Periodic Table Trends: Atomic & Chemical Properties
The variation in properties like Atomic Radius, Ionization Energy, and Electronegativity across periods and groups forms the backbone of Inorganic Chemistry. Mastering these trends is essential for predicting chemical behavior.
1. Atomic & Ionic Radius
Atomic radius is the distance from the center of the nucleus to the outermost shell of electrons.
Due to increased Effective Nuclear Charge ($Z_{eff}$).
Due to addition of new principal shells.
Ionic Radius Rule
For the same element:
Reason: Cations have fewer electrons (higher $Z_{eff}$), while Anions have more electrons (increased repulsion/screening).
Figure: Summary of Periodic Trends for Main Group Elements
2. Ionization Enthalpy ($\Delta_i H$)
The energy required to remove the most loosely bound electron from an isolated gaseous atom.
Trend: Increases across a period, Decreases down a group.
Critical Exceptions
- Half-filled/Full-filled Stability: Elements with stable configurations have abnormally high $\Delta_i H$.
Example: $\text{N} (2p^3) > \text{O} (2p^4)$ and $\text{Be} (2s^2) > \text{B} (2p^1)$. - Successive IE: Always increases: $\text{IE}_3 > \text{IE}_2 > \text{IE}_1$.
3. Electron Gain Enthalpy ($\Delta_{eg} H$)
The energy change when an electron is added to a neutral gaseous atom. It generally becomes more negative across a period.
The Halogen Exception
Although Fluorine is the most electronegative, Chlorine has the most negative Electron Gain Enthalpy.
- Reason: Fluorine's small size leads to strong inter-electronic repulsion, making it harder to accept an incoming electron compared to the larger Chlorine atom.
4. Electronegativity (EN)
The tendency of an atom to attract the shared pair of electrons in a chemical bond.
Trend: Increases across a period ($\text{F}$ is max: 4.0), Decreases down a group ($\text{Cs}$ is min: 0.7).
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