Which one of the following ions is not iso-electronic with F~ ?

Isoelectronic species are atoms or ions that possess the same number of electrons and identical electronic configurations. The fluoride ion (F⁻) is formed when a fluorine atom (atomic number 9) gains one electron, resulting in a total of 10 electrons. This configuration (1s²2s²2p⁶) matches the noble gas neon (Ne), which naturally has 10 electrons. Similarly, the oxide ion (O²⁻) gains two electrons (8+2=10) and the sodium ion (Na⁺) loses one electron (11-1=10), making them both isoelectronic with F⁻. However, nitrogen (atomic number 7) has 7 electrons [2]. A nitrogen anion (N⁻) would have only 8 electrons (7+1), which does not match the 10-electron configuration of F⁻. Therefore, N⁻ is not isoelectronic with the fluoride ion.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.1 BONDING IN CARBON – THE COVALENT BOND > p. 60