Atomic Physics: History, Modern Development and FAQs - Testbook

Rutherford’s experiments led to the depiction of atoms as entities with a dense, positively charged core surrounded by a negatively charged gas. Earlier atomic models, such as J.J Thomson’s, did not provide a solid explanation of positive and negative entities. The discovery of the neutron by James Chadwick clarified the issue of isotopic composition and the relationship between nuclear spin and atomic weight. This discovery of atomic building blocks like neutrons and protons marked the birth of modern atomic physics.

In 1913, Niels Bohr proposed his atomic model, which portrayed electrons orbiting the dense nucleus in quantised orbits. The work of physicists like Werner Heisenberg and Erwin Schrödinger in studying subatomic interactions led to the development of quantum mechanics. The uncertainty principle and Schrödinger’s equation became fundamental in defining the properties of subatomic particles and interactions.

Hideki Yukawa proposed the existence of a unique intermediate elementary particle to act as a glue holding protons and neutrons together in the nucleus. The discovery of the pi and mu mesons by Cecil Powell solved the mystery of the particle that did not seem to interact strongly with nuclei. These discoveries underscored the importance of theoretical induction and completed the first phase of the microscopic representation of the nuclear structure. Since then, a host of elementary particles have been discovered, each with its unique role in the quantum world.

The discovery of the fission process by Otto Hahn and Fritz Strassmann opened the doors for the development of atomic bombs and fission reactors for power generation. The fusion process, which powers the stars, was the inspiration for the design of the hydrogen bomb in the 1950s. Extensive research since then has focused on harnessing thermonuclear fusion as a viable power source. Techniques for identifying isotopes and nuclear magnetic resonance, which have found applications in fields like archaeology, geology, medical diagnosis, and many others, have been developed from this science.

The advent of power lasers has breathed new life into the field of atomic physics. Lasers have greatly improved the precision with which an atom’s characteristic wavelengths can be measured. Modern standards of frequency and time are based on the calculations of transition frequencies in atomic caesium clocks. The definition of the unit ‘metre’ is linked to the measurement of frequency through the speed of light. Beyond these examples, lasers have been used in techniques for isolating individual atoms and cooling them close to absolute zero. When atoms are held in an electromagnetic trap, they can undergo a quantum mechanical phase change to form a unique superfluid called Bose-Einstein condensation, while still remaining in a gaseous state. In this state, all atoms are in the same coherent quantum state. As a result, the atoms lose their individual characteristics, and the quantum mechanical wave characteristics become more prominent. The entire condensate then reacts to external influences as a single coherent entity rather than a collection of individual atoms. Recent research has shown that a coherent beam of atoms can be extracted from the trap to create an “atom laser” similar to the coherent photon beam in a conventional laser. This type of laser is still in development but holds great promise as a tool in the development of advanced technologies for creating microelectronic and nanoscale devices.

The electronic configuration of atoms represents the arrangement of electrons among the subatomic shells and subshells. While the electron configuration is generally used for this purpose, it can also be helpful in depicting atoms that have ionised into an anion or cation by accounting for the gain or loss of electrons in their adjacent orbitals. The distinct chemical and physical properties of elements are tied to their unique electronic configurations. The electrons in the outermost shell play a crucial role in determining the unique chemistry of the elements.

Electrons form the subatomic shells around the core of protons and neutrons. They are usually in a ground state but can be excited by absorbing energy from light, interacting with colliding particles, or magnetic fields. Electrons that form the shell are considered to be in a bound state. The energy required to remove an electron from its shell is known as the binding energy. Any energy absorbed by the electrons in excess of this amount is converted into kinetic energy (as per the conservation of energy). The atom is considered to have undergone ionisation. If the electrons absorb energy greater than the binding energy, they will be elevated to an excited state. After a certain period, the electrons in excited states will drop to a lower energy state. In the case of a neutral atom, it will emit a photon of the difference in energy, as energy is always conserved.

When an electron absorbs much more energy than the binding energy, another outer electron may transition to fill the inner orbital. In this case, a characteristic x-ray or a visible photon is emitted. Alternatively, a process known as the Auger effect could occur, where the emitted energy is transferred to another bound electron. There are strict conditions for achieving different types of electronic configurations that can be achieved by light excitation. In the case of collision processes, there are no such rules.