Potassium is so reactive water because when mixed with water Potassium forms Potassium Hydroxide and H2 gas. H2 gas is highly flamable, and the reaction that forms Potassium Hydroxide is very exothermic, which creates enough heat to ignite. This causes an explosion in effect. If you have seen those videos on youtube about the people putting the different alkali metals in water, you will notice that the heavier elements like Rubidium and Cesium cause an even more explosive reaction because their reaction with water is even more exothermic and produces a more violent reaction.
The potassium atom has only one electron in its valence (outer) shell and has a low ionization energy. Since it wants to get rid of the lone electron (octet rule), it doesn’t take much energy to remove the electron. When it comes in contact with water, it reacts quickly and violently to form potassium hydroxide (KOH) and hydrogen. The hydrogen released reacts with the oxygen in the air, which causes the ignition.
Well, the above answer is a description of what happens when you mix K and H20, but it does not account for why such a reaction happens in the first place.
The reason for this is actually quite simple. Potassium, like all other alkali metals (such as Na, Rb, Cs, and Fr), has only one valence electron. Since atoms tend toward stability–and since an atom is generally most stable with eight valence electrons–potassium and other alkali metals tend to be exceptionally willing to “give up” this one electron to moderately or highly electronegative compounds. Since H20 has an electronegativity of 1.24 (you can google how to find this and what this means)–which is considered moderately electronegative–K will react with it.
What this means in layman’s terms is that potassium reacts violently with a great many things–including anything from the halogen group. Water, having a tendency to “pull electrons away” and potassium, wanting to “give its electron away” are the perfect pair for a chemical reaction.
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