Question

what is the orbital filling rules in the aufbau principle​

Answer 1

first ground state elctron should fill

Answer 2

$\tt{\bold{\Large{\underline {Aufbau\:Principle}}}}$ :

➡️ ' Aufbau' is a ' German word ' which means $\tt{\underline {"building\:up" }}$. According to this principle,

$\sf{In\:the\:ground\:state\:of\:an\:element\:the\:electrons\:are\:filled\:in\:the\:increasing\:order\:of\:their\:energy\:levels}$.

$\underline{\sf{Orbital\:Filling\:Rules}}$ :

For Hydrogen and Hydrogen like species, the energy can be decided by the value of $\bold{'n'}$.

To represent, n $\propto {E}$ ( Greater the value of n, Greater will be the energy.)

⚫For multi electronic species, Energy can be decided by $\bold{\underline {(\:n\:+\:l\:)}}$ rule.

According to $\bold{\underline {(\:n\:+\:l\:)}}$ rule,

➡️ ( 1 ). Greater the value of $\sf{\underline {sum\:of\:n\:and\:l \:}}$, greater will be the energy.

$\sf{\fbox{(\:n\:+\:l\:) {\propto{E}}}}$
$\sf{\underline {For\: example}}$ : Refer the attached picture.

➡️ ( ii ). If the sum of 'n' and 'l' is same, then greater the value of 'n' greater will be the energy.

$\sf{\fbox{(\:n\:+\:l\:) {\propto{E}}}}$
Or
$\sf{\fbox{n{\propto{E}}}}$.

$\sf{\underline {For\: example}}$ : Refer the attached picture.