Core Logic3.1.2

Periodic Trends (The "Big Three")

Mastering the interplay of Nuclear Charge and Shielding.

The "Zeff" Core Logic

To score a 7, you must explain every trend using the interplay of Nuclear Charge (Z) and Shielding (S).

Z_{eff} \approx Z - S

Interactive Periodic Trends

Select a property to visualize its trend across the table.

Select a trend above to visualize arrows and explanations.

1. Atomic Radius

Across a Period (→)

Decreases

Why? Protons ( $Z$ ) increase, but shielding ( $S$ ) remains constant (electrons added to same shell). Therefore, Zeff increases, pulling the valence shell closer.

Down a Group (↓)

Increases

Why? The number of occupied shells ( $n$ ) increases. The valence electrons are physically further from the nucleus, dominating the increase in Zeff.

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.

Example:

N^{3-}, O^{2-}, F^-, Na^+, Mg^{2+}, Al^{3+}

Trend: As Proton number ( $Z$ ) increases, Radius Decreases drastically.
Logic: $N^{3-}$ (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).

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