Atoms and Molecules
Subject: General Science — Chemistry | Topic: Atoms and Molecules Exam: AP Group 2 (APPSC) | Sources: NCERT Class IX-X, Lucent's GS
Introduction
This chapter covers atomic structure, the periodic table, chemical bonding, and the mole concept. APPSC typically asks 1-2 questions — subatomic particle discoveries, atomic models, and periodic table trends are the most tested areas.
Atomic Theory and Structure
Dalton's Atomic Theory (1808): All matter is made of indivisible atoms. Atoms of the same element are identical. Atoms combine in whole-number ratios. Atoms cannot be created or destroyed in reactions. (Modern update: atoms ARE divisible; isotopes exist.)
Subatomic Particles
| Particle | Discoverer | Charge | Mass |
|---|---|---|---|
| Electron | J.J. Thomson (1897, cathode rays) | −1.6 × 10⁻¹⁹ C | 9.1 × 10⁻³¹ kg |
| Proton | Goldstein (canal rays) | +1.6 × 10⁻¹⁹ C | 1.67 × 10⁻²⁷ kg |
| Neutron | James Chadwick (1932) | Zero | ≈ proton mass |
Atomic Models
| Model | Key Feature | Limitation |
|---|---|---|
| Thomson (Plum Pudding) | Positive charge spread uniformly, electrons embedded | Disproved by Rutherford |
| Rutherford (1911) | Tiny dense positive nucleus; electrons orbit; mostly empty space | Could not explain electron stability |
| Bohr (1913) | Fixed orbits (K, L, M, N) with definite energy; electrons jump between orbits | Only valid for hydrogen-like atoms |
Atomic Number and Mass Number
- Atomic number (Z) = number of protons = number of electrons (neutral atom). Defines the element.
- Mass number (A) = protons + neutrons.
- Isotopes: Same Z, different A. Example: Carbon-12, Carbon-13, Carbon-14. Hydrogen: Protium (H-1), Deuterium (H-2), Tritium (H-3).
- Isobars: Different elements, same A. Example: Ca-40 and Ar-40.
Electronic Configuration
- Maximum electrons in a shell = 2n² (K=2, L=8, M=18, N=32).
- Electrons fill innermost shell first (Aufbau principle).
- Valence electrons: Outermost shell electrons. Determine chemical properties.
- Valency: Combining capacity. Equal to valence electrons (1-4) or 8 minus valence electrons (5-7).
- Noble gases (He, Ne, Ar): completely filled outermost shells — chemically inert.
Laws of Chemical Combination
| Law | Scientist | Statement |
|---|---|---|
| Conservation of Mass | Lavoisier (1789) | Total mass of reactants = total mass of products |
| Definite Proportions | Proust | A compound always has elements in fixed mass ratio |
| Multiple Proportions | Dalton | Ratios of masses combining are small whole numbers |
The Mole Concept
| Quantity | Value |
|---|---|
| 1 mole | 6.022 × 10²³ particles (Avogadro's number, Nₐ) |
| Molar mass | Atomic/molecular mass in grams |
| Number of moles | Given mass / Molar mass |
| Volume at STP (0°C, 1 atm) | 22.4 litres for any gas |
Example: 1 mole of H₂O = 18 g. Molecular mass of H₂O = 2(1) + 16 = 18 u.
Chemical Bonding
| Bond Type | Mechanism | Example | Properties |
|---|---|---|---|
| Ionic | Transfer of electrons (metal → non-metal) | NaCl | High MP, conduct in solution/molten, soluble in water |
| Covalent | Sharing of electrons (non-metal + non-metal) | H₂O, CO₂, CH₄ | Low MP, poor conductors, often insoluble |
Periodic Table
- Mendeleev: Properties are periodic function of atomic mass.
- Modern Law (Moseley): Properties are periodic function of atomic number. 7 periods, 18 groups.
- Same group = same valence electrons = similar chemical properties.
- Same period = same number of shells.
- Metals (left), Non-metals (right), Metalloids along zig-zag line (B, Si, Ge, As, Sb, Te).
| Trend | Down a Group | Across a Period (L→R) |
|---|---|---|
| Atomic size | Increases | Decreases |
| Metallic character | Increases | Decreases |
Likely Exam Questions
-
Who discovered the neutron? Ans: James Chadwick (1932)
-
The maximum number of electrons in the M shell is: Ans: 18 (2 × 3² = 18)
-
Avogadro's number is: Ans: 6.022 × 10²³
-
Isotopes have the same ___ but different ___: Ans: Atomic number; Mass number
-
The Law of Conservation of Mass was given by: Ans: Lavoisier (1789)
-
The Modern Periodic Law was proposed by: Ans: Moseley (properties are periodic function of atomic number)
-
Atomic size increases: Ans: Down a group (more shells added)
-
An ionic bond is formed by: Ans: Transfer of electrons from metal to non-metal
-
At STP, 1 mole of any gas occupies: Ans: 22.4 litres
-
Rutherford's gold foil experiment revealed that: Ans: The atom has a tiny, dense, positive nucleus with mostly empty space