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I would understand it if the metallic activity series Li, Cs, Rb, ... Pt, Au corresponded to electronegativity or ionization energy. However, the way it is written, lithium will displace caesium, while caesium will displace rubidium (it goes from the top of family 1 to the bottom and then up again).

This order seems so random. Is there any order to it or is it just determined experimentally through trial and error? In addition, wouldn't different valences on the metals affect their placement in the series?

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  • $\begingroup$ There is an order but not all metals obey that order. We need to experimentally verify the theoretical order. Of course, the electronegativity and ionization energy are closely related to the order. The higher the tendency of the metal to get oxidised, the more reactive it will be and hence a more active metal. The ability to get easily oxidised depends on the factors you mentioned. $\endgroup$ – Yashas Jun 19 '16 at 8:40
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The metals in the activity series are ordered by their oxidation potentials; there is nothing random about it. The displacement reactions, though, are but one of the applications of these potentials, and not the most important one at that. Moreover, with active metals it does not make much sense anyway. The oxidation potentials are defined for water solutions; their significance ultimately stems from the fact that we live in a world dominated by water. Well, you can't react lithium or rubidium with anything in water, because they would rather react with water itself. And if you try a reaction in other media than water, the oxidation potentials do not apply and the outcome can't be predicted that easily. For example, magnesium would reduce sodium from its oxide, despite being nominally "less active".

Indeed, the very concept that some things are "more active" than others is just an approximation; admittedly, a pretty good one when we talk about metals, but still an approximation.

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