[ELI5] Why alkaline metals explode when they touch water
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They’re called alkali metals. These are reduction-oxidation reactions, which occur when one reactant (in this case the alkali metal) loses an electron and the other (water) gains. The alkali metal starts as having no charge (we write this as, for example Na(s) or Na with a little zero where the plus or minus usually goes if it’s an ion). It is oxidized and the water is reduced (remember, LEO says GER – lose electron oxidation; gain electron reduction). So in the case of sodium, a sodium atom goes from Na(zero, if I could write that here) to Na+. It loses one electron to become a sodium ion. The reason the alkali metal is so easily oxidized is that its lone electron in its outer shell is unstable. This is true of all alkali metals, which is why they’re all lined up in the left column on the periodic table. Electrons like to at least be paired, and preferably have eight in an outer shell (an octet). Given the opportunity, a lone electron in an outer shell is not going to stay that way. When the alkali metal loses that electron (oxidation), it moves to a lower energy state, giving off energy in the form of heat. In a reaction with water, the energy change is huge, and the reaction is violent, melting and ripping apart the alkali metal. This only speeds the reaction, as a steadily increasing metal surface area is exposed to water.
Reacting with water, the alkali metal donates its lone electron to a hydrogen on a water molecule, stripping it off the water molecule and leaving the electron pair that the hydrogen shared with the rest of the molecule behind, turning the water molecule into OH-, a hydroxide ion. Do this on two hydrogen atoms from two different water molecules, and you immediately put together H2, which is, as you know, flammable. The massive heat from the reaction causes the H2 to combust in the presence of the oxygen in the air. The oxygen for the combustion comes from air, not water.
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