Each molecule of a gas has f degrees of freedom. The radio $ { C_P \over C_V} = \gamma $ for the gas is
$C_p = \left( { f \over 2 } + 1 \right) R, C_v = {f \over 2} R $ $ { C_p \over C_v }= { \left( { f \over 2 } + 1 \right) R \over { f \over 2} R } = { {f \over 2} + 1 \over { f \over 2} } = { f+2 \over f} = 1 + { 2 \over f } $