Periodic Trends
Chemistry · आवर्ती प्रवृत्तियां
📋Quick Overview
As we move across a period (left to right) or down a group (top to bottom) in the periodic table, certain properties change in a predictable pattern. The four key trends are: Atomic Radius, Ionization Energy, Electron Affinity, and Electronegativity. Understanding these trends helps predict element behavior.
📖Four Key Periodic Trends
| Property | Definition | Across Period (→) | Down Group (↓) |
|---|---|---|---|
| Atomic Radius | Size of the atom (distance from nucleus to outermost electron) | DECREASES (more protons pull electrons closer) | INCREASES (new shells added) |
| Ionization Energy (IE) | Energy needed to remove outermost electron from neutral atom | INCREASES (smaller atom holds electrons tighter) | DECREASES (outermost electron is farther, easier to remove) |
| Electron Affinity (EA) | Energy released when atom gains an electron | INCREASES (atom wants to complete octet) | DECREASES (larger atom, less attraction for extra electron) |
| Electronegativity (EN) | Tendency of atom to attract shared electrons in a bond | INCREASES (smaller atom attracts more) | DECREASES (larger atom, weaker pull) |
📝Extreme Values — Important for Exams
- •Smallest atom → Helium (He). Smallest among reactive elements → Hydrogen (H)
- •Largest atom → Francium (Fr) — bottom of Group 1
- •Highest ionization energy → Helium (He)
- •Lowest ionization energy → Francium (Fr) or Caesium (Cs)
- •Highest electronegativity → Fluorine (F) = 4.0 on Pauling scale
- •Highest electron affinity → Chlorine (Cl) — NOT Fluorine (due to small size, electron repulsion)
- •Noble gases have very high IE and zero EA (complete octet, no need for electrons)
📖Summary of Trend Directions
| Property | Across Period (L→R) | Down Group (T→B) | Easy Memory |
|---|---|---|---|
| Atomic Radius | Decreases ↓ | Increases ↑ | Opposite of other three |
| Ionization Energy | Increases ↑ | Decreases ↓ | Same as EN and EA |
| Electron Affinity | Increases ↑ | Decreases ↓ | Same as IE and EN |
| Electronegativity | Increases ↑ | Decreases ↓ | Same as IE and EA |
| Metallic Character | Decreases ↓ | Increases ↑ | Same direction as atomic radius |
| Non-metallic Character | Increases ↑ | Decreases ↓ | Same as EN |