Are molecules only held together by Covalent bonds?

[Ben Sanders]

Hi there,
I'm studying for IGCSE Chemistry and I'm a little confused with Bonding/Molecules.
My textbook says:

"Molecules are held together by covalent bonds"

I'm a little confused by this as I thought a molecule was formed when two or more atoms (same or different elements) join together. From my understanding also, a compound is also a molecule because it has two or more different atoms joined together. Because ionic bonding occurs between two different elements it must therefore also be a molecule.

So aren't molecules also held together by Ionic bonds?

Cheers,

Ben

 

[BvU]

Hello Ben,

What is a compound according to you ?
In a crystal lattice it is rather difficult to speak of molecules: in kitchen salt (NaCl), for example, Na ions are surrounded by six Cl ions, so there are no NaCl molecules to speak of.

You could read (e.g. here) about ionic bonding and disagree with me, though. But be careful: their language isn't completely accurate (they also speak of a sodium molecule!)

 

[JeremyG]

Because ionic bonding occurs between two different elements it must therefore also be a molecule.

In short, no.

In chemistry, what is referred to as molecules can be further divided into two categories: simple covalent molecules and macromolecules.

Simple covalent molecules are formed with covalent bonds keeping the atoms together, and as the name suggests, they tend to be simple and small (compared to macromolecules). Some examples include O₂, CH₄ and C₆H₁₂O₆ (glucose).

Macromolecules, on the other hand, are giant lattice structures that are also formed from covalent bonds, but form extended structures such as in diamond or graphite.

Ionic compounds (note: not molecules), are called as such because they do not fall within these categories. It is not customary to call an ionic compound a molecule. A better explanation would be that ionic compounds are actually lattice structures of ions, held together by electrostatic forces of attraction between the ions.

So aren't molecules also held together by Ionic bonds?

Nope. Molecules are held together by covalent bonds, as explained above.

 

[DrDu]

Nope. Molecules are held together by covalent bonds, as explained above.

So how would you call the bonding in diatomic NaCl in the gas phase?

 

[BvU]

So how would you call the bonding in diatomic NaCl in the gas phase?

Strong point ! We could call it ionic, we could call it exotic, we could call it far-fetched, we could call it the exception that proves the rule.

Or we could say that ionic bonds and covalent bonds differ only quantitatively in the measure of attraction of the electron(s) that is (are) exchanged c.q. shared. The consequences are enormous, to be sure; so big that totally different perceptions have developed for the two categories.
I suppose there is an in-between category as well, but not so well known that it gets a lot of attention (anyone?)

But DD is correct in the sense that in gaseous NaCl the bonding is just as ionic as in the lattice, but one Na on one Cl. The textbook is still right if you consider this molecule as an extreme form of covalent bonding, I would venture to say.

I've been 'making' NaCl for half my life, as crystals from evaporating a brine solution. Much healthier.
I did study the orange Na fluorescence in flames in the first half of my life. The NaCl was atomized in the heat of the flame, but here must have been NaCl molecules around as well.

Come to think of it, there must be NaCl molecules in brine as well: just not very many of them.

--

 

[DrDu]

Strong point ! We could call it ionic, we could call it exotic, we could call it far-fetched, we could call it the exception that proves the rule.

--

I don't agree on that. Diatomic molecules in the gas phase have been studied extremely well and their dissociation energy forms the basis for the Pauling electronegativity scale, which we mostly use to classify a bond as mainly ionic or mainly covalent.

How about ionic bonds in proteins, e.g. between lysine and glutamate?

 

[BvU]

Well, goes to show one's view on the world is rather limited. Thanks for the scale, I'd completely forgotten its name.
I can only concede utterly!

So to Ben we just say that yes, you are absolutely right, good post ! The textbook tried to keep it simple for the majority of students that don't think so far ahead. Physics is interesting: there's always a next layer of complexity to explore !

drB