Why do atoms start moving when heated?

Students should infer that the speed of the atoms in the metal ball has increased. This increased motion competes with the attractions the atoms have for each other, causing the atoms to move slightly further apart. This is why the heated ball is too big to fit through the ring. read more

Atoms and molecules in materials are always moving. Adding internal energy to a system of atoms increases the average kinetic energy of the atoms and molecules so that they on average are moving faster, but it doesn't start them moving. The motion never stops because there isn't anything like friction on atomic scales. read more

Full Answer. When any substance is heated, its molecules and atoms start moving relatively faster and this causes an increase in the space between each one of them. read more