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I searched on Wikipedia for reactivity series but it showed me Caesium as the most reactive metal. I know and have studied that reactivity increases as we go down the first group. Then shouldn’t it be Francium more reactive than Caesium than the other way around.

PS: I study in class 10 so please explain in that way only.

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  • $\begingroup$ Oscar, if they are mere predictions then can’t we predict that it will be more reactive than Caesium. If you want to you can explain in more detail. $\endgroup$ – Aryan Mar 30 at 2:25
  • $\begingroup$ Francium is less reactive due to relativistic effects. This earlier Q&A entitled "Why do the trends in reactivity not apply for francium?" may be helpful. $\endgroup$ – ron Mar 30 at 15:05
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    $\begingroup$ For chemical purposes, francium pretty much does not exist. $\endgroup$ – Ivan Neretin Mar 30 at 16:25
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The reactivity series consists of elements whose properties are FULLY known and understood. It is on the basis of whether an element can displace another from its salt solution (this is what you've been told at grade 10 Indian schooling level I guess?). Francium, on the other hand is a radioactive element, with a half life of only 22 minutes for its most stable isotope (Francium-223). Therefore, we can only make predictions, and not observations, about the nature of its salts and whether they would displace other elements from their salt solution or not.

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  • $\begingroup$ Sorry for the bad English, it's my second language. $\endgroup$ – Micelle Mar 30 at 6:20
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We do not really know. Francium is known only as highly radioactive, short-lived isotopes and we have very little experimentally based chemical information about it. Most of its properties are mere predictions.

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Caesium is more reactive. Generally as you go down the periodic table, electro-positivity increases. However, for the really heavy elements, the presence of so many positively charged protons in the nucleus has the affect of causing the electrons to move round at incredibly fast speeds approaching the speed of light. As Einstein realised at such speeds strange things begin to happen. The electrons become a little closer to the nucleus than expected and they also become slightly harder to remove than expected.

Thus Caesium is more reactive than Francium.

For More Information https://www.quora.com/Which-is-most-reactive-element-francium-or-cesium

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