Which of the following electronic configurations is NOT possible for any neutral atom?
2, 10, 9
The maximum number of electrons that a shell can hold is given by 2n2, where n is the shell number. So the K shell (n=1) holds up to 2, the L shell (n=2) up to 8, and the M shell (n=3) up to 18.
The configuration 2, 10, 9 places 10 electrons in the L shell, which is impossible because L can hold a maximum of 8. The other three configurations (2, 8, 3 – aluminium; 2, 8, 2 – magnesium; 2, 8, 8 – argon) all respect the 2n2 rule and correspond to real neutral atoms.
Hence, the answer is 2, 10, 9.
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