Atomic Structure

Atoms are the smallest component of matter and are indivisible. They are the building blocks for the matter. “Atom” is derived from the Greek word, ‘A-tomio’, meaning uncuttable or indivisible. In the initial days, all ideas regarding the matter were merely theoretical, they could not be carried out experimentally. John Dalton, a British school teacher, was the first one to formulate a theory about the atom, called Dalton’s Atomic Theory. 

What is atomic structure – history?

The atomic theory proposed by John Dalton led to the following hypothesis. 

• Atoms of one element are completely different from atoms of other elements. Dalton came to this hypothesis, based on the different properties exhibited by different elements. 
• His second hypothesis was, for forming compounds we require a particular proportion of the required elements. This law basically is an extension of Proust’s law of definite proportions. It is important to note that Dalton’s second hypothesis also satisfies the law of multiple proportions.
• The third hypothesis given by Dalton is what we call the ‘law of conservation of mass’. The law states that mass is neither created nor destroyed in a chemical reaction. It is constant.

Atomic Structure Definition

It is essential to understand that Dalton’s theory only helped us define atoms as the building blocks for matter, but it did not reveal the accurate atomic structure definition. A series of investigations and experiments post the proposal of Dalton’s atomic theory, led to the discovery of subatomic particles (electrons, protons, and neutrons). 

Experiments that led to the discovery of Subatomic Particles:

• Electrons (Negatively charged particles)- Discovered by J.J. Thomson in 1897, which was actually a result of the pursuit involving the search of cathode rays for the carrier that helps in transporting electronic properties of a matter made by a group of scientists including Thomson.  

• Protons (Positively Charged Particles)- Discovered by Electrical discharge in a modified Cathode Ray Tube, in Ernest Rutherford in 1919 (Petrucci et al., 2017).

• After discovering a positive and a negative subatomic particle, scientists felt the need for a neutral particle to exist in the atom, which led to the discovery of the neutron by James Chadwick in 1932 

After the discovery of the electron , J.J Thomson conducted an experiment to determine the charge by mass ratio of the electron which turned out to be 1.758820 * 1011  C Kg-1. And further, using this constant RA Millikan had formulated the mass and charge of the electron (Petrucci et al., 2017).

Mass of Electron, me =9.1094×10-31  Kg

Charge of electron, e= -1.6× 10-19 C 

Different Atomic Structures:

Over the late 19th and complete 20th centuries, different scientists have come up with different atoms by making modifications to the previous models or by correcting the limitations of the previous ones. The different atomic models proposed by different scientists are as follows:

1. J.J. Thomson’s Atomic Model: Thomson had suggested a plum-pudding model for the atom. He considered the entire cake to be protons (positively charged particles) and the plum pieces in it to be the electrons (negatively charged particles). His structure described the atom to be in a neutral state as the protons and electrons were equal and opposite in magnitude. However, his model could not describe the stability of the atom, which is a fundamental drawback of this model. Also, he did not account for the future discoveries of new sub-atomic particles. 
2. Rutherford’s Atomic Model: His model was theorized after his discovery of the nucleus of the atom. He conducted the Alpha-Ray Experiment to formulate his atomic model. Based on the inferences from his experiment, he proposed that the atom is spherical in shape and its nucleus lies in the center. The electrons in the atom revolve around the nucleus in a circular orbit, like the solar system. Later, scientists suggested that the electrons would lose their energy due to revolution and fall into the nucleus leading to the collapse of the atom, which is not possible. 
3. Bohr’s Atomic Theory:  Neils Bohr proposed a new structure of the atom in 1915, after the discovery of all subatomic particles, and it is based on Planck’s Quantization Theory. In the structure proposed by Bohr, electrons in the atoms exist in stationary orbits around the nucleus, whose energy levels could be represented by quantum numbers. An electron staying within the same orbit would have constant energy, but they would jump to higher and lower energy levels upon attaining or losing energy respectively. Electrons revolve only in their stationary orbits and the energy levels of these levels are quantized. The limitation with Bohr’s model was that it could be applicable for only single electron species and it could not explain the Stark and Zeeman effects.

Conclusion

The structure of the atom is the best way of describing the atom and the subatomic particles. It describes the properties of atoms and helps us understand the different behaviors and characteristic features of the atom. Over the past century, many different physicists and scientists proposed different atomic models, but the model proposed by Neils Bohr satisfies most principles and properties of the atom and this model is considered the most appropriate atomic model.