Periodic Trends (The "Big Three")
Mastering the interplay of Nuclear Charge and Shielding.
The "Zeff" Core Logic
To score a 7, explain every trend using the interplay of Nuclear Charge (Z) and Shielding (S).
1. Atomic Radius
Across a Period (→)
Decreases
Why? Protons (\(Z\)) increase, but shielding (\(S\)) stays constant (same shell). Zeff increases, pulling the valence shell closer.
Down a Group (↓)
Increases
Why? The number of occupied shells (\(n\)) increases. Valence electrons are physically further from the nucleus.
2. Ionic Radius
Cations (+)
Smaller than parent atom. (Loss of valence shell + increased effective pull).
Anions (−)
Larger than parent atom. (Same Z, but increased electron-electron repulsion expands the cloud).
The Isoelectronic Trap
Isoelectronic Series: Ions with the same electron configuration.
\( N^{3-},\; O^{2-},\; F^-,\; Na^+,\; Mg^{2+},\; Al^{3+} \)
Trend: As proton number (\(Z\)) increases, radius decreases drastically.
\(N^{3-}\) (7 protons) pulls weakly on 10 electrons; \(Al^{3+}\) (13 protons) pulls strongly on 10 electrons.
3. Ionization Energy & Electronegativity
First Ionization Energy (IE)
Energy to remove 1 mol of electrons from 1 mol of gaseous atoms: \( M(g) \to M^+(g) + e^- \)
- Trend: Increases across a period (higher Zeff). Decreases down a group (more shielding).
Electronegativity
Ability to attract a shared pair of electrons.
- Trend: Matches IE. Fluorine is most electronegative (4.0); Francium is least (0.7).