Answer : The correct option is, Ar
Explanation :
(1) The element is, [tex]Na^+[/tex]
Atomic number of Na = 11
The general electronic configuration of Na is, [tex]1s^22s^22p^63s^1[/tex]
The electronic configuration of [tex]Na^+[/tex] is, [tex]1s^22s^22p^6[/tex]
The number of electrons in [tex]Na^+[/tex] = 10
(2) The element is, [tex]Mg^{2+}[/tex]
Atomic number of Mg = 12
The general electronic configuration of Mg is, [tex]1s^22s^22p^63s^2[/tex]
The electronic configuration of [tex]Mg^{2+}[/tex] is, [tex]1s^22s^22p^6[/tex]
The number of electrons in [tex]Mg^{2+}[/tex] = 10
(3) The element is, [tex]F^-[/tex]
Atomic number of F = 9
The general electronic configuration of F is, [tex]1s^22s^22p^5[/tex]
The electronic configuration of [tex]F^-[/tex] is, [tex]1s^22s^22p^6[/tex]
The number of electrons in [tex]F^-[/tex] = 10
(4) The element is, Ar
Atomic number of Ar = 18
The general electronic configuration of Ar is, [tex]1s^22s^22p^63s^23p^6[/tex]
The number of electrons in Ar = 18
(5) The element is, [tex]O^{2-}[/tex]
Atomic number of O = 8
The general electronic configuration of O is, [tex]1s^22s^22p^4[/tex]
The electronic configuration of [tex]O^{2-}[/tex] is, [tex]1s^22s^22p^6[/tex]
The number of electrons in [tex]O^{2-}[/tex] = 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
