Beryllium, magnesium, strontium, barium, or radium. Hope this helped :))
<span>subshells have two reasons of stability-
</span><span> 1) half filled and full filled subshells lead to symmetry as symmetry always increases stability in the nature.
</span>2)<span> in half filled and full filled subshells there are maximum no Of exchanges so exchanges releases energy so it increase stability.. Exchange of e-s also can b understood as delocalization of e-s which is similar like resonance and resonance always increases stability</span>
Oxygen and selenium both have 6 valence electrons. They're in the same group hope this helps
