Two solid metal blocks are placed inside an insulated container. if there is a net flow of heat between the blocks, they must have different what?

They must have different temperatures.

Heat is only transferred if two objects have a different temperature, which also means that their particles move at different speed and have different kinetic energy. In addition, heat must be transferred from the hotter object to the cooler one.

Usually in metals, since they have a large number of free electrons, heat can be transferred when the free electrons move and collide with other particles and pass the energy on and on. The faster they move, the faster heat is transferred. Metals can be very efficient in transferring heat compared to other materials where there are less free electrons as free electrons can travel a long distance before colliding with other particles. 

So next time if you try and touch a metal spoon, you feel coldness because heat is transferred from your hand to the spoon as they have different temperatures.

In any atom of any element, you will find protons and neutrons in the nucleus of any atom