Chapter 4: Structure of the Atom
4.1 Charged Particles in Matter
An atom is divisible and consists of charged particles. Key sub-atomic particles:
- Electron (eā»): Discovered by J.J. Thomson. Negligible mass, charge -1.
- Proton (pāŗ): Discovered by E. Goldstein (Canal Rays). Unit mass, charge +1.
- Neutron (n): Discovered by J. Chadwick. Unit mass, no charge.
4.2 Atomic Models
Similar to a "Christmas Pudding" or "Watermelon". Positive charge is spread all over, and electrons are embedded like seeds.
Nuclear model based on the alpha-particle scattering experiment. Identified the Nucleus as a tiny, dense, positively charged center.
Electrons revolve in discrete orbits called energy levels (K, L, M, N). They do not radiate energy while in these orbits.
4.3 Electron Distribution
Bohr-Bury Scheme: Maximum electrons in a shell = 2nĀ².
- K shell (n=1) ā 2 electrons
- L shell (n=2) ā 8 electrons
- M shell (n=3) ā 18 electrons
4.4 Valency
The combining capacity of an atom is called valency. Atoms react to achieve an octet in their outermost shell.
4.6 Isotopes and Isobars
Isotopes: Atoms of same element with same atomic number but different mass numbers (e.g., Protium, Deuterium, Tritium).
Isobars: Atoms of different elements with same mass number but different atomic numbers (e.g., Calcium and Argon both have mass 40).
Properties Table
| Particle | Mass (u) | Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/2000 approx | -1 | Orbits |