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From what I was taught in middle school, cations are those ions that move towards the cathode, likewise anions are those ions which move towards the anode.

I didn't have issues with this back then, since all we studied were electrolytic cells. But now that we've crossed over to electrochemical cells, I'm having doubts.

Every piece of chemistry literature I've come across so far, always deals with positive ions as cations and negative ions as anions; even my school textbook does it! That makes sense with respect to electrolytic cells, since the cathode's negative and anode's positive therefore positive ions would move to the cathode (hence 'cations') and negative ions would move to the anode (hence 'anions').

But in an electrochemical cell, the cathode's positive and the anode's negative. So if ions are to be classed according to the electrodes they move over to, then positive ions would be anions and negative ions would be cations, which is exactly opposite to the first case (on the basis of electrolytic cells).

This is really confusing....

So should I refer to positive and negative ions as cations and anions respectively or as anions and cations respectively? Or are both acceptable, depending on the scenario?

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2 Answers 2

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Yes, cations always have a positive charge and anions always have a negative one.

The difficulty is that the term cathode and anode do not always correspond to the same pole. The cathode is that pole of an electrolytic/electrochemical cell where reduction takes place (cathodic reduction) while the anode is where oxidation takes place (anodic oxidation). Since it depends on the type of cell which will be oxidised and which will be reduced, the terms cathode and anode mean different things in different cells.

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    $\begingroup$ You’re German, right? My mnemonic was anodische Oxidation ist das A und O. $\endgroup$
    – Jan
    Commented Sep 27, 2016 at 9:57
  • $\begingroup$ @AaronAbraham Yeah, you are correct, it means anodic oxidation is the alpha and omega; in German, alpha and omega is typically written as A und O to the point that many people don’t know what it actually means. $\endgroup$
    – Jan
    Commented Sep 27, 2016 at 11:59
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    $\begingroup$ To potentially add to the "anode/cathode" confusion, in the battery community, it is conventional to refer to an electrode as the "anode" or "cathode" based on whether oxidation or reduction occurs during discharge. (Otherwise, it would flip depending on whether you are charging or discharging, and would get confusing in a paper) $\endgroup$
    – PGnome
    Commented Sep 27, 2016 at 16:10
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From what I was taught in Middle-school, cations are those ions that move towards the cathode, likewise anions are those ions which move towards the anode.

Nope, the definitions are as follows (from the IUPAC Goldbook):

cation

A monoatomic or polyatomic species having one or more elementary charges of the proton.

anion

A monoatomic or polyatomic species having one or more elementary charges of the electron.

So, yes, cations are always positively charged, while anions are always negatively charged.

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