For a monatomic gas, 

molar heat capacity at constant volume, $C_{V,\mathrm{m}}=(3/2)R$

molar heat capacity at constant pressure,  $C_{p,\mathrm{m}}=(3/2)R$

the ratio of $C_{p,\mathrm{m}}$  and $C_{V, \mathrm{m}}$ is 3R

the difference between $C_{p, \mathrm{m}}$ and $C_{V, \mathrm{m}}$ is 2R