Options (a) and (d) : (a) 1p and (d) 2d represent impossible combinations of n and l.
Explanation:
n refers to the principal quantum number, and l refers to the angular momentum or Azimuthal quantum number.
Principal quantum number (n) is used to indicate the main energy level of the electron. It may take whole numbers: 1, 2, 3, 4, 5, 6, 7, ...
Angular momentum or Azimuthal quantum number refers to the kind (shape) of the orbital. It can take numbers from 0 to n - 1.
So, if n = 1, l can only be 0; if n = 2, l can be either 0 or 1; if n = 2, l can be either 0, 1 or 2.
On the other hand, the shape of the orbitals is also representd by a letter. then there is a unique relation between the letter that represents the orbital and the angular quantum number which is:
letter l number
s 0
p 1
d 2
f 3
The previous information is summarized in the next table:
n possible l numbers
1 0
2 0, 1
3 0, 1, 2
4 0, 1, 2, 3
5 0, 1, 2, 3, 4
6 0, 1, 2, 3, 4, 5
7 0, 1, 2, 3, 4, 5, 6
As per the choices given in the question you have:
(a) 1 p is not possible because when n = 1 the only l number is 0 and it is an s orbital, but p ⇒ l = 1. Thus, this is a correct choice.
(b) 4s is possible since n = 4 permits l to be 0, 1, 2, and 3.
(c) 5f is possible since n = 5 permits l to be 0, 1, 2, 3, and 4.
(d) 2d is impossible since n = 2 permits l to be 0, and 1, but d ⇒ l = 2. Thus, this is other right choice.
