A BRIEF HISTORY OF ATOMIC THEORY

Early beginnings: break down into the ratio of 2:1 Hydrogen to Oxygen atoms. Dalton’s findings were further supported by Antoine Lavoisier, when he formulated the law of conservation of mass, proving that matter could not be created or destroyed, hence atoms being indivisible.

In an early pursuit to understand the contents of the universe, Greek philosopher Democritus first proposed the foundations of atomic theory over 2600 years ago during 400 BCE. Democritus theorized that all matter is composed of tiny, indivisible particles, undetectable to the naked human eye. He called these particles ‘atomos’ which loosely translates to ‘unbreakable’ from Ancient Greek [2]. Democritus was often ridiculed by influential philosophers of the same era, who followed Aristotle’s theory of the four elements1 [1]. Democritus’ theory would be neglected for centuries to come, until John Dalton, a quaker teacher would make revolutionary breakthroughs in atomism, sparking new ideas for years to come.

An age of discovery:

Almost 2200 years later, atomic theory was kickstarted again by quaker teacher John Dalton, who was seeking to challenge the four elements theory and further develop atomism. Democritus, although formulating a conclusive hypothesis, left his work as simply theoretical. Dalton, on the other hand, was able to find concrete evidence to support atomic theory, by proving that all amounts of the same chemical compound broke down into the same proportion of elements2 [2]. For example, all amounts of water molecules

1 Aristotle and his followers believed that all matter was made up of four major elements – earth, air, fire, and water. The amount of each element used to create an object would affect its properties.

Nearly a century later, the next major advancement in atomism took place when Physicist J.J. Thomson produced the ‘plum pudding’ model of the atom, which also proved that atoms were made up of subatomic particles [2]. By conducting a series of experiments involving a cathode ray tube, Thomson was able to determine that all atoms have subatomic particles that are negatively charged (the electron or e-). Thomson used his discovery of the electron to design the plum pudding model which stated that atoms were circles with a radius of roughly 10-10m, scattered with negatively charged electrons and other unknown subatomic particles which had positive charges to provide the atom with an overall neutral charge. Thomson’s model was accurate to some extent; however, it didn’t account for the correct shape of the atom, and neither did it show the correct distributions of subatomic particles throughout the atom

Modern findings:

2 Elements of the periodic table. Not to be confused with Aristotle’s four elements.

Thomson’s work was continued by two of his brightest students, Ernest Rutherford and Niels Bohr. First, Rutherford, in what is now known as the ‘Gold Foil’ experiment, concluded that atoms had a dense centre which caused waves (specifically X-Rays) to be reflected back, and less dense surroundings, which allowed the waves to be transmitted through the atom. Rutherford was able to prove, through this experiment, that an atom contains a concentrated, positively charged centre, which accounts for the majority of the atom’s mass. He called this the ‘nuclei’ [2]

In 1913, one of the latest and most pivotal discoveries in the course of atomic theory would be made by another of Thomson’s students, Niels Bohr. Bohr, drawing on previous works from Max Planck and Albert Einstein, demonstrated that electrons existed in fixed energy levels, known as shells, orbiting the nuclei of atoms; shells were arranged in decreasing energy levels, with a shell closest to the nucleus of an atom containing the most energy [2]. Furthermore, Bohr formulated electrons may jump into different shells when gaining/losing energy. The following equation notes that the difference in energy levels is equal to the energy absorbed or emitted by the electron [1]:

Bohr’s model of the atom was developed and challenged further when physicists recently proved that electrons have wave-like characteristics, such as producing vibrating oscillations causing them to move about instead of staying in a fixed orbit path around the nucleus.

Conclusion:

Be it the basic building blocks of all matter, atoms are everywhere around us. What started with Democritus’ underlying predictions has evolved after two millennia of discoveries, including those of John Dalton’s atomic proof, J.J. Thomson’s electron, Ernest Rutherford’s nucleus, Niels Bohr’s electronic configurations/shells and many more, into a developed field of science that we are yet to fully grasp.

References

[1] Eric, S. (2007). The Periodic Table: Its Story and Its Significance. Oxford University Press.

[2] Theresa, D. The 2400-year search for the atom. TED Ed. 8th December 2014 (https://ed.ted.com/lessons/the-2-400-yearsearch-for-the-atom-theresa-doud)

Aarush Vir Banerjee Kharbanda 9SOR