The element expected to form the largest ion to achieve the nearest noble gas configuration is:

To achieve the nearest noble gas configuration (Neon, with 10 electrons), the given elements form the following ions:

1. Nitrogen (Z=7): Gains 3 electrons to form the nitride ion, $N^{3-}$ (10 electrons).
2. Oxygen (Z=8): Gains 2 electrons to form the oxide ion, $O^{2-}$ (10 electrons).
3. Fluorine (Z=9): Gains 1 electron to form the fluoride ion, $F^{-}$ (10 electrons).
4. Sodium (Z=11): Loses 1 electron to form the sodium ion, $Na^{+}$ (10 electrons).

These ions ($N^{3-}, O^{2-}, F^{-}, Na^{+}$) are isoelectronic species because they all possess the same number of electrons (10). According to NCERT Section 3.7.1(b), for isoelectronic species, the ionic radius decreases as the nuclear charge (atomic number) increases. This is because a higher nuclear charge pulls the same number of electrons more strongly towards the nucleus.

• $N^{3-}$ has the smallest nuclear charge (+7) among the group, so the electrons are held least tightly, resulting in the largest radius.
• $Na^{+}$ has the highest nuclear charge (+11), resulting in the smallest radius.

Order of size: $N^{3-} > O^{2-} > F^{-} > Na^{+}$.