Because of how there IS only one electron in
The added electrons in multi-electron atoms are intrinsically correlated in such a way that each electron is influenced by the motions of the others.
This electron correlation introduces the effect of electron-electron repulsion. That in turn generates an energy splitting of the orbitals with the same
For example, for just the
$"H"$ atom:
$ul(color(white)(uarr darr))" "ul(color(white)(uarr darr))" "ul(color(white)(uarr darr))" "ul(color(white)(uarr darr))"$
$" "" "" "underbrace(" "" "" "" "" "" "" "" "" ")$
$3s" "" "" "" "" "" "3p$
For multi-electron atoms:
$" "" "" "ul(color(white)(uarr darr))" "ul(color(white)(uarr darr))" "ul(color(white)(uarr darr))"$
$" "" "" "underbrace(" "" "" "" "" "" "" "" "" ")$
$ul(color(white)(uarr darr))" "" "" "" "" "3p$
$3s$
We had that the
In multi-electron atoms, higher