Answer : The correct option is, Ar
Explanation :
(1) The element is, \(Na^+\)
Atomic number of Na = 11
The general electronic configuration of Na is, \(1s^22s^22p^63s^1\)
The electronic configuration of \(Na^+\) is, \(1s^22s^22p^6\)
The number of electrons in \(Na^+\) = 10
(2) The element is, \(Mg^{2+}\)
Atomic number of Mg = 12
The general electronic configuration of Mg is, \(1s^22s^22p^63s^2\)
The electronic configuration of \(Mg^{2+}\) is, \(1s^22s^22p^6\)
The number of electrons in \(Mg^{2+}\) = 10
(3) The element is, \(F^-\)
Atomic number of F = 9
The general electronic configuration of F is, \(1s^22s^22p^5\)
The electronic configuration of \(F^-\) is, \(1s^22s^22p^6\)
The number of electrons in \(F^-\) = 10
(4) The element is, Ar
Atomic number of Ar = 18
The general electronic configuration of Ar is, \(1s^22s^22p^63s^23p^6\)
The number of electrons in Ar = 18
(5) The element is, \(O^{2-}\)
Atomic number of O = 8
The general electronic configuration of O is, \(1s^22s^22p^4\)
The electronic configuration of \(O^{2-}\) is, \(1s^22s^22p^6\)
The number of electrons in \(O^{2-}\) = 10
From this we conclude that, the option 1, 2, 3 and 5 have the same number of electrons as the neutral atom of neon. While in 'Ar' there are 18 number of electrons.
Hence, the correct option is, Ar
