The units of K_p for the reaction  ${\mathrm{P}}_4\left(\mathrm{s}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\leftrightarrow {\mathrm{P}}_4{\mathrm{O}}_{10}\left(\mathrm{s}\right) \text{is:}$

Units of K_p = (atm)^Δn 
      Δn = n_p – n_r =  0 – (0 +5) = -5, hence units of K_p is (atm)^-5
As P₄ and P₄O₁₀ are solids, these do not appear in K_p expression.