The number of electrons present in all the completely filled...

(a) Number of sub-shells associated with n = $4$:

• A given value of  $n$, $l$ can have values from zero to $( n – 1 ) . i . e . , 0, 1, 2, 3$
• The four sub-shells are associated with n=$4$, are s, p, d, and f.
• Where,

The number of electrons in the sub-shells having an m_s value of$+ 1 / 2$for n =$4$: can have,

$= 1 + 3 + 5 + 7 = 16$