a. How many sub-shell are associated with `n = 4`?
b. How many electron will be present in the sub-shell having `m_(s)` value of `-1//2` for `n = 4`?

Explanation

(a) phosphorus (P):
Atomic number = 15
the electronic configuraion of P is
`1s^(2) 2s^(2) 2p^(6)3s^(2) 3p^(3)`
the orbital picture of P can be represented as :
from the orbital picture , phophorus has three unpaired electrons.
(b) silicon (Si)
Atomic number = 14
the elctronic configuration of Si is
` 1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(2)`
the orbital picture of Si can be represent as .

from the orbital picture , iron has four unparried electrons.
(e) keypton (Kr).
Atomic number =36
the electronic configuration is
`1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6) 4s^(2) 3d^(10)4p^(6)`
the orbital picture of Krypton is :
since all orbitals are fully occupied , there are not no unpaired electrons in krypton.