You probably learned in high school chemistry class that core electrons don’t participate in chemical bonding.

They’re thought to be too deep inside an atom and close to the nucleus to meaningfully interact with the electrons of other atoms, leaving the outer valence electrons to get all the chemical bonding glory in textbooks.

The actual science is more complicated, as some elements’ core electrons are theorized to activate when squeezed hard enough, like at the pressure levels found deep inside Earth.