Why do heavier elements have smaller specific heat values.

In the periodic table the trend is that as the mass of each element increase's the specific heat tends to go down. This seems to be counter-intuitive because if I am not mistaken as mass increase's the amount of heat that the substance can hold goes up. So why does the specific heat of these elements go down?

• There is no reason to assume specific heat is a nuclear property. – Lighthart Jan 4 '15 at 0:44

2 Answers

The more particles in a system, the more modes of vibration (and rotation), so one gram of $$\ce{H2}$$ has about $$235 \times$$ more particles than one gram of U. The heat is "stored" in the motions of the atoms and electrons, until reaching millions of kelvin, at which point dissociation of the nucleus would be significant in specific heat. See these links and for more information.

As the amount of a substance increases the Heat capacity of that substance does go up. However, Specific Heat is a correction for this because it is the Heat Capacity divided by the amount of the substance.

For Specific Heat the amount of a substance is irrelevant and its value is largely determined by the degrees of freedom for the constituent particles. The more degree of freedom, other than transnational, the higher the Specific Heat. An example of another degree being rotational energy which does not contribute to Temperature changes.

• This doesn't say anything about the trends in the periodic table, which is what the question is about. – bon Feb 16 '16 at 19:08
• This post does not answer the question and it is also incorrect since specific heat capacity c is heat capacity C divided by mass m (not heat capacity C divided by amount of substance n). – user7951 Feb 16 '16 at 19:34