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METALLOIDS
Metalloids are chemical elements with properties between metals and nonmetals, forming a jagged line on the periodic table (e.g., Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium) and sometimes Polonium and Astatine.
They typically have a metallic look but are brittle solids, possess moderate electrical conductivity (making them semiconductors), and are crucial in electronics,

Key Characteristics
Appearance: Metallic luster (shiny).
Conductivity: Fair to good semiconductors, better than nonmetals but not as good as metals.
Brittleness: Brittle and fragile.
State: Solid at room temperature.
Bonding: Can form covalent bonds (like nonmetals).
Common Metalloids
Boron (B)
Silicon (Si)
Germanium (Ge)
Arsenic (As)
Antimony (Sb)
Tellurium (Te)
Polonium (Po) (sometimes)
Astatine (At) (sometimes)
Uses
Electronics: Silicon in semiconductors for computers and solar panels.
Alloys: Used with metals.
Other: Catalysts, flame retardants, and optical storage.

*Oxides* are chemical compounds where *oxygen is combined with another element* (metal or non‑metal). The oxygen usually has a -2 oxidation state (O²⁻).

- *Metal oxides* → Basic (or amphoteric)
_Example:_ Na₂O (sodium oxide), Fe₂O₃ (iron(III) oxide – rust), CaO (quicklime).

- *Non‑metal oxides* → Acidic (or neutral)
_Example:_ CO₂ (carbon dioxide), SO₂ (sulfur dioxide), NO₂ (nitrogen dioxide).

- *Amphoteric oxides* → Can act as both acid and base
_Example:_ Al₂O₃ (aluminium oxide), ZnO (zinc oxide).

Simple formation
*Element + O₂ → Oxide*
- 4 Na + O₂ → 2 Na₂O (sodium oxide)
- S + O₂ → SO₂ (sulfur dioxide)

Properties (in short)
- *Basic oxides* + water → bases (Na₂O + H₂O → 2 NaOH)
- *Acidic oxides* + water → acids (SO₂ + H₂O → H₂SO₃)

- *Insoluble oxides* (like MgO, Fe₂O₃) don’t dissolve in water but react with acids/bases.

1. Classification of Oxides
Type , Typical Elements , Reaction with Water, Example

Basic, (Metallic) Oxides Alkali (Na, K) & Alkaline Earth ,(Ca, Mg) Form hydroxides (alkalis) Na₂O → 2 NaOH, CaO → Ca(OH)₂

Acidic ,(Non‑metal) Oxides Non‑metals (S, P, C, N) ,Form acids SO₂ → H₂SO₃, CO₂ → H₂CO₃

Amphoteric, Oxides Some metals (Al, Zn, Sn, Pb) ,React with both acids & bases Al₂O₃ + 6 HCl → 2 AlCl₃ + 3 H₂O
Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄

Neutral Oxides CO, NO, N₂O, H₂O No acid/base reaction CO – doesn’t form acid or base

Peroxides O₂²⁻ (O–O bond) Form H₂O₂ with water/acid Na₂O₂, H₂O₂

Superoxides O₂⁻ (KO₂, RbO₂, CsO₂) React with water → O₂ + OH⁻ 2 KO₂ + 2 H₂O → 2 KOH + H₂O₂ + O₂

2. Physical Properties
- *Appearance:* Mostly solid (MgO, Fe₂O₃), gases (CO₂, SO₂), or liquids (Cl₂O₇, rare).
- *Melting/Boiling Points:* High for ionic metal oxides (MgO mp 2852 °C), low for covalent non‑metal oxides (CO₂ sublimes ‑78 °C).
- *Solubility:*
- *Basic oxides* (Na₂O, K₂O) → very soluble → strong bases.
- *Acidic oxides* (SO₂, P₄O₁₀) → soluble → acids.
- *Some (e.g., Al₂O₃, Fe₂O₃)* → insoluble, amphoteric

3. Key Reactions (Quick Ref)
A. Formation
- *Metal + O₂* → Metal oxide (4 Al + 3 O₂ → 2 Al₂O₃)
- *Non‑metal + O₂* → Non‑metal oxide (S + O₂ → SO₂)

B. Acid–Base Reactions
- *Basic oxide + Acid → Salt + Water*
CaO + 2 HCl → CaCl₂ + H₂O
- *Acidic oxide + Base → Salt + Water*
CO₂ + 2 NaOH → Na₂CO₃ + H₂O
- *Amphoteric oxide* (both sides)
ZnO + 2 HCl → ZnCl₂ + H₂O
ZnO + 2 NaOH + H₂O → Na₂[Zn(OH)₄]

C. Redox (Reduction–Oxidation)
- *Metal oxide reduction (smelting)*
Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂ (blast furnace)
- *Combustion of non‑metal oxides*
2 CO + O₂ → 2 CO₂

D. Thermal Decomposition
- CaCO₃ → CaO + CO₂ (lime‑kiln)
- 2 HgO → 2 Hg + O₂ (heating mercury(II) oxide)

4. Industrial / Environmental Angle
- *CO₂* – greenhouse gas, ocean acidification.
- *SO₂* – precursor to acid rain, used to make H₂SO₄.
- *NOx (NO, NO₂)* – pollutants, smog formation.
- *Fe₂O₃ (hematite)* – ore for iron/steel.
- *Al₂O₃ (corundum)* – abrasive, in ceramics.

*Quick Quiz (Test yourself)*
1. *Which oxide is amphoteric?* ZnO, CO₂, Na₂O?
2. *What acid forms when SO₃ reacts with H₂O?*
3. *Write the reaction of CaO with H₂O.*