We know that $${C_P} - {C_V} = R$$ is same for all gases. For a diatomic gas, we have

$${C_V} = {{5R} \over 2}$$

Therefore, $${C_P} = {C_V} + R = {{7R} \over 2}$$

For a monoatomic gas, we have

$${C_V} = {{3R} \over 2}$$

$${C_P} = {{5R} \over 2}$$

Therefore,

$${C_P} + {C_V} = 6R$$ (diatomic)

$${C_P} + {C_V} = 4R$$ (monoatomic)

$${{{C_P}} \over {{C_V}}} = {7 \over 5} = 1.4$$ (diatomic)

$${{{C_P}} \over {{C_V}}} = {5 \over 3} = 1.67$$ (monoatomic)

$${C_P}\,.\,{C_V} = {{35{R^2}} \over 4}$$ (diatomic)

$${C_P}\,.\,{C_V} = {{15{R^2}} \over 4}$$ (monoatomic)