here,
Cv - molar specific heat at constant volume
Cp - molar specific heat at constant pressure
where, according to the first law of thermodynamics,
Cp = (1/n)(dU/dT) + R
or for ideal monoatomic gases
Cp = (5/2)R
and
Cv = (1/n)(dU/dT)
or Cv = (3/2)R
thus Cp - Cv = (5/2)R - (3/2)R
or
Cp - Cv = R
hope it helps!!