Introduction
To arrange O²-, Mg²+, and Na+ in order of increasing ionic radius, we need to consider their respective charges and electron configurations.
Understanding Ionic Radii
- Cation vs. Anion: Cations (positively charged ions) are generally smaller than their neutral atoms due to the loss of electrons and increased nuclear charge affecting the remaining electrons. Anions (negatively charged ions) are larger than their neutral atoms due to the gain of electrons, which increases electron-electron repulsion.
Electron Configurations
- O²-: The oxide ion has gained two electrons, resulting in a total of 10 electrons (1s² 2s² 2p⁶).
- Mg²+: The magnesium ion has lost two electrons, resulting in a total of 10 electrons as well (1s² 2s² 2p⁶).
- Na+: The sodium ion has lost one electron, resulting in 10 electrons (1s² 2s² 2p⁶) but has a lower effective nuclear charge compared to Mg²+.
Order of Ionic Radius
1. Mg²+: Smallest ionic radius due to higher positive charge (12 protons) pulling the same number of electrons closer to the nucleus.
2. Na+: Larger than Mg²+ since it has a lower positive charge (11 protons) and thus a weaker pull on the electron cloud.
3. O²-: Largest ionic radius due to the addition of two electrons, leading to increased electron-electron repulsion.
Final Arrangement
- Increasing order of ionic radius: Mg²+ < na+="" />< />.