Ionic Bonding (1) Evidence for the existence of Ions

Ionic Bonding (1) Evidence for the existence of Ions

In this blog, I want to show you the evidence that exists to suggest that ions are real particles.

You might have carried out an experiment like this in your laboratory:—

•Take a microscope slide and cover with wet filter paper.

•At the mid point place a crystal of potassium manganate(VII).

•Connect each of the short sides of the slide to a 20v dc power source and wait.

You can see a similar experiment done here

You can find instructions and further discussion of the manganate(VII) experiment here. Or here and a good example of the experiment from the Royal Society of Chemistry here.

Here is a picture (its fairly stylised) of the kind of result you might expect:

If the purple colour moves towards only one of the electrodes (the positive electrode) what does that tell us about the particles that make up the purple colour?

Yes, that's right, they could only be negatively charged.

And we are pretty sure potassium particles are colourless given how many colourless salts there are of potassium.   

Here is another experiment you can do fairly easily though it takes time to develop and see the results:—

•Place a paste of copper(II)chromate in the bottom of a U tube.

•Top up each arm of the U tube with conducting liquid and insert charcoal electrodes

•Set up a 20v dc potential difference across the electrodes and watch.

After 20mins or so you might see movement of colour blue to the negative terminal and yellow to the positive terminal.

These diagrams illustrate the results of this demonstration:

Here’s further evidence, as if we needed it, to show that copper ions (blue) are positively charged species and that chromate(VI) ions (yellow coloured) are negatively charged species.

One final piece of evidence to suggest that charges atoms called ions do really exist is the phenomenon of the electrolysis of molten salts like lead bromide, PbBr₂

You can find details of the experiment and how to carry it out here

And you can see the experiment itself on video here

Here is a diagram of what we think is happening in the electrolysis of lead bromide:

What I like about this picture is that it is not too fussy and crowded with detail.

Lead is produced at the negative cathode suggesting that lead ions carry a positive charge.

Bromine gas (highly toxic) is evolved at the anode suggesting bromide ions carry a negative charge,

The magnified diagram on the left shows that we think bromide ions lose electrons to the anode and the resulting atoms pair up to form bromine molecules:

2Br^—     =      Br₂(g) +     2e^–

On the right, the magnified diagram shows positive lead ions each accepting two electrons.

Pb²⁺      +    2e^–      =   Pb(l)

The evidence for the existence of ions is pretty convincing.

The word ION (he derived it from the Greek for “going”) was first conceived by Michael Faraday in 1834 in his early experiments with electrolysis in the 19^th century. 

Cations (positive ions) Faraday so named because they go to the negative cathode and anions (negatively charged) because they go to the positive anode.

This is a brief introduction into the existence of ions but there is much more to say about how they enable the formation of compounds and all that will be found in forthcoming blogs. 

Best now go and do an ion experiment and see them move for yourself. 

Happy experimenting!!