# In what order do electrons fill orbitals within a sublevel?

May 17, 2016

The energy of a given subshell or sublevel is decided by the energy of the sublevel .

#### Explanation:

The relative energy of sublevels is decided by ($n + l$) value of a given subshell . Lower the $n + l$ value , lower is the energy. Lower energy subshells are filled first .

Second shell has two subshells ( $s$ and $p$ ) also termed as $2 s$ and $2 p$ subshell

for $2 s$ subshell, $n + l$ value is $2 + 0 = 2$( $l = 0$ for $s$ subshell)

and for $2 p$ subshell, $n + l$ value is $2 + 1 = 3$ ( $l = 1$ for $p$ subshell)

$2 s$ subshell will be filled first and $2 p$ subshell will be filled, once $2 s$ subshell is filled.

Third shell has three subshells ( $s , p$ and $d$ ) also termed as $3 s , 3 p$ and $3 d$ subshell

for $3 s$ subshell, $n + l$ value is $3 + 0 = 3$ ( $l = 0$ for $s$ subshell)

for $3 p$ subshell, $n + l$ value is $3 + 1 = 4$ ( $l = 1$ for $p$ subshell)

for $3 d$ subshell, $n + l$ value is $3 + 2 = 5$ ( $l = 2$ for $d$ subshell)

$3 s$ subshell will be filled first and $3 p$ subshell will be filled, once $3 s$ subshell is filled and then $3 d$ subshell.

If two subshells have same $n + l$ value, the one with lower $n$ , value will be filled first.

for $2 p$ subshell, $n + l$ value is $2 + 1 = 3$ , and $3 s$ subshell , $n + l$ , value is $3 + 0 = 3$, since 2p subshell has lower $n$ value , so it will be first filled.