$K+Cl\to KCl$. This is an example of

In the given reaction $\mathrm{K}+\mathrm{Cl}\to \mathrm{KCl}$, the reactants elemental potassium ($K$) and chlorine ($\mathrm{Cl}$) are in zero oxidation state, whereas in products potassium oxidation state changed from $0$ to $+1$ and chlorine oxidation state changed from $0$ to $–1$. Thus it is concluded that potassium gets oxidized and chlorine gets reduced. So it is an redox reaction.