1 hour ago, HbWhi5F said:

Last time I provided an answer I got no acknowledgement from you, so I have no idea whether my response was helpful to you or not (assuming you are a person and not a robot).

I'm tempted not to bother this time.

Edited September 1Sep 1 by exchemist

2 hours ago, exchemist said:

Last time I provided an answer I got no acknowledgement from you, so I have no idea whether my response was helpful to you or not (assuming you are a person and not a robot).

I'm tempted not to bother this time.

Well you got one from me +1

13 hours ago, HbWhi5F said:

OK, here is the link to what the Libretext source has to say about this: https://chem.libretexts.org/Courses/University_of_California_Davis/Chem_124A%3A_Fundamentals_of_Inorganic_Chemistry/05%3A_Molecular_Orbitals/5.02%3A_Homonuclear_Diatomic_Molecules/5.2.02%3A_Orbital_Mixing

If you read that and then go on to the next section, it explains why you get a different order of filling the orbitals in diatomic molecules as you go across the period. It is not really as the diagram you posted says. There is mixing in all cases but, because electrons in σ orbitals spend more time close to the atomic nuclei than those in π orbitals ( π orbitals have a node in the plane of the molecule i.e. zero electron density at the nucleus) they are more exposed to nuclear charge - which increases as you go across the period. So they get pulled down in energy (more electrostatic attraction). There is a nice diagram showing the progression:

Notice how the σ orbital derived from the 2p(z)atomic orbitals comes down in energy and crosses over the level of the π orbitals between N and O.

I never knew this, I'm sure. I think it's cool stuff! It's another manifestation of the topic of "penetration" and "shielding", which crops a lot in explaining various features of inorganic chemistry.

Edited September 1Sep 1 by exchemist