are not bonded* together."   This is even more puzzling because Silberberg states that the processes in ionic and covalent compounds "generate chemical bonds (my emphasis), the forces that hold the atoms together in a compound".

[Borek]

Let me start by saying that "covalent compound" and "ionic compound" are idealizations - no compound is actually "perfectly" ionic or covalent. That's why I added the starred comment.

You wrote, "Even in crystals...but they are not bonded* together."   This is even more puzzling because Silberberg states that the processes in ionic and covalent compounds "generate chemical bonds (my emphasis), the forces that hold the atoms together in a compound".

Perhaps the problem here stems from the fact that bonds in ionic and covalent molecules - while technically both connect atoms - have different properties?

Bonds in covalent compounds keep atoms together and keep them in (more or less) stable relative positions. In methane molecule there is a a central carbon atom and four hydrogen atoms arranged in such a way, the angle between H-C-H is always 109.5° and the distance between carbon and any hydrogen is around 109 pm (they do vibrate slightly, but for now let's just assume the covalent molecule is mostly rigid). When the methane freezes these molecules create a crystal, when the crystal melts, exactly the same methane molecules are recreated.

In ionic compounds there is no rigid structure, unless they are in solid, crystalline form. When they get melted ions become separated, a bit like Lego bricks. When they are crystallized back they will create structure that has exactly the same parameters, but every ion will occupy some new, random position. It is a bit like using the same Lego bricks each time to build exactly the same looking structure, but not paying attention which exact Lego bricks goes where, after all they are all identical, so it doesn't matter.

In the ideal ionic crystal the only way ions are kept together is because they are charged spheres that attract each other. This attraction is a form of bonding, but it is completely different from the way atoms are kept together in covalent molecules.

[Orcio_87]

In the example of table salt (sodium chloride) if the bond between sodium and chlorine ions is broken then we no longer have the properties common to table salt.

Why?  If you disolve NaCl in water you will still have Na and Cl ions - not in crystal lattice, but in solution. If you dissolve sugar you will get the same - sugar molecules not in crystal lattice, but in solution.

But their chemical properties will not change.

[Borek]

But their chemical properties will not change.

This is a bit tricky. Properties of a solid, crystalline NaCl, are not identical to properties of NaCl solution, even if we are talking about the same compound. The first step should be to define what we mean by "properties" and "chemical properties", to make sure we talk about the same thing.

[Ian000Bell]

@Borek

Part of the problem is one of communication - with chemistry, I am literally a novice, having just started to read this text book and the material in question is covered in only the second of 23 chapters.   

It would seem that, for now, I am just going to have accept the statement that "there are no molecules in an ionic compound"  and come back to it at some later point. 

What is obvious to you is a puzzle for me.   I will reflect on what you have said and come back to this issue at another time.

Thank you for your patience. 

Ian

[Orcio_87]

It would seem that, for now, I am just going to have accept the statement that "there are no molecules in an ionic compound"  and come back to it at some later point.

Why? Is not true that at 1465 C NaCl evaporates into NaCl molecules?

[Ian000Bell]

@Orcio_Dojek

My apologies, but I have only high school chemistry that has been forgotten over the decades and I am only becoming reacquainted again with very basic concepts.   I do not have your experience and so I cannot confirm or refute what you are saying.

From Silberberg's textbook, my understanding is, if the sodium and chlorine bonds are broken and the chlorine ion reverts to a gas and this gas has physical and chemical properties that are very different from table salt.

I fully acknowledge my understanding in this matter is less than rudimentary and will often be incorrect.  This is going to be a steep learning curve...

Ian

[Borek]

if the sodium and chlorine bonds are broken and the chlorine ion reverts to a gas and this gas has physical and chemical properties that are very different from table salt.

That's not what happens. When the NaCl melts (or gets dissolved) chlorine ions will not convert to a gas, they will be still chlorine ions, just like Na ions will not become a metal - they will be still Na⁺ ions. No matter what you do to NaCl, it is always an equimolar mixture of Na⁺ and Cl^- ions, and their presence in this particular ratio makes NaCl NaCl. But in typical circumstances these ions are not bound, they just coexist in no particular structure (which is characteristic for covalent compounds).