Evolution of Atomic Models
From Dalton's solid sphere to Bohr's quantized orbits.
The concept of the atom has evolved significantly over the last two centuries. This article explores the progression of atomic theory through the groundbreaking models proposed by Dalton, Thomson, Rutherford, and Bohr.
1. Dalton's Atomic Theory (1808)
John Dalton proposed the first scientific theory of the atom, treating it as the fundamental building block of matter.
Key Postulates:
- All matter consists of indivisible atoms.
- Atoms of a given element are identical in mass and properties.
- Compounds are formed when atoms of different elements combine in fixed, simple whole-number ratios.
- Chemical reactions involve the reorganization of atoms; atoms are neither created nor destroyed.
2. Thomson's Model (1898)
Proposed by J.J. Thomson after his discovery of the electron. Known as the Plum Pudding Model or Watermelon Model.
Key Features:
- The atom is a positively charged sphere.
- Electrons are embedded into it like plums in a pudding or seeds in a watermelon.
- The total positive charge equals the total negative charge, making the atom electrically neutral.
3. Rutherford's Nuclear Model (1911)
Ernest Rutherford conducted the famous $\alpha$-particle scattering experiment by bombarding a thin gold foil with high-energy alpha particles.
Observations:
- Most $\alpha$-particles passed straight through the foil (99.9%).
- Some were deflected by small angles.
- Very few (1 in 20,000) rebounded back by $180^\circ$.
Conclusions (The Planetary Model):
1. Most of the atom is empty space.
2. The positive charge and mass are concentrated in a tiny center called the Nucleus.
3. Electrons revolve around the nucleus in circular paths (orbits) like planets around the sun.
4. Bohr's Atomic Model (1913)
Neils Bohr overcame Rutherford's limitations by applying Planck's Quantum Theory. This model is valid primarily for hydrogen-like species ($H, He^+, Li^{2+}$).
Key Postulates:
- Stationary Orbits: Electrons revolve in fixed circular orbits without radiating energy.
- Quantization Condition: Electrons can only exist in orbits where their angular momentum is an integral multiple of $\frac{h}{2\pi}$. $$ mvr = \frac{nh}{2\pi} $$ Where $n = 1, 2, 3...$ (Principal Quantum Number).
- Energy Transitions: Energy is absorbed or emitted only when an electron jumps from one orbit to another ($\Delta E = E_2 - E_1 = h\nu$).
Energy of n-th Orbit:
Radius of n-th Orbit: $ r_n = 0.529 \times \frac{n^2}{Z} \, \mathring{A} $
Knowledge Check
Test your understanding of Atomic Models
No comments:
Post a Comment