1. Subatomic Particles
Atoms are made of three subatomic particles: Electrons (negative, discovered by J.J.
Thomson), Protons (positive, discovered by E. Goldstein), and Neutrons
(neutral, discovered by J. Chadwick).
2. Atomic Models
- Thomson's Model: Compared atom to a Christmas pudding or watermelon.
- Rutherford's Model: Alpha-particle scattering experiment led to
discovery of the Nucleus.
- Bohr's Model: Electrons revolve in discrete orbits called energy levels
or shells (K, L, M, N).
3. Valency and Atomic Number
- Valency: The combining capacity of an atom. Determined by electrons in
the outermost shell.
- Atomic Number (Z): Number of protons in the nucleus.
- Mass Number (A): Sum of protons and neutrons.
4. Isotopes and Isobars
Isotopes: Atoms of the 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.