Question

1. think about the modes of substances in the gas phase and explain why potential energy is included in the thermal energy of some substances in the gas phase, but not in other substances in the gas phase.

2. PE= E-bond + (1/2)E-thermal             and           KE = (1/2)Ethermal

when a monatomic solid/liquid turns into a gas, what are the PE and KE equal to then?

Answer 1

1) The thermal energy may include both the Kinetic energy and Potential energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles in plasmas. It originates from the individually random, or disordered, motion of particles in a large ensemble, as consequence of absorbing heat. In ideal monatomic gases, thermal energy is entirely kinetic energy. In other substances, in cases where some of thermal energy is stored in atomic vibration, this vibrational part of the thermal energy is stored equally partitioned between potential energy of atomic vibration, and kinetic energy of atomic vibration. Thermal energy is thus equally partitioned between all available quadratic degrees of freedom of the particle

2) The phase transition changes the negative PE of the solid or liquid to zero PE, and the temperature, and thus the KE, does not change. So all the heat taken up by a transition to a gaseous state goes to internal PE. A KE change requires a temperature change, which does not happen in a phase transition. See the ref.
PE is negative in a solid or liquid because of molecular attraction. It takes work to separate the molecules. In a gas, the molecules are free, with zero PE.