Atomic and Ionic Sizes - Class 12 Chemistry
1. Introduction to Atomic and Ionic Sizes
Atomic size refers to the distance from the nucleus to the outermost electron shell. Ionic size, on the
other hand, is the radius of an ion, which may be smaller or larger than its atomic size due to loss or
gain of electrons.
2. Factors Affecting Atomic Size
- Nuclear Charge: Higher nuclear charge pulls electrons closer, reducing size.
- Number of Electron Shells: More shells mean larger atomic size.
- Shielding Effect: Inner electrons shield outer electrons from nuclear attraction, increasing size.
- Effective Nuclear Charge (Z_eff): Higher Z_eff leads to smaller atomic size.
3. Trends in Atomic Radius Across Periods and Groups
- **Across a Period:** Atomic size decreases from left to right due to increasing nuclear charge.
- **Down a Group:** Atomic size increases due to the addition of new electron shells.
4. Ionic Radius and Factors Affecting It
- **Cations (Positive Ions):** Smaller than their parent atoms because they lose electrons and have
higher Z_eff.
- **Anions (Negative Ions):** Larger than their parent atoms because they gain electrons, reducing
Z_eff.
- **Isoelectronic Species:** Ions with the same number of electrons; their size depends on nuclear
charge (e.g., O2- > F- > Na+).
5. Comparison of Atomic and Ionic Radii
- Neutral atoms generally have a balanced size.
- Cations are smaller, while anions are larger than their neutral atoms.
- Periodic trends affect both atomic and ionic radii similarly.
6. Applications and Importance
- Helps understand chemical bonding and reactivity.
- Explains periodic trends and element properties.
1. Introduction to Atomic and Ionic Sizes
Atomic size refers to the distance from the nucleus to the outermost electron shell. Ionic size, on the
other hand, is the radius of an ion, which may be smaller or larger than its atomic size due to loss or
gain of electrons.
2. Factors Affecting Atomic Size
- Nuclear Charge: Higher nuclear charge pulls electrons closer, reducing size.
- Number of Electron Shells: More shells mean larger atomic size.
- Shielding Effect: Inner electrons shield outer electrons from nuclear attraction, increasing size.
- Effective Nuclear Charge (Z_eff): Higher Z_eff leads to smaller atomic size.
3. Trends in Atomic Radius Across Periods and Groups
- **Across a Period:** Atomic size decreases from left to right due to increasing nuclear charge.
- **Down a Group:** Atomic size increases due to the addition of new electron shells.
4. Ionic Radius and Factors Affecting It
- **Cations (Positive Ions):** Smaller than their parent atoms because they lose electrons and have
higher Z_eff.
- **Anions (Negative Ions):** Larger than their parent atoms because they gain electrons, reducing
Z_eff.
- **Isoelectronic Species:** Ions with the same number of electrons; their size depends on nuclear
charge (e.g., O2- > F- > Na+).
5. Comparison of Atomic and Ionic Radii
- Neutral atoms generally have a balanced size.
- Cations are smaller, while anions are larger than their neutral atoms.
- Periodic trends affect both atomic and ionic radii similarly.
6. Applications and Importance
- Helps understand chemical bonding and reactivity.
- Explains periodic trends and element properties.
