1. Occurrence
s-Block Elements
- Highly reactive, hence never found in free state. Occur mainly as salts in minerals, sea water, and earth’s crust.
- Group 1 (Alkali): NaCl (rock salt), KCl (sylvine), Li in spodumene.
- Group 2 (Alkaline earth): CaCO₃ (limestone), Mg in magnesite & sea water, Be in beryl.
p-Block Elements
- Found in free as well as combined state due to variable reactivity.
- Non-metals: O₂, N₂ in atmosphere; S in native form; C as diamond/graphite/coal.
- Metals: Al in bauxite, Sn in cassiterite, Pb in galena.
- Metalloids: Si in quartz/sand, Ge, As in ores.
Trend: s-block elements occur only in combined state; p-block shows both free and combined forms.
2. Electronic Configuration
| Property | s-Block | p-Block |
|---|---|---|
| General Configuration | ns¹ (Group 1) or ns² (Group 2) | ns² np¹⁻⁶ (Group 13–18) |
| Valence electrons | 1 or 2 | 3 to 8 |
| Example | Li: 1s² 2s¹ Na: [Ne] 3s¹ |
B: 1s² 2s² 2p¹ Cl: [Ne] 3s² 3p⁵ |
Trend: s-block always ends with s-orbital. p-block shows progressive filling of p-orbitals, leading to variable oxidation states.
3. Atomic & Ionic Radii
s-Block
- Largest atomic radii in their respective periods due to lowest effective nuclear charge (Zeff).
- Atomic radius increases down the group (Li → Cs, Be → Ba).
- Ionic radii: M⁺ (Group 1) > M²⁺ (Group 2) for same period due to higher charge in Group 2.
p-Block
- Smaller than s-block in the same period because of higher Zeff.
- Decreases across the period (B → Ne); increases down the group.
- Exception: Ga has slightly smaller radius than Al due to d-block contraction.
Trend: Atomic radius → s-block > p-block (same period). Increases down the group in both blocks.
4. Density
s-Block
- Low density (Li is lightest metal). Increases down the group except K < Na due to atomic size increase.
- Group 2 has higher density than Group 1 in the same period.
p-Block
- Generally higher than s-block. Non-metals have very low density (C, N, O, F gases/solids).
- Metals (Al, Sn, Pb) have high density; increases down the group.
Trend: Density increases down the group in both blocks due to increase in atomic mass.
5. Ionization Potential (IE)
s-Block
- Very low IE due to large size and low Zeff. Decreases down the group (Li > Na > K > Rb > Cs).
- Group 2 has higher IE than Group 1 (higher nuclear charge).
p-Block
- Higher IE than s-block. Increases across the period; decreases down the group.
- Exception: Group 15 (N, P) have higher IE than Group 16 due to half-filled p³ stability.
Trend: IE: s-block << p-block (same period). Lowest for Cs (s-block), highest for noble gases (p-block).
Read more: Ionization Potential
6. Metallic Behaviour
s-Block
- All are highly metallic (good conductors, malleable, ductile, lustrous).
- Metallic character decreases slightly from Group 1 → Group 2.
p-Block
- Shows gradation: metals (left) → metalloids → non-metals (right).
- Metallic character increases down the group (B non-metal → Tl metal).
Trend: s-block → 100% metallic. p-block shows complete range of metallic to non-metallic behaviour.
7. Electropositive Nature
s-Block
- Highly electropositive (tendency to lose electrons). Decreases down the group? No → increases down the group (Cs most electropositive).
- Group 1 > Group 2.
p-Block
- Less electropositive than s-block. Decreases across the period; increases down the group.
Trend: Electropositivity: s-block >> p-block. Cs is the most electropositive element.
8. Electronegativity (Pauling Scale)
s-Block
- Very low (Li = 1.0, Cs = 0.7). Decreases down the group.
p-Block
- Higher than s-block. Increases across the period (F = 4.0 is highest). Decreases down the group.
Trend: Electronegativity: s-block << p-block. Highest in halogens, lowest in alkali metals.
Read more: Electronegativity
9. Electron Affinity
s-Block
- Low (sometimes endothermic for Group 2). Group 1 has small positive EA.
p-Block
- Higher (more exothermic) especially halogens (Cl has highest EA). Group 15 and noble gases have low/positive values due to stable configuration.
- Decreases down the group.
Trend: Electron affinity is low in s-block and high (negative) in p-block non-metals.
Read more: Electron Affinity
10. Hydration Energy
s-Block
- Very high due to small size & high charge. Decreases down the group (Li⁺ highest).
- Group 2 > Group 1 for same period.
p-Block
- Lower than s-block cations of same charge. Small ions like Al³⁺, Be²⁺ show very high hydration energy (anomalous).
Trend: Hydration energy ∝ charge / size. Highest for small highly charged ions (Be²⁺, Al³⁺).
Read more: Solvation Energy
11. Flame Colouration
s-Block
- Characteristic flame colours due to low IE (electrons easily excited).
- Li: Crimson red, Na: Golden yellow, K: Violet, Rb: Red-violet, Cs: Blue.
- Group 2: Ca: Brick red, Sr: Crimson, Ba: Apple green.
p-Block
- Generally no characteristic flame colour (high IE). Some exceptions like Cu (green) but Cu is d-block.
Trend: Only s-block (especially alkali & alkaline earth) show prominent flame colouration.
12. Photoelectric Effect
s-Block
- Alkali metals (K, Rb, Cs) show photoelectric effect because of very low IE. Cs is used in photocells.
p-Block
- No photoelectric effect (higher IE).
Trend: Observed only in highly electropositive s-block metals (K, Rb, Cs).
Read more: photoelectric effect
13. Polarization Power (Fajans' Rule)
s-Block
- Low for larger ions (K⁺, Rb⁺). High for small Li⁺ and Be²⁺ → covalent character (LiCl, BeCl₂ covalent).
p-Block
- Higher polarization by small highly charged cations (Al³⁺, Sn⁴⁺) leading to covalent compounds.
Trend: Polarizing power ∝ charge / size. Small size + high charge → high polarization (Li⁺, Be²⁺, Al³⁺).
Realted Topics:
Fajans' Rule
Polarization and Polarizibility
14. Boiling and Melting Point
s-Block
- Low melting & boiling points due to weak metallic bonding. Decreases down Group 1; irregular in Group 2.
- Li has highest mp in Group 1 due to small size.
p-Block
- Wide variation: Non-metals have low mp/bp (N₂, O₂ gases); metals have high (Al, Sn, Pb); metalloids intermediate.
- Decreases down the group for metals due to weaker metallic bonds.
Trend: s-block → low mp/bp. p-block shows large variation (gases → high-melting solids).
Basic Properties of Iodine (I)
Iodine is a chemical element with the symbol I and atomic number 53. It is the heaviest of the stable halogens and exists as a lustrous, purple-black non-metallic solid under standard conditions.
1. Physical Properties
| Property | Value / Description |
|---|---|
| Atomic Number | 53 |
| Atomic Mass | 126.904 u |
| Phase at STP | Solid |
| Melting Point | 113.7°C |
| Boiling Point | 184.3°C |
| Appearance | Lustrous, dark grey/purple-black crystals |
2. Chemical Properties
- Reactivity: It is the least reactive of the stable halogens, though it still forms compounds with many elements.
- Sublimation: One of its most notable features is its ability to undergo sublimation, where it transitions directly from a solid to a violet gas when heated.
- Electronegativity: It has an electronegativity of 2.66 on the Pauling scale.
- Solubility: It is only slightly soluble in water but dissolves easily in organic solvents like chloroform and carbon tetrachloride, producing violet solutions.
3. Biological Importance
Iodine is an essential trace element for humans. It is a key component of thyroid hormones, which regulate metabolic rate, protein synthesis, and proper skeletal and central nervous system development in fetuses and infants.