shielding electrons and energy states contradiction

[The Boatox]

Hey friends, quick question(s) on Effective Nuclear Charge.

• Shielding electrons are those that are in subshells closer to the nucleus (lower in energy) than the electron we are interested in.
• The higher the effective nuclear charge for an electron, the more stable it is (higher ionization energy, not easily knocked off).

To my understanding, this is saying that shielding electrons are lower in energy and more stable than other electrons (such as valence)? If someone can explain this whole subject regarding ENC I would appreciate it. I can get the answer through memorization but I can't understand this phenomena conceptually.

• As you go down a group (Na to K), the atomic size increases even though the effective nuclear charge stays the same, because higher shells have a larger radius than lower shells.

I understand that as you fill a shell (going to the right on the periodic table) the effective nuclear charge increases because the increasing number of protons acting on the electrons increases thus becoming more stable; however, why is it when you go down a group the effective nuclear charge stays the same even though the radii increases?

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[charistivity]

I am not sure of you are asking about as your first question but yes. The idea is that as you move further away from the nucleus, n=1 ==> n=2 ==> n=3, etc, the electrons in higher subshells are less tightly pulled on by the nucleus. So it would require more energy to rip off electrons from the atom as you advance towards the nucleus. More difficult to remove ==> more stable

Side note: I would only take the first assertion you have at face value. Shielding capabilities are not exclusive to lower subshell electrons. electrons in the same subshell can have shielding effects on each other. The first and second ionization energies of Mg are not exactly the same. Probably not relevant to your question but just putting it out there

Regarding ENC trends in a group, keep in mind that the number of protons increases as well as radii increases. You would expect the ENC to be weaker if there was no concurrent increase in positive charge density in the nucleus as radius increases. For example, Na has 11 protons while K has 19. So increasing atomic radius would not necessarily mean that ENC should be weaker or stronger for a given electron. Actually the first ionization energies decrease as you go down form Li to Na to K but only slightly