General Chemistry
Periodic Table Trends
Patterns across the periodic table — atomic size, ionization energy, electronegativity
Periodic trends are predictable patterns in atomic properties across rows (periods) and columns (groups) of the periodic table. Atomic radius decreases across a period (increased nuclear charge pulls electrons closer) and increases down a group (more shells). Ionization energy and electronegativity show opposite trend: increase across, decrease down. These patterns derive from electron configuration and effective nuclear charge. Mendeleev arranged elements by these patterns in 1869, predicting undiscovered elements (Ga, Sc, Ge) from gaps.
- Atomic radiusDecreases across period; increases down group
- Ionization energyIncreases across; decreases down
- ElectronegativityIncreases across; decreases down (Pauling scale)
- Electron affinityGenerally increases across (with exceptions)
- Metallic characterDecreases across; increases down
- Most electronegativeFluorine (3.98 Pauling)
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Why trends matter
- Predicting reactivity. Group properties guide reactions.
- Bond polarity. Electronegativity differences.
- Ion formation. Low IE → cations; high EA → anions.
- Material properties. Conductivity, hardness.
- Synthesis. Element selection for desired properties.
- Education. Foundation for understanding chemistry.
- Drug design. Halogen substitution affects activity.
Common misconceptions
- Trends are absolute. Many exceptions.
- Larger atoms are more reactive. Depends on electron config.
- Electronegativity is a constant. Varies by oxidation state, environment.
- Cs is most reactive metal. Fr is, technically; Cs effectively.
- Periodic table arranged by mass. By atomic number now.
- Trends apply to all blocks equally. Transition metals deviate.
Frequently asked questions
Why does atomic radius decrease across a period?
Effective nuclear charge increases. Going from Li to Ne: protons added (Li=3, Ne=10), but electrons added go to same shell (n=2). Inner shells don't shield outer electrons well — outer electrons feel stronger pull. Result: shell shrinks. Li radius: 152 pm. Ne radius: 38 pm. Same shell, much smaller.
Why does atomic radius increase down a group?
New shells added. Each row adds principal quantum number n. n=1, 2, 3... orbitals are progressively larger. Inner shells shield outer electrons from nucleus. Outer electrons see effectively reduced charge. Combined with larger orbitals: radius grows. Li: 152 pm. K: 227 pm. Cs: 265 pm.
What's electronegativity?
Tendency of an atom in a bond to attract shared electrons. Pauling scale: F=3.98 (highest), Cs=0.79 (lowest among stable). Higher electronegativity → more polar bond. Difference 0-0.5: nonpolar covalent. 0.5-2.0: polar covalent. >2.0: ionic. Used to predict bond character and molecular polarity.
What's ionization energy?
Energy to remove an electron from gaseous atom. Generally increases across (more nuclear charge); decreases down (electrons farther from nucleus). First IE: easiest electron. Second IE: next-hardest, especially after losing a noble-gas configuration. Big jumps in IE correspond to electron shell completions.
Why is fluorine most electronegative?
Smallest p-block atom that can accept an electron. High effective nuclear charge (9 protons), small radius (42 pm). Strong attraction for electrons. F⁻ has noble gas configuration ([Ne]). Highest Pauling electronegativity (3.98). Hydrogen exceptions: H is unusual but not most electronegative.
Are there exceptions?
Yes. (1) Atomic radius — slight bumps. (2) Ionization energy — Be > B (filled 2s vs unstable single 2p), N > O (half-filled stability). (3) Electron affinity — N is anomalous. (4) Transition metals — d-block trends complicated by d-electron interactions. Rules are guides, not rigid laws.
How did Mendeleev use trends?
Arranged 63 known elements (1869) by atomic mass and properties. Saw periodic patterns. Left gaps for undiscovered elements (Ga, Sc, Ge). Predicted properties accurately based on neighbors. When discovered, prediction matched. This validated the periodic table approach. Modern table arranged by atomic number (Moseley, 1913).