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After combining all elements of physics, chemistry, and mathematics, the scientists came up with a scientific explanation of the basic unit of matter which is the atom. The theory that all matter is made up of atoms as their building blocks are known as the atomic theory. After that various scientists proposed their own theory to scientifically describe the structure of an atom and its nature. The atomic theory also suggested that an element is made up of identical atoms and is different from atoms of other elements. Atoms, after combining with other atoms in fixed proportion form molecules and compounds.
With time, the theory has evolved and so did the structure of the atom. With each theory, some merits and demerits came, and the problems were solved with the explanation of the next theory that came. Few significant examples of atomic structures and the associated theories are described in the article.
Atomism Theory
The theory of atomic structure was first a philosophical concept proposed in India and Greece. The term atom was derived from the word “atomos” which meant indivisible. Thus, they believed that matter is made up of small discrete units called atoms.
Problems
- It was just a concept and wasn’t really a theory based on scientific data which is why it wasn’t acknowledged by the scientific community.
Billiard Ball Theory
After the formulation of the law of constant proportion and the law of conservation of mass by Pruist and Lavoiser respectively, an English chemist and physicist, John Dalton proposed Dalton’s atomic theory and the billiard ball model of the structure of the atom. He constructed upon them to formulate the law of multiple proportions, which states that the ratios of masses of elements in a compound are small whole numbers. His theory was based on experimental data and was approved initially.
Problems
- However, the problems with this theory were multiple, as the discovery of isotopes, isobars, azeotropes, etc., made it impossible for the theory to be universal.
- The structure of the atom as proposed by his billiard ball model was that the atom is a hard, sphere-like, and indivisible particle, which was later disapproved by J.J Thompson’s experiment with the discovery of subatomic particles.
Plum Pudding Model
In 1897, JJ Thompson discovered the electron by conducting an experiment on a cathode ray tube. Thomson positioned two oppositely charged electric plates around a cathode ray to test the properties of a particle. The selection of the cathode ray from negatively charged plate and towards the positively charged plate demonstrated the presence of negatively charged particles in the cathode rays.
Afterward, he proposed that an atom is neutral and the positively charged atom is balanced by the electron’s negative charge. Further, Thomson proposed that atoms could be explained as negative particles floating within a matrix of diffuse positive charge like a plum pudding with electrons as dry fruits in it.
Problem
- This model was later discarded due to its inability to explain the stability of the structure of an atom and failed to explain the results of Rutherford’s experiment.
Planetary Model
According to the plum pudding model, Rutherford predicted that in his experiment, most of the α particles would not pass straight through the gold foil due to diffused positive charge throughout the structure of the atom. Therefore, the electric field from the positive charge wouldn’t be significant to affect the path of the fast-moving α particles.
But during the experiment, most of the particles passed the atom, only a small fraction of it was deflected. Drawing from his experiment, Rutherford proposed the nuclear model, in which an atom consists of a very small, positively charged nucleus surrounded by negatively charged electrons, and the size of the nucleus is very small as compared to the atomic size.
Problem
- The nuclear model explained Rutherford’s experimental results but created other problems which couldn’t be explained like the stability of the structure, what keeps the electron from falling into the nucleus, the absorption, and the emission of spectra.
Bohr Model
In 1913, Niels Bohr proposed the Bohr model that proposed, an atom includes a small, undoubtedly charged nucleus that’s surrounded through electrons that revolves in round orbits due to electrostatic forces across the nucleus. According to Bohr’s principle, one and the best spectral line can originate from an electron among any given electricity.
Problems
- Bohr’s model of an atom was unable to explain the Zeeman Effect which is the effect that the magnetic field has on the spectra of atoms.
- It also could not explain the Stark effect which is the effect that the electric field has on the spectra of atoms.
- It violates the Heisenberg Uncertainty principle as Bohr assumed that an electron in an atom is placed at a particular detachment from the nucleus and is revolving spherically with a specific speed.
- Bohr`s version of an atom couldn’t explain the broad spectra of atoms containing multiple electrons known as multi-electron atoms. Bohr’s principle didn’t account for the impact of magnetic discipline on the spectra of atoms or ions.
Quantum Theory Of The Structure Of The Atom
After the theory given by Bohr, several discoveries led to the understanding of atoms. For example, Federick Soddy described isotopes, the discovery of neutrons. After that, Louis de Broglie’s wave theory for the particles was proposed which Erwin Schrödinger described using Schrödinger’s equation. This led to the proposal of Werner Heisenberg’s uncertainty principle.
All of this formed the stepstone for quantum mechanics based on atomic theory in which subatomic particles exist. The electron can possibly be found anywhere in an atom but the greatest probability of finding it is in an atomic orbital or energy level. In modern atomic theory, orbitals may be spherical, dumbbell-shaped, etc. For atoms with a high number of electrons, relativistic effects play an important role as the particles are moving with high speed.
Currently, this theory is widely accepted by scientific communities all over the world.
Conclusion
Although our understanding of the particle began with an atom which was an indivisible building block for an element but gradually, with scientific developments and discovery of various other constituents of an atom known as the subatomic particles, our theories evolved. The subatomic particles i.e., the proton, neutron, and electron now form the bases for the newer theories and models to describe the structure of an atom. A few examples of the atomic structure include the first billiard ball model by Dalton, the plum pudding model by JJ Thompson, the nuclear model by Rutherford, and now the quantum model.
