Chemical bonds are formed or broken. This is true of a chemical change, but not a physical change. A physical change is usually something that you can see, such as changing of shape or form or staying the same. However, because you cannot visibly see chemical bonds being formed or broken and because this would affect the chemical composition, it is not a physical change.
Chemicals are bound together like two pieces of bread
One electron is shared between ethane, H3-C-C-H3
Heat is change in thermal energy. So, thermal energy is the net potential energy of the system. while, internal energy is the net potential energy + the net kinetic energy of the system.