Which of the following electron configurations is not consistent with Hund's rule?
1) 1s^2 2s^2 2p^2 3s^1
2) 1s^2 2s^2 2p^6 3s^2 3p^5
3) 1s^2 2s^2 2p^6 3s^3 3p^6
4) 1s^2 2s^2 2px^2 2py^1
Hund's rules has to do with the electrons being paired only when necessary; i.e., after all of the orbitals are half filled, the next electron pairs with one already there. In answering this question, don't confuse Hund's rule with the Aufbau Principle. In other words, don't pick #1 for the correct answer.
I thought it was #3 because there is no
such thing as 3s^3. Am I right?
The electron configuration that is not consistent with Hund's rule is option 4) 1s^2 2s^2 2px^2 2py^1.
Hund's rule states that when filling orbitals of equal energy, electrons will occupy empty orbitals singly, before pairing up. In option 4, the electron configuration shows that the 2p subshell is not completely filled before electrons start to pair up. According to Hund's rule, the 2p subshell should be filled with one electron in each orbital (2px^1, 2py^1, and 2pz^1) before any pairing occurs. Therefore, option 4 is not consistent with Hund's rule.
To determine which electron configuration is not consistent with Hund's rule, let's briefly review Hund's rule. Hund's rule states that when filling orbitals of equal energy, electrons will fill each orbital with parallel spins before pairing up.
Now let's analyze each option:
1) 1s^2 2s^2 2p^2 3s^1
This configuration follows Hund's rule. Each orbital is occupied with parallel spins before pairing up.
2) 1s^2 2s^2 2p^6 3s^2 3p^5
This configuration also follows Hund's rule. Each of the p orbitals is occupied with parallel spins before pairing up.
3) 1s^2 2s^2 2p^6 3s^3 3p^6
This configuration follows Hund's rule as well. Each of the s and p orbitals is occupied with parallel spins before pairing up.
4) 1s^2 2s^2 2px^2 2py^1
This configuration does not follow Hund's rule. According to Hund's rule, the electron in the 2py orbital should have a parallel spin with the electrons in the 2px orbitals, before pairing up. However, in this configuration, the 2px orbitals are already paired up before the 2py orbital is filled.
Therefore, option 4) 1s^2 2s^2 2px^2 2py^1 is the electron configuration that is not consistent with Hund's rule.