The energy associated with the random motion of molecules or atoms within a substance is called what?

Thermal energy.

Explanation:

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In this case, since there are many types of energy that could be involved in a substance's energetic molecular behavior, it is important to notice that the energy associated with the random motion of molecules or atoms within a substance is called thermal energy. Such energy defines how the molecules or atoms move since the higher the thermal energy, the particles will have more energy to move rapider than when the thermal energy is lower. In addition, such energy is related with the kinetic energy which is also related with the motion.

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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.