Atomic Models Evolution: Dalton, Thomson, Rutherford, Bohr

Evolution of Atomic Models: Limitations and Contributions

Limitations of Early Atomic Models

Dalton's Atomic Theory: Unexplained Phenomena

• Cathode and Positive Rays Behavior

  1. Cathode ray deflections in the presence of magnetic and electrical fields.
  2. The constancy of the quotient q/m in the particles that constitute cathode rays.

• Existence of Isotopes

  The concept of an element was changed according to Rutherford's model, which Dalton's theory did not account for.

Rutherford's Atomic Model: Theoretical Instability

• Atoms are theoretically unstable systems according to the laws of electromagnetism.
• This model could not explain atomic spectra.

Bohr's Atomic Model: Incomplete Explanations

• It could not explain the spectra of poly-electronic atoms.
• It did not fully account for the Zeeman effect.

Enduring Key Concepts from Atomic Models

Dalton's Contributions to Atomic Theory

• Atoms are present in all matter.
• Atoms are conserved in chemical reactions; they merely rearrange themselves (explaining Lavoisier's Law).
• Compounds are formed by different atoms (elements) in constant or definite proportions (explaining Proust's Law).

Thomson's Insights on Atomic Structure

• Atoms are divisible and can split up.
• Atoms are composed of electrons.
• Electrons are common to all atoms, acting as universal particles.
• Electrons can be extracted from an atom.

Rutherford's Revolutionary Atomic Structure

• The atomic nucleus is composed of protons and neutrons and occupies a minute part of the atom.
• Atoms are largely empty space.
• A new element concept: atomic number (Z).
• The existence of isotopes.

Bohr's Quantum Leap in Atomic Understanding

• Energy is released when an electron in an excited state (where its energy is greater than that of the ground level) jumps down to a lower energy level. This process results in the emission of light, specifically a photon.
• The energy of the electron is quantized.

Fundamental Principles of Atomic Models

Dalton's Indivisible Atom

• Atoms are solid, indestructible, and indivisible particles.
• Elements are characterized by their masses.

Thomson's Plum Pudding Model

• All atoms are composed of minute, negatively charged particles called electrons, which are embedded in a sphere of positive charge. Atoms are neutral.
• Atoms are divisible, meaning they can split up, and electrons can be extracted from them.
• Electrons are common, universal particles found in all atoms.

Rutherford's Nuclear Atom

• Atoms consist of a central, positively charged nucleus where almost all the atom's mass is concentrated.
• Inside the nucleus are two types of particles: protons and neutrons.
• The atom's size is approximately 10⁴ times the size of its nucleus, making the atom largely empty space.
• An equal number of electrons orbit the nucleus as there are protons.

Bohr's Quantized Orbits

• In the hydrogen atom, electrons orbit the nucleus in stable, circular paths.
• The radii of all possible orbits (r_n = r₁·n²) and the energy (E_n = E₁/n²) corresponding to each state are quantized.
• Electrons can absorb energy, jumping to higher energy levels.
• When an electron jumps back down between two levels, the released energy is emitted as light, specifically photons (E = hν).