Trends
High-Yield Summary
- Effective nuclear charge (Zeff) = nuclear charge minus inner-shell shielding — the driver behind every periodic trend.
- Across a period (left→right): Zeff rises steadily (new electrons don't add much shielding). Down a group: Zeff stays roughly constant (added shielding offsets growing nuclear charge).
- Atomic radius: decreases across a period, increases down a group. Cations are smaller than their neutral atom; anions are larger; ionic radius increases down a group.
- Ionization energy (endothermic; always first < second < third...): increases across a period, decreases down a group.
- Electronegativity (Pauling scale) and electron affinity (exothermic) both increase across a period, decrease down a group — generally track together, except noble gases (high ionization energy, negligible electronegativity).
- The octet rule (tendency toward 8 valence electrons) explains much of this behavior but has many exceptions, especially transition metals and heavier elements.
Key Terms
- Effective nuclear charge (Zeff)
- Net positive charge valence electrons actually experience; nuclear charge minus inner-shell shielding.
- Shielding
- Inner-shell electrons partially blocking the nucleus's pull on valence electrons.
- Ionization energy
- Energy required to remove an electron from a gaseous atom or ion; always endothermic.
- Electron affinity
- Energy change when a neutral atom gains an electron to form a negative ion; typically exothermic.
The Five Trends: Across a Period (Left → Right)
| Trend | Direction |
|---|---|
| Atomic radius | Decreases |
| Ionic radius | Cations shrink, anions grow (relative to neutral atom) |
| Ionization energy | Increases |
| Electronegativity | Increases |
| Electron affinity | Increases (more exothermic) |
The Five Trends: Down a Group (Top → Bottom)
| Trend | Direction |
|---|---|
| Atomic radius | Increases |
| Ionic radius | Increases |
| Ionization energy | Decreases |
| Electronegativity | Decreases |
| Electron affinity | Decreases (less exothermic) |
Key Groups and Their Properties
| Group | Valence Electrons / Oxidation State / Behavior |
|---|---|
| Alkali metals (Group 1) | 1 valence e⁻, +1 state; largest radii in period, very reactive with water |
| Alkaline earth metals (Group 2) | 2 valence e⁻, +2 state; reactive with water (less than Group 1) |
| Chalcogens (Group 16) | 6 valence e⁻, −2 (nonmetal) or +6 (metal) state; O and S biologically essential |
| Halogens (Group 17) | 7 valence e⁻, −1 state; highest electronegativity/electron affinity |
| Noble gases (Group 18) | Full valence shell; very high ionization energy, essentially unreactive |
| Transition metals (Groups 3–12) | Multiple oxidation states; low ionization energy/electronegativity/electron affinity vs. nonmetals |
Common MCAT Trap
- Zeff stays roughly CONSTANT down a group (shielding offsets rising nuclear charge) — it doesn't decrease, even though ionization energy and electronegativity both decrease down a group (due to increasing distance, not decreasing Zeff).
- Noble gases have very HIGH ionization energy but NEGLIGIBLE electronegativity — the one clear break in the 'electronegativity tracks ionization energy' pattern.
- Ionic radius trend: cations are smaller and anions are larger than their neutral parent atom — a frequently flipped fact under time pressure.
Quick Recall
Why does effective nuclear charge stay roughly constant down a group instead of increasing with the growing nuclear charge?
Is ionization energy exothermic or endothermic, and why?
Which group has the highest electronegativity, and which has essentially none despite high ionization energy?
Is a cation larger or smaller than its neutral parent atom, and why?