Kr

[Jesse_torres2H]

Does Kr have a greater first ionization than Mg because its valence shell is complete? I figured since Mg is much smaller it would have a larger first ionization energy.

[Curtis Wong 2D]

Krypton is a noble gas and has a complete valence shell. For it to be ionized, or to achieve ionization, it would take an insane amount of energy to remove an electron from a complete shell. It is easier for Mg to remove an electron than Krypton.

[Cassandra Mullen 1E]

Yes, Kr is stable since it has a filled valence shell, so it holds onto its electrons much more strongly than Mg does. Even though Mg is smaller, it only has 2 valence electrons, and prefers to lose those valence electrons and form a cation that resembles the fully stable octet of neon. Since it's harder to remove the valence electrons of Kr, it has a greater first IE.

[Tessa Lawler 1A]

As a general rule, elements with a full outer valence shell want to stay that way. Taking away an electron from a full outer valence shell is very difficult due to the amount of energy required to kick the element out of its ideal state (a full outer valence shell).