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Internal energy of a system composed of molecules or atoms is the mean energy of all these particles in the system (energy as kinetic energy, potential energy etc..). But what I am seeing that we are not taking into consideration the energy levels of electron inside the atom. My question is: Why we are excluding the energy of electrons of atoms (since electrons are part of the atom structure). Hope somebody can clarify it for me

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  • $\begingroup$ The electron itself has a weak mass 511 keV/c² , the energy levels are of the magnitude of 1/2 to some 10 eV/c². Sometimes, it accounts, but most time not $\endgroup$
    – user46925
    Commented Jan 24, 2016 at 17:10
  • $\begingroup$ but in chemical reaction, it is due to energy levels of electron that light is emitted no? $\endgroup$
    – Tonylb1
    Commented Jan 24, 2016 at 17:21
  • $\begingroup$ yes , it's a matter of electrons levels. Are you particularly thinking to the photochemistry ? $\endgroup$
    – user46925
    Commented Jan 24, 2016 at 17:42
  • $\begingroup$ I am not into chemistry but when we calculate the difference of gibbs free energy of a chemical reaction, do we take into consideration electrons levels ? $\endgroup$
    – Tonylb1
    Commented Jan 24, 2016 at 18:05
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    $\begingroup$ if you compute it in a chemical reaction with a tool like QFT, all accounts, furhermore if you know the distributions differences of particular states like the electrons levels $\endgroup$
    – user46925
    Commented Jan 24, 2016 at 18:39

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I do not think many people attempt to find the absolute total internal energy of a system. Rather they will pick and choose what needs to be counted to suit their purposes. If interested in statistical mechanics then treated atoms as billiard balls which can rotate and vibrate, have translational kinetic energy and potential energy due to the inter atomic forces is good enough. For ideal gases just stick to the translational (thermal) kinetic energy.
If you are designing a heat engine then microscopic parameters are not need and the internal energy will be quantified via parameters such as temperature, mass, specific heat capacity etc. When discussing the energy changes for 1 kg of steam I doubt whether the binding energy of the electrons is ever mentioned?

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