Learning Objectives
- Learn about the discovery of subatomic particles
- Understand various atomic models (Thomson, Rutherford, Bohr)
- Learn about distribution of electrons in different shells
- Understand atomic number, mass number, isotopes, and isobars
Key Concepts
Discovery of Subatomic Particles
Electron (e⁻): Discovered by J.J. Thomson using cathode ray experiment. Charge: -1.6 × 10⁻¹⁹ C. Mass: 9.1 × 10⁻³¹ kg.
Proton (p⁺): Discovered by E. Goldstein using canal rays (anode rays). Charge: +1.6 × 10⁻¹⁹ C. Mass: 1.67 × 10⁻²⁷ kg.
Neutron (n): Discovered by James Chadwick. No charge (neutral). Mass: approximately equal to proton.
Thomson's Model (Plum Pudding Model)
The atom is a positively charged sphere with electrons embedded in it like plums in a pudding. This model could not explain Rutherford's scattering experiment.
Rutherford's Alpha Particle Scattering Experiment
Rutherford bombarded a thin gold foil with alpha particles. Observations:
- Most alpha particles passed straight through — atom is mostly empty space.
- Some were deflected by small angles — positive charge in the atom.
- Very few (1 in 20,000) bounced back — positive charge is concentrated in a very small, dense region.
Rutherford's Nuclear Model
The atom has a small, dense, positively charged centre called the nucleus. Electrons revolve around the nucleus in circular orbits. The size of the nucleus is very small compared to the atom.
Limitation: According to electromagnetic theory, revolving electrons should continuously radiate energy and spiral into the nucleus. This model could not explain the stability of the atom.
Bohr's Model of the Atom
- Electrons revolve in certain discrete orbits (energy levels/shells) without radiating energy.
- These orbits are called K, L, M, N (or n = 1, 2, 3, 4).
- Each orbit has a fixed energy. Electrons can jump between orbits by absorbing or emitting energy.
Distribution of Electrons in Shells
Maximum number of electrons in a shell = 2n² where n is the shell number.
- K shell (n=1): 2 × 1² = 2 electrons
- L shell (n=2): 2 × 2² = 8 electrons
- M shell (n=3): 2 × 3² = 18 electrons
- N shell (n=4): 2 × 4² = 32 electrons
The outermost shell can hold a maximum of 8 electrons. Electrons fill shells from the innermost outward.
Valency
The combining capacity of an atom is its valency. It is determined by the number of electrons in the outermost shell. If outermost shell has ≤ 4 electrons, valency = number of electrons. If > 4, valency = 8 - number of outermost electrons.
Atomic Number and Mass Number
Atomic number (Z): Number of protons in the nucleus. Z = number of protons = number of electrons (in a neutral atom).
Mass number (A): Total number of protons and neutrons. A = Z + number of neutrons.
Isotopes and Isobars
Isotopes: Atoms of the same element with the same atomic number but different mass numbers (different neutrons). Example: ¹H, ²H (Deuterium), ³H (Tritium).
Isobars: Atoms of different elements with the same mass number but different atomic numbers. Example: ⁴⁰Ar and ⁴⁰Ca.
Summary
Atoms consist of three subatomic particles: protons, neutrons, and electrons. The atomic models evolved from Thomson's plum pudding model to Rutherford's nuclear model to Bohr's shell model. Electrons are distributed in shells following the 2n² rule. Atoms are characterised by their atomic number and mass number. Isotopes share atomic number while isobars share mass number.
Important Terms
- Nucleus: The dense, positively charged centre of an atom containing protons and neutrons
- Atomic Number (Z): Number of protons in the nucleus
- Mass Number (A): Sum of protons and neutrons
- Isotopes: Atoms of the same element with different mass numbers
- Isobars: Atoms of different elements with the same mass number
- Valency: The combining capacity of an atom
Quick Revision
- Electron: J.J. Thomson; Proton: E. Goldstein; Neutron: Chadwick
- Rutherford: atom has dense nucleus, mostly empty space
- Bohr: electrons in discrete energy levels (shells)
- Max electrons in shell n = 2n²
- Atomic number Z = number of protons; Mass number A = protons + neutrons
- Isotopes: same Z, different A; Isobars: same A, different Z