The energy of a molecule has what sign?

[Lotto]

TL;DR Summary

Is it negative or positive? I saw it with negative sign. If it is correct, then why is the energy negative?

And have all molecules or even atoms negative energies? So when a molecule have energy let's say -70 Ha and the other -75 Ha, does it mean that the second molecule has a lower energy?

 

[DaveC426913]

I saw it with negative sign.

Where did you see it? Show us.

 

[PeroK]

TL;DR Summary: Is it negative or positive? I saw it with negative sign. If it is correct, then why is the energy negative?

Energy is generally given relative to some arbitrary zero point. E.g. in Newtonian gravity, the zero point for a system of two masses is usually taken to be when the masses are "infinitely" far apart. In which case, the potential energy of the system is given by:$$U = -\frac{Gm_1m_2}{r}$$where ##r## is the distance between the masses. There is no physical significance of this being negative. You could equally take the potential energy to be:$$U = U_0 -\frac{Gm_1m_2}{r}$$for some constant ##U_0##. This would give a value of zero for some finite value of ##r = r_0## and the potential energy would be positive or negative, depending on the value of ##r##.

What's of physical significance is the change in potential energy between two configurations.

And have all molecules or even atoms negative energies? So when a molecule have energy let's say -70 Ha and the other -75 Ha, does it mean that the second molecule has a lower energy?

Likewise, the electrostatic energy of an atom is generally taken to be relative to some zero point and is often negative. And, yes, ##-75 \ Ha## is less than ##-70 \ Ha##. You need to put ##+10 \ Ha## into the system to change it from the first energy level to the second energy level, in this case.

 

[Lotto]

Where did you see it? Show us.

Here for example https://www.researchgate.net/figure...-of-squares-of-the-initial-CSF_fig5_221729111