Writing pressure equilibrium constant expression for a reaction with a single solid product How would you write the $K_\mathrm p$ expression for the reaction: $$\ce{4 Ag(s) + O2(g) \rightarrow 2 Ag2O(s)}$$

I know normally it would be products over reactants, but there is a solid on the product side and they aren't included, so I don't know how you would write it?
The expression $\mathrm{K_p}$ is an expression of the equilibrium of partial pressures. So, it just includes the the partial pressure of the oxygen because that's the only gas.

$$\mathrm{K_P} = \frac{1}{\ce{P_{O2}}}$$

More information about the equilibrium constant $\mathrm{K_p}$ can be found here.