I'm confused about filling 3d and 4s subshells. When writing electrons configuration for transition metal (for example), why do we fill 4s before 3d, yet when this same metal atom gets ionized, 4s electrons are removed first (assuming that 4p isn't filled)?
Also, why do subshells become more stable when they are half-filled or fully-filled with electrons?
What I think:
- 4s is higher than 3d, but 3d extends more away from the nucleus than 4s does.
- Half filled: because they don't face repulsion force from another electron in the same orbital maybe?