Atomic Models: From Thomson to Quantum Theory
Atomic Models: A Historical Perspective
Thomson’s Atomic Model
Thomson’s atomic model proposed that an atom is a uniform sphere of positive electricity with electrons dispersed throughout, held apart by repulsive forces. This model attempted to explain the neutrality of atoms and the ionization process when electrons are lost.
Rutherford’s Discovery of the Atomic Nucleus
Thomson’s model failed to explain the alpha particle scattering experiments conducted by Geiger and Marsden under Rutherford’s direction. The fact that most alpha particles passed through a gold sheet undeflected suggested that atoms are mostly empty space. The deviations and strong repulsions experienced by some particles indicated a concentration of positive charge within the atom, leading to the concept of the nucleus.
According to Rutherford’s model, an atom consists of two zones: the nucleus, where almost all the mass and positive charge are concentrated, and the electron cloud, where electrons revolve around the nucleus.
Rutherford also observed that the mass of an atom was less than the sum of the individual masses of its protons and electrons. This led him to predict the existence of neutral particles, which were later discovered and named neutrons.
However, Rutherford’s model did not address the behavior of electron spin or explain why electrons didn’t lose energy and spiral into the nucleus.
The Modern Atomic Model
The current atomic model describes the nucleus as composed of protons and neutrons, surrounded by an electron cloud.
Quantum Theory and Atomic Spectra
The quantum theory explains that emission spectra originate from the radiant energy emitted by excited atoms. The discontinuous nature of spectra indicates that atoms emit energy in discrete amounts.
Absorption spectra also show that atoms only absorb energy at specific frequencies and wavelengths, indicating that energy absorption by atoms is also discontinuous.
The fact that atoms are responsible for emission and absorption spectra, meaning matter can emit or absorb electromagnetic radiation, is explained by quantum theory, which was initiated in 1900 by Max Planck.
According to quantum theory, matter emits or absorbs radiant energy in discrete packets called photons or quanta. Light carries energy in these photons.
Bohr’s Atomic Model
Bohr proposed that electrons move around the nucleus in specific circular orbits (energy levels). While in these orbits, electrons do not absorb or emit energy (stationary states). The energy of each level is the sum of kinetic and potential energies, and the total energy is quantized.
Each energy level corresponds to a specific quantum number. The lowest energy state is the ground state, while higher energy states are excited states. When an electron is completely separated from the atom (ionization), its energy level is considered to be zero.
When an electron transitions from a higher energy level to a lower one, it emits energy in the form of a photon. The absorption of energy to move to a higher energy level is also discontinuous and quantized.
Limitations of Bohr’s Model
The centripetal force that keeps electrons in their orbits is the electrostatic attraction with the nucleus. However, classical physics predicted that accelerating charges (electrons in circular motion) should radiate energy, causing them to spiral into the nucleus. Bohr’s model did not fully address this issue.