The ionic radius of a nonmetal is typically larger than its atomic radius.
This difference in size arises from the nonmetal atom gaining one or more electrons to achieve a stable electron configuration, often fulfilling the octet rule. Here's a detailed breakdown:
Why Nonmetals Form Anions
Nonmetals, found on the right side of the periodic table, generally have a high electronegativity. This means they have a strong tendency to attract electrons. To achieve a stable valence shell (usually eight electrons, similar to a noble gas configuration), nonmetal atoms will gain electrons, forming negative ions called anions.
Effect of Gaining Electrons on Radius
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Increased Electron-Electron Repulsion: When a nonmetal atom gains electrons, the number of electrons increases while the number of protons in the nucleus remains the same. This leads to a greater electron-electron repulsion within the electron cloud.
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Expansion of the Electron Cloud: The increased repulsion causes the electron cloud to expand outwards, resulting in a larger ionic radius compared to the original atomic radius.
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Reduced Effective Nuclear Charge: The effective nuclear charge is the net positive charge experienced by an electron in the atom. When electrons are added, they partially shield the existing electrons from the full force of the nucleus. This reduces the effective nuclear charge experienced by each electron, further allowing the electron cloud to expand.
Example: Chlorine
Consider chlorine (Cl). Its atomic number is 17, meaning it has 17 protons and 17 electrons. Its electron configuration is [Ne] 3s² 3p⁵. To achieve a stable octet, chlorine gains one electron to become Cl⁻, with the electron configuration [Ne] 3s² 3p⁶.
- Atomic radius of Cl: ~100 pm
- Ionic radius of Cl⁻: ~181 pm
The significantly larger ionic radius of Cl⁻ is a direct result of the added electron and the subsequent increase in electron-electron repulsion and reduced effective nuclear charge.
Table Summarizing the Difference
| Feature | Atomic Radius (Nonmetal) | Ionic Radius (Nonmetal Anion) |
|---|---|---|
| Number of Electrons | Lower | Higher |
| Electron-Electron Repulsion | Lower | Higher |
| Effective Nuclear Charge | Higher | Lower |
| Size | Smaller | Larger |
Conclusion
In summary, the ionic radius of a nonmetal is larger than its atomic radius because gaining electrons increases electron-electron repulsion, reduces the effective nuclear charge, and causes the electron cloud to expand.
