Chemistry — Atomic Structure, Reactions & Key Elements
1. Atomic Structure — Deeper
Bohr's Model
Electrons orbit the nucleus in FIXED SHELLS. Shell 1 (K): max 2. Shell 2 (L): max 8. Shell 3 (M): max 8 (for first 20). Shell 4 (N): max 18.
Valence Electrons
Electrons in the OUTERMOST SHELL. 'Valence electrons determine an element's CHEMICAL PROPERTIES.'
Valency
The COMBINING CAPACITY of an atom. For metals: valency = number of valence electrons. For non-metals: valency = 8 — number of valence electrons. 'Atoms react to ACHIEVE a stable configuration of 8 electrons in the outermost shell (OCTET RULE).'
Atomic Number (Z) and Mass Number (A)
- Z = Number of PROTONS. Defines the element.
- A = Protons + Neutrons.
- Number of neutrons = A — Z.
Isotopes
Atoms of the SAME ELEMENT with the SAME number of protons but DIFFERENT number of neutrons. Same atomic number. Different mass numbers. Example: Carbon-12, Carbon-13, Carbon-14. Same chemical properties. Different physical properties.
2. Language of Chemistry — Chemical Equations
Writing Chemical Equations
Word Equation: Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen. Symbol Equation: Zn + 2HCl → ZnCl₂ + H₂.
Balancing Chemical Equations
Number of atoms of EACH element must be the SAME on BOTH sides. 'Mass is conserved. Atoms are neither created nor destroyed — only REARRANGED.'
- Use COEFFICIENTS (numbers before formulas) — never change SUBSCRIPTS.
- Balance: metals first. Non-metals next. Hydrogen and oxygen LAST.
3. Types of Chemical Reactions
| Type | Description | Example |
|---|---|---|
| Combination | Two or more → ONE product | 2H₂ + O₂ → 2H₂O |
| Decomposition | ONE reactant → Two or more products | CaCO₃ → CaO + CO₂ (heating limestone) |
| Displacement | More reactive element DISPLACES less reactive | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Displacement | Exchange of ions between compounds | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
4. Hydrogen
Properties
- LIGHTEST element. Colourless. Odourless. HIGHLY FLAMMABLE.
- Combustion: 2H₂ + O₂ → 2H₂O (burns with a 'pop' sound — test for hydrogen).
Uses
- In HABER PROCESS to manufacture AMMONIA (fertilisers)
- As a CLEAN FUEL (water is the only product). 'Hydrogen is the fuel of the future.'
5. Carbon — The Element of Life
Allotropes of Carbon
| Allotrope | Structure | Properties & Uses |
|---|---|---|
| Diamond | Each C bonded to 4 others. RIGID 3D network. | HARDEST natural substance. Insulator. Used in cutting tools. |
| Graphite | Layers of hexagons. Weak forces between layers. | Layers SLIDE — used as lubricant. CONDUCTS electricity — used in electrodes. |
| Buckminsterfullerene (C₆₀) | Soccer-ball shaped molecules | Nanotechnology. Discovered 1985. |
Carbon Dioxide
- Formed by: combustion of carbon. Respiration. Decomposition.
- Test: Limewater TURNS MILKY. Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
- Properties: Colourless, odourless. Does NOT support combustion. Used in FIRE EXTINGUISHERS.
Carbon Monoxide (CO)
COLOURLESS. ODOURLESS. DEADLY POISONOUS. 'CO binds to haemoglobin MORE STRONGLY than oxygen — starving the body of oxygen.'
