Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, H2Y, HX, Y2, and X2 for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se2, where the AoC-DHF value was used due to strong multireference effects in Se2 for the SO calculations. The calculated results show that the At2 BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. H2Po is not predicted to be stable to dehydrogenation to Po + H2 in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H2SO4. The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.