Classification of Elements and Periodicity
Periodic law, atomic radius, ionization energy, electron affinity, electronegativity, periodic trends.
Atomic radius and ionic radius
Trends across period and down group.
Five trends to master. All follow from effective nuclear charge (Z_eff) and shell number (n).
1. Atomic radius.
- Decreases across period (L → R): more protons pulling same shell.
- Increases down group: new shells added.
- Smallest: He, then F. Largest in period 2: Li.
2. Ionic radius.
- Cation < parent atom (lost electrons, sometimes lost a shell).
- Anion > parent atom (gained electrons, same shell, more repulsion).
- Isoelectronic species (same electron count): radius decreases as Z increases. N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺.
3. Ionization energy (IE). Energy to remove the most loosely bound electron.
- Increases across period: higher Z_eff.
- Decreases down group: outer electron farther from nucleus.
- Anomalies (memorize):
- Be > B: removing 2p (from B) is easier than removing 2s (from Be).
- N > O: half-filled 2p³ of N is stable; removing one electron from 2p⁴ of O is easier.
- IE₁ < IE₂ < IE₃ < ... always (each successive electron harder).
- Big jumps after a noble gas configuration.
4. Electron affinity (EA). Energy released when an electron is added.
- Mostly increases across period.
- Decreases down group.
- Cl > F (smaller F has more electron-electron repulsion in already crowded 2p).
- Noble gases: EA ≈ 0 (or slightly endothermic).
5. Electronegativity (EN). Tendency to attract a shared bonding pair (Pauling scale).
- Increases across period (small + high Z_eff).
- Decreases down group.
- F > O > N > Cl > Br ≈ C ≈ S ≈ I > P > H... (memorize the top 5).
- Most electronegative: F (4.0).
- Least: Cs (0.7).
Quick comparison patterns (JEE love):
- Metallic character: decreases across, increases down.
- Non-metallic character: increases across, decreases down.
- Acidic strength of oxides: increases across (Na₂O basic → SiO₂ amphoteric → Cl₂O₇ strongly acidic).
- Basic strength of hydroxides: decreases across, increases down.
Trends summary diagram:
← Atomic radius increases
← Metallic character increases
← Ionization energy decreases ↓
← Electronegativity decreases ↓
The arrow ↓ also means: from top to bottom of the table, those properties decrease/increase as written.
Ionization energy and electron affinity
Trends, exceptions (Be vs B, N vs O).
Five trends to master. All follow from effective nuclear charge (Z_eff) and shell number (n).
1. Atomic radius.
- Decreases across period (L → R): more protons pulling same shell.
- Increases down group: new shells added.
- Smallest: He, then F. Largest in period 2: Li.
2. Ionic radius.
- Cation < parent atom (lost electrons, sometimes lost a shell).
- Anion > parent atom (gained electrons, same shell, more repulsion).
- Isoelectronic species (same electron count): radius decreases as Z increases. N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺.
3. Ionization energy (IE). Energy to remove the most loosely bound electron.
- Increases across period: higher Z_eff.
- Decreases down group: outer electron farther from nucleus.
- Anomalies (memorize):
- Be > B: removing 2p (from B) is easier than removing 2s (from Be).
- N > O: half-filled 2p³ of N is stable; removing one electron from 2p⁴ of O is easier.
- IE₁ < IE₂ < IE₃ < ... always (each successive electron harder).
- Big jumps after a noble gas configuration.
4. Electron affinity (EA). Energy released when an electron is added.
- Mostly increases across period.
- Decreases down group.
- Cl > F (smaller F has more electron-electron repulsion in already crowded 2p).
- Noble gases: EA ≈ 0 (or slightly endothermic).
5. Electronegativity (EN). Tendency to attract a shared bonding pair (Pauling scale).
- Increases across period (small + high Z_eff).
- Decreases down group.
- F > O > N > Cl > Br ≈ C ≈ S ≈ I > P > H... (memorize the top 5).
- Most electronegative: F (4.0).
- Least: Cs (0.7).
Quick comparison patterns (JEE love):
- Metallic character: decreases across, increases down.
- Non-metallic character: increases across, decreases down.
- Acidic strength of oxides: increases across (Na₂O basic → SiO₂ amphoteric → Cl₂O₇ strongly acidic).
- Basic strength of hydroxides: decreases across, increases down.
Trends summary diagram:
← Atomic radius increases
← Metallic character increases
← Ionization energy decreases ↓
← Electronegativity decreases ↓
The arrow ↓ also means: from top to bottom of the table, those properties decrease/increase as written.
Electronegativity and metallic character
Pauling scale, period and group trends.
Five trends to master. All follow from effective nuclear charge (Z_eff) and shell number (n).
1. Atomic radius.
- Decreases across period (L → R): more protons pulling same shell.
- Increases down group: new shells added.
- Smallest: He, then F. Largest in period 2: Li.
2. Ionic radius.
- Cation < parent atom (lost electrons, sometimes lost a shell).
- Anion > parent atom (gained electrons, same shell, more repulsion).
- Isoelectronic species (same electron count): radius decreases as Z increases. N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺.
3. Ionization energy (IE). Energy to remove the most loosely bound electron.
- Increases across period: higher Z_eff.
- Decreases down group: outer electron farther from nucleus.
- Anomalies (memorize):
- Be > B: removing 2p (from B) is easier than removing 2s (from Be).
- N > O: half-filled 2p³ of N is stable; removing one electron from 2p⁴ of O is easier.
- IE₁ < IE₂ < IE₃ < ... always (each successive electron harder).
- Big jumps after a noble gas configuration.
4. Electron affinity (EA). Energy released when an electron is added.
- Mostly increases across period.
- Decreases down group.
- Cl > F (smaller F has more electron-electron repulsion in already crowded 2p).
- Noble gases: EA ≈ 0 (or slightly endothermic).
5. Electronegativity (EN). Tendency to attract a shared bonding pair (Pauling scale).
- Increases across period (small + high Z_eff).
- Decreases down group.
- F > O > N > Cl > Br ≈ C ≈ S ≈ I > P > H... (memorize the top 5).
- Most electronegative: F (4.0).
- Least: Cs (0.7).
Quick comparison patterns (JEE love):
- Metallic character: decreases across, increases down.
- Non-metallic character: increases across, decreases down.
- Acidic strength of oxides: increases across (Na₂O basic → SiO₂ amphoteric → Cl₂O₇ strongly acidic).
- Basic strength of hydroxides: decreases across, increases down.
Trends summary diagram:
← Atomic radius increases
← Metallic character increases
← Ionization energy decreases ↓
← Electronegativity decreases ↓
The arrow ↓ also means: from top to bottom of the table, those properties decrease/increase as written.