Question

Which of the following electron configurations is not possible?
(A) n = 3, l = o, m = 0
(B) n = 3, l = 1, m = -1
(C) n = 2, l = o, m = -1
(D) n = 2, l = 1, m = 0

Answer 1

Hint: Allowed values for azimuthal quantum number (l) is 1 to n-1, where n is principal quantum number. Allowed values for magnetic quantum number (m) are ranging from ( –l) to (+l), where l is azimuthal quantum number.

Complete answer:
Here, n = principal quantum number
l = azimuthal quantum number
m = magnetic quantum number
Now let’s see all the configurations step-by-step and find which one is not possible.
(A) n = 3, l = o, m = 0
Here n = 3 and l = 0 stands for 3s orbital and allowed values for m is from –l to +l.
But here l=0, so only the allowed value for m is zero which is given in the option.
So, this is a true electronic configuration.
(B) n = 3, l = 1, m = -1
Here, n = 3 and l = 1 suggest 3p orbital and p orbital has allowed values of m starting from –l to +l.
Here l=1, so possible values of m could be -1 , 0 and +1. Here-1 is given.
So, this is also a true electronic configuration.
(C) n = 2, l = o, m = -1
Here, n = 2 and l = 0 stands for 2s orbital and as l=0 for s orbitals, allowed values of m for s orbitals are –l to +l. So, only the allowed m value for 2s orbital will be zero, but here -1 is given. So, this is not possible.
So, this electronic configuration is not possible.
(D) n = 2, l = 1, m = 0
Here, n = 2 and l = 1 stand for 2p orbital and it has three allowed values for m, which are -1, 0 and +1. Zero is given as a m value which is possible.
So, this electronic configuration is possible.

Hence we can say that correct answer is (C) n = 2, l = o, m = -1.

Additional Information:
There are some rules that govern how electrons are arranged in orbitals.
- Aufbau’s principle states that electrons always arrange in an order starting from orbitals that have low energy to highly energized orbitals.
- Hund’s rule states that electrons always arrange in orbitals in a way that they attain maximum spin multiplicity.

Note:
Remember that m value cannot be other than zero in any s orbital. Whenever these types of configurations are given in the question, make sure that you allot orbitals according to configuration and see which orbital corresponds to get a clearer idea.