Ionic Bonding (2) Dot and cross diagrams/Lewis structures

Ionic Bonding (2) Representing ionic bonds with dot-cross diagrams (Lewis structures)

So here we are again trying to give a few words of wisdom about ionic bonds.

Let’s start by being controversial and say that the ionic bond does not exist in its raw form. 

Unlike say hydrogen gas where we can see a discrete covalent bond of two electrons perfectly shared between two atoms you don’t get that sort of thing with ionic bonding.

Ionic bonds exist in massive clusters called structures.

An ionic bond never occurs in isolation.

Rather its better to talk about moles of ionic bonds.

Which reminds me to mention in the blogs I’m going to write for the next few weeks that Mole Day will be upon us soon.

Nevertheless you are going to find the ionic bond spoken about pretty frequently. 

You’ll have been taught that its formed by the donation of electrons from a metal atom to a non metal atom. 

Here’s a typical diagram illustrating this process which you can find everywhere on the Internet:

Here’s your first diagram that shows why you shouldn’t believe the Internet ever.

All is going fine with this diagram until we get to the second line “to form a molecule of sodium chloride” 

No, 1000 times no!!!! someone has to speak out here at this …p.

I suggest you try that line (about the molecule!) on your chemistry tutor and see what their reaction is!!

I would annihilate you actually if it were me. 

And here is another dodgy diagram:

I just don’t like the red splodge labelled ionic bond.

It smacks too much of a covalent bond picture like an σ bonding molecular orbital.

 Let’s leave this stuff off our Lewis Structures and dot and cross diagrams.

Although this next diagram doesn’t give enough information at least it gives us the accompanying structure of the ionic compound formed. 

But let’s have the sodium labelled as an “Ion” and same with the chloride it’s also an “ion”.

The colours are immaterial.

There’s no colour at the atomic level. 

But the structure is correct for sodium chloride as its has 6:6 coordination. 

What that means is that around each sodium ion are 6 chloride ions and vice versa. 

So you don’t just get one positive ion attached to one negative ion.

The formula of an ionic compound just gives you the overall ratio of the numbers of positive to negative ions. 

The stoichiometric ratio…..

That’s 1:1 in the case of sodium chloride hence NaCl.

And as the ratio is 1:1, there are no subscript stoichiometric numbers required. 

What you find with these diagrams is that they don’t show ionic bonds but the process of how they are formed. 

So you have the transfer of outer shell electrons from the Group 1, 2 or 3 element (usually a metal) to the outer shell of the Group 5, 6 or 7 element in order to fill the outer shell with sufficient electrons. 

Here is a classic diagram you have probably seen more times than sliced bread. 

Now the reason I like this diagram is not for the pink and pale blue blobs that are supposed to represent electrons. 

Nor the distinguishing dots and crosses.  Very sad!!

Nor the  correct numbers of electrons in each shell of the atoms and the ions, good one that!!

But I like the brackets (or parentheses for some of you) that encase the ion arrangements with the appropriate charge outside the bracket. 

I also like the electron in the chloride in outer shell keeping its original colour and cross because that’s what examiners like to see. 

I also like the statement about 1 electron transferred.

There’s none of this nonsense about atoms “wanting” electrons to fill their outer shells.

Atoms are inanimate objects without personality or personal traits so they can’t want anything.  Its crazy!!!

You really need to watch your use of language in this topic. 

But best of all is the comment in the middle of the diagram between the ions.

Here the attraction between ions is labelled correctly with a word not well known or remembered by students of chemistry at school or college level.

The attraction between ions is termed electrostatic attraction.  Yes!!!

Now, we like this!!

We can also see that each ion has a Noble gas electron configuration once it has formed. 

Sodium that of Neon and the chloride ion that of Argon.

So finally, let me challenge you to draw up a few of these dot and cross diagrams or Lewis structures to illustrate the ionic bonding in some simple binary ionic compounds like magnesium chloride, potassium oxide and lithium fluoride.

Some humorous diagrams from the Internet with the odd sexist joke thrown in.  

More on Ionic bonding in the next blog especially on the Born Haber Cycle.