A theoretical study of the methyl and ethyl derivatives of C, Si, Ge, Sn, and Pb has been conducted. All-electron basis sets have been used and the relativistic effects taken care with the relativistic elimination of small components (RESC) method, and electron correlation has been taken into account with density functional theory (DFT). A comprehensive set of fully optimized geometries and reaction energies are presented for both methyl and ethyl families. For the methyl derivatives, a more detailed study has been conducted and the following results are juxtaposed: those obtained with two different functionals (BOP and B3LYP) and relativistic vs nonrelativistic. Second, for the methyl derivatives, we also present common thermodynamic quantities as a function of temperature, such as enthalpy, Gibbs' energy, and heat capacity, all of which have been obtained from computing Hessians and using harmonic oscillator and rigid rotor models. Good agreement with experiment is achieved, within a few picometers in optimized geometries and several kcal/mol for reaction energies.