The Story Of Atoms
Science walks with two legs, Theory and Experiment, in the quest for the discovery of knowledge. Sometimes theory is ahead and other times observations made by experiments are ahead. This is the story, in chronological order, of how science walked through centuries powered by continuous human curiosity that led to the discovery of atoms.
The story of atoms began as soon as ancient philosophers started wondering about what everything in this world is made up of. Around 400 BC Greek philosopher Democritus thought and proposed that everything in this world should be made up of something indivisible called ‘atomos’. In Greek ‘atomos’ means indivisible.
Fifty years later another Greek philosopher named Empedecles proposed that all matter is made up of four different ‘Elements’, fire, earth, water, and air. One hundred years later, Aristotle, one of the most influential philosophers of Ancient Greece, believed in the four-element theory and that became more popular. Democritus’s hypothesis was possibly ignored for a long time because of that.
But, it wasn’t just Democritus who imagined atoms. Many philosophers around the world from various cultures also hypothesized the atom as the fundamental building block of matter.
Avvaiyar (The First), a poet from Tamil Nadu, India, who lived in the 3rd century BC wrote the following metaphorical praise for Thirukkural poems. Since Thirukkural has fit vast knowledge into poems of just seven words. “அணுவை துளைத்து ஏழ் கடலை புகட்டி குறுக தரித்த குறள்.” The literal translation for this is, “Thirukkural poems have fit seven oceans into a single atom.” Another poet, Avvaiyar (The Third), who lived in the 12th century AD wrote the following to express how Lord Ganesha is both omnipresent and infinite, “அணுவிற்கு அணுவாய், அப்பாலுக்கு அப்பாலாய்.” This means, [Lord Ganesha is the] building block of every atom, and [he] extends beyond the limits of the universe.
In 1590 Zaccharias Janssen invented the microscope and in 1609 AD Galileo Galilei perfected it. This invention allowed science to take a peek at the tiny invisible microscopic world. Scientists were able to see tiny bacterias and amoebas but not the atoms. Atoms are 100 millionth the size of a bacteria, even the most modern technology scanning tunnelling microscopes can not peek and look into an atom.
In 1803, the theory of atoms was picking up heat again. John Dalton, an English chemist, also believed that atoms existed. His atomic theory had 4 basic parts to it. Today we know that some of these parts are only mostly true.
- All matter is made up of tiny, indivisible particles called atoms.
- All atoms of a given element are identical in mass and properties, but different to atoms of another element.
- Compounds are formed when two or more different types of atoms combine in fixed, whole-number ratios. He based this on the law of constant composition, which states that a given pure compound will always have the same proportion of the same elements.
- A chemical reaction is just a simple rearrangement of atoms. He based this on the law of conservation of mass, which states that matter can neither be created nor destroyed, only changed.
In 1827, Robert Brown, a Scottish Botanist, Observed using a microscope that pollen grains suspended in still water were continuously jittering and moving. He first thought this movement was because there were bacteria on the pollen grains. To confirm, he suspended some dust particles on the water instead. And to his surprise, the dust also moved randomly. The movement he was seeing is called Brownian motion. He was not able to explain why this was occurring at the time. It was Albert Einstein in 1905, who explained that the pollen grains were moving because the individual water molecules were pushing them around.
In 1897, J.J. Thomson, a British physicist, was experimenting using cathode rays. Cathode rays are formed when a high voltage is applied across two electrodes: an anode, which is negatively charged, and a cathode, which is positively charged. When he directed the cathode ray through two electrically charged plates, Thomson observed that the cathode ray deflected away from the negative plate and went closer to the positive plate. Using this fact, he was able to understand that the cathode ray was made up of particles that are negatively charged. He called them ‘corpuscles’ and calculated that they were approximately 1/2000th the size of a hydrogen atom. This meant that the atom is not the smallest unit of matter.
In 1904, he proposed a new model for the atom. It was known through experiments that atoms consisted of both positively charged and negatively charged matter. Since there was an overall neutral charge for the atom, he proposed that the electrons must be floating in a positively charged matter. This model is called the plum-pudding model, after a popular English dessert plum pudding that had plum pieces floating on a pudding layer.
In the same year, Hantaro Nagaoka, a Japanese physicist, rejected J.J. Thomson’s atomic model because he believed that opposite charges were impenetrable. In place of the plum-pudding model, he proposed the planetary model for the structure of the atom. In which, there is a positively charged center, while the electrons are circling the center analogous to how a planet would circle a star.
In 1908, Ernest Rutherford, a British-New Zealand physicist, was testing the plum-pudding model with an experiment. He dir ected positively charged alpha particles towards a gold foil that was surrounded by a luminescent screen that can detect alpha particles. Based on the plum-pudding model, Rutherford predicted that all the alpha particles would be able to pass straight through it because of the electric field from the spread-out positively charged matter (a.k.a the pudding in the model) in the model would be too weak to change the trajectory of the alpha particles.
But the results of the experiment were shocking. While most of the alpha particles passed straight through the gold foil, about 1 in 20,000 got deflected more than 90 degrees from their path. Rutherford’s analogy:
“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
This means that there needs to be two modifications/ additions to Hantaro Nagaoka’s planetary model of the atom.
- The positive charge needs to be very concentrated because only a few alpha particles get deflected drastically by the atom.
- Since most of the alpha particles passed through the gold foil, most of the atom must be empty.
And slowly the visual of the atom, as we perceive now, began to emerge. This is only the beginning of a long and exciting story of atoms. There have been a lot more discoveries made since then and there will be a lot more to come in the future.
Thanks for Reading!
