For the homogeneous gas-phase reaction $A_{(g)} + B_{(g)} \rightleftharpoons C_{(g)} + D_{(g)}$ at equilibrium, the expression for the equilibrium constant $K_c$ is:

Equilibrium constant in terms of molar concentrations is written products over reactants, with each species raised to its stoichiometric coefficient. Here all coefficients are 1, so $K_c = \dfrac{[C][D]}{[A][B]}$. This convention is the load-bearing rule: products on top, reactants on the bottom, exponents from the balanced equation. Reversing the order is the most common student error and gives the reciprocal.