Why is fluorine more reactive than chlorine?

Why is fluorine more reactive than chlorine despite chlorine having a higher electron affinity?




It depends on what reaction you are talking about.


* In the context of organic SN1 reactions, the reactivity is $\ce{R-F}$ (least) $\ce{< R-Cl < R-Br < R-I}$ (most). [1] This reflects the leaving group ability, with larger atoms better able to accommodate the extra electron.
* In the context of radical homolysis of $\ce{X-X}$ the order of reactivity is $\ce{F2 < Cl2 < Br2 < I2}$ as the sigma bonds become weaker as you move down the period.[2]
* In polar aprotic solvents (DMSO) nucleophilicity follows $\ce{F- > Cl- > Br- > I-}$. This order reverses in polar protic solvents due to solvation: [*J. Chem. Educ.,* **1970,** *47* (6), p 473](https://doi.org/10.1021/ed047p473)
* Transition metal $\ce{MX\_n}$ bond enthalpies have been measured empirically and calculated: [*J. Chem. Phys.,* **1957,** *26* (6), 1644–1647.](https://doi.org/10.1063/1.1743597). It would be inappropriate to over-generalize, as any discussion here needs to also account for metal oxidation states, HSAB theory, electronic configurations, and ligand-field theory


**References:**


1. Clayden, Greeves and Warren *Organic Chemistry*, p. 348.
2. Clayden, Greeves and Warren *Organic Chemistry*, p. 971.


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There are three things to consider every single time relative reactivity is unknown; atomic radius, shielding, and number of electrons. The reactivity is the halogens ability to gain an electron, so number of electrons already in the atom plays a vital role. Chlorine has more electrons so repels a reacting electron with greater force than fluorine, making it less likely to react. 


Fluorine also has fewer electron shells than chlorine, so there are fewer electrons between the positive nucleus and the reacting electron to essentiallly block, or weaken, the electromagnetic attraction. This is shielding. 


Lastly, fluorine is much smaller molecule than chlorine, and the shorter distance, or radius, between the nucleus and the electron again makes it more likely to attract the electron and react to gain a noble gas configuration. 


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Reactivity is an elements ability to gain an electron. So the better it is at "stealing" electrons, the more reactive it will be. The thing that makes fluorine so reactive is its electronegativity. Electronegativity is the tendency of an atom to attract a shared pair of electrons. In simple terms, think of this as tug of war, if fluorine went up against chlorine, it would always win the tug of war due to its higher electronegatvity. There is a periodic trend for electronegativity that you can look up. Basically it starts at the bottom left with francium having the lowest electronegativity and goes diagonally left to right all the way up to fluorine having the highest electronegativity. Electronegativity is probably the biggest thing that plays into reactivity. Therefore, since fluorine has a higher electronegatvity than chlorine, fluorine is more reactive. 


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Fluorine molecule has a small atomic radius with 5 electron in 2p orbital. This means it has a high charge density on its atom. When fluorine molecule breaks up into F- ions it remains highly unstable and just wants to bond with other element. Due to this charge density fluorine has high polarizability which makes it more reactive.


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