Key Atomic Models: Thompson, Rutherford, Planck, and Bohr

Foundational Atomic Models and Quantum Theory

J.J. Thompson's Atomic Model (1904)

Thompson proposed a particularly simple atomic model based on the discovery of the electron through a series of experiments conducted in low-pressure gas tubes. These experiments confirmed the emission of negatively charged particles, which he called electrons.

Thompson's model (often referred to as the "Plum Pudding Model") considers the atom as a homogeneous sphere of positive charge in which electrons are embedded on its surface, neutralizing the positive charges.

Ernest Rutherford's Nuclear Model (1911)

Rutherford developed his model based on experiments carried out by bombarding thin gold foil with alpha particles. Observations from the experiment included:

• Almost all particles passed through the foil without suffering deviation.
• A few particles suffered only minor changes in direction.
• A very small number of particles were reflected back.

These results indicated that almost the entire volume of the atom is empty. Furthermore, they suggested the existence of a small, dense, positively charged region where the positive charge is concentrated, which was the source of most of the deviations.

The Rutherford atomic model proposed distinct areas:

1. A positively charged center, called the nucleus, containing 99.9% of the atom's concentrated mass.
2. An outer region, often called the electron shell or crust, where negatively charged electrons orbit the nucleus.
3. The atom must be electrically neutral; therefore, the number of positive charges in the nucleus must be equal to the number of electrons in the shell.

Max Planck's Quantum Hypothesis

Planck hypothesized that the energy absorbed or emitted by a body in the form of electromagnetic radiation is not continuous but consists of small, discrete packets of energy called quanta.

The hypothesis states:

"The energy absorbed or emitted by a body in the form of electromagnetic radiation is always a multiple of a number $h$, called Planck's constant."

The quantum is the minimum amount of energy that radiation can carry. This concept established that energy is quantized.

Niels Bohr's Atomic Model (1913)

Bohr built upon Rutherford's atomic model and applied Planck's hypothesis. The Bohr atomic model was proposed to address the failures of the Rutherford model, particularly its inability to explain the stability of the atom and the discrete spectrum of the hydrogen atom. It is based on three fundamental assumptions:

Bohr's Postulates

1. First Postulate: Stable Orbits

   The atom has a nucleus where the positive charge and almost all the mass are located. The electron describes circular orbits around the nucleus. The centrifugal force of the electron's rotation is compensated by the force of electrostatic attraction between the electric charges.

2. Second Postulate: Quantized Angular Momentum

   The electron cannot turn around the nucleus in any orbit; it may only do so in specific orbits where the angular momentum is quantized.

3. Third Postulate: Energy Transitions

   The electron, when moving in a stable orbit, does not radiate energy. Emission or absorption of a quantum of energy occurs only when the electron moves from one orbit to another, according to Planck's equation ($E = h\nu$).