NEET 2023 Chemistry Trends in Ionic Sizes MCQ Question

To determine which element is expected to form the largest ion to achieve the nearest noble gas configuration, we need to analyze the elements given in the options based on their positions in the periodic table, their electronegativity, and the trends in ionic sizes.

Analysis of Options

1. Fluorine (F):

   • Atomic number: 9
   • Electron configuration: $1s^2 2s^2 2p^5$
   • To achieve the nearest noble gas configuration (Neon), fluorine needs to gain 1 electron to form the fluoride ion ($F^-$).
   • The size of the ion is relatively small because as we go from a neutral atom to an anion, there is an increase in electron-electron repulsion, but the nucleus has the same positive charge attracting more electrons, leading to a smaller ionic radius.

2. Nitrogen (N):

   • Atomic number: 7
   • Electron configuration: $1s^2 2s^2 2p^3$
   • Nitrogen needs to gain 3 electrons to achieve the nearest noble gas configuration (Neon). It forms the nitride ion ($N^{3-}$).
   • The nitride ion is larger than a neutral nitrogen atom due to increased electron repulsion, but it is still not as large as the sodium ion.

3. Sodium (Na):

   • Atomic number: 11
   • Electron configuration: $1s^2 2s^2 2p^6 3s^1$
   • Sodium needs to lose 1 electron to achieve the nearest noble gas configuration (Neon). It forms the sodium ion ($Na^+$).
   • The sodium ion is relatively large compared to anions because it has lost an electron, which reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus. However, it is not the largest among the options provided.

4. Oxygen (O):

   • Atomic number: 8
   • Electron configuration: $1s^2 2s^2 2p^4$
   • Oxygen needs to gain 2 electrons to achieve the nearest noble gas configuration (Neon), forming the oxide ion ($O^{2-}$).
   • Although the oxide ion has increased electron-electron repulsion due to added electrons, it is still not as large as the nitride ion because nitrogen's larger charge state leads to a greater ionic radius over oxygen.

Conclusion

The correct answer should be C) Na.

• Sodium ($Na^+$) forms the largest ion among the options listed because, despite being a cation, it has a larger radius than the anions formed by nitrogen and oxygen due to the lower effective nuclear charge experienced by the outer electrons after losing one electron.

Summary of Ionic Sizes

• Cations (positive ions) are generally smaller than their neutral atoms because they lose electrons, which reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus.
• Anions (negative ions) are larger than their neutral atoms because they gain electrons, increasing electron-electron repulsion.

Key Concept

The relative sizes of ions can be summarized by the following trend:

• Ionic size increases down a group in the periodic table due to the addition of electron shells.
• Ionic size decreases across a period from left to right due to increasing nuclear charge attracting electrons more strongly.

In this case, among the given options, Sodium (Na) forms the largest ion to achieve the nearest noble gas configuration, thus making C) Na the correct answer.