EVOLUTION OF PHYSICS- THE MODERN INTERPRETATIONS OF PHYSICS (1)

No other discipline of science has grown and developed as rapidly as Physics. In the blogs that follow, I will try my level best to describe the modern notions of this subject. But wait... do you know that Newton's Laws and their numericals, Newton's Universal Law of Gravitation, Bohr's Model of the Atom, etc., which you studied so hard for your exams, do not actually hold universally and are considered 'false statements' according to modern research and scientific understanding?

Then why did we learn all these statements in the first place? This is a very relevant question. We solved countless questions based on these so-called 'false statements' and even lost marks for not solving them correctly — when the 'law' itself is wrong!

The reason we learned them is because understanding where they went wrong and how they were corrected is crucial. For example, the earliest understanding of nature was that the universe was made up of matter. Later, we learned that matter is made up of atoms. Democritus, an ancient Greek philosopher, postulated that atoms were the tiniest particles in the universe, indivisible and fundamental. Later, we discovered that atoms themselves contain protons, neutrons, and electrons. Further exploration into atomic structure revealed the presence of a positively charged nucleus, confirmed by Rutherford through his famous ⍺-particle scattering experiment.

Bohr then proposed that electrons revolve around the nucleus in circular paths called 'orbits.' However, the contributions of scientists like de Broglie, Heisenberg, and Schrödinger led to the modern understanding of 'orbitals,' which exist within these orbits. This is now the currently accepted model of the atom.

See what just happened? We traced the evolution of scientific thought — from the broad concept of matter to the tiniest details of atomic structure. Who knows? Within a decade or so, the next generation might be learning about particles even smaller than electrons and protons!

This is just the story of the 'atom.' Science is full of such examples, showing that science itself is a living entity — constantly refined, improved, and reshaped over time. It will forever remain one of the greatest ‘products’ of human intellect and curiosity.

Alright then... let’s meet some of the brilliant scientists of the modern era. I will try to feature as many as possible in this series of blogs because their passion for science is unparalleled, and their contributions led to incredible breakthroughs in their respective fields.

Let us begin today with the story of the 'atom'...

The atom is the smallest particle in the universe. All matter is made up of atoms, making them the basic units of everything, from the macroscopic world to the microscopic realm.

Scientists have always been curious about understanding the structure of the atom, and over the years, numerous models have been proposed — from Thomson's Model to the currently accepted quantum mechanical model. In this blog, we will explore Thomson's and Rutherford's Atomic Models.

PLUM PUDDING MODEL OF AN ATOM

                                                        

Image 1: Sir J.J Thomson

Sir J.J. Thomson was an English physicist who is often credited with the discovery of the negatively charged particles present within the atom, now known as electrons. He discovered these electrons while conducting the famous cathode ray experiment.

The cathode rays themselves consisted of electrons, which moved from the cathode (the negative end) to the anode (the positive end). For this groundbreaking discovery, J.J. Thomson was awarded the Nobel Prize in Physics in 1906.

The Thomson Model of the Atom is given below for your reference:

                                               

Figure 2: Plum Pudding Model of an Atom

J.J. Thomson’s famous cathode ray experiment played a crucial role in the discovery of the electron, the first known subatomic particle. In this experiment, he used a discharge tube — a glass tube containing gas at low pressure with metal electrodes at both ends, connected to a high-voltage source. 

Figure 3: Cathode Ray experiment

When the voltage was applied, a beam called the cathode ray was emitted from the negatively charged cathode and moved toward the positively charged anode. Thomson placed charged plates and magnetic fields around the tube and observed that the cathode rays were deflected toward the positively charged plate, proving that they carried negative charge. 

He further found that the nature of these particles did not depend on the type of gas in the tube or the material of the electrodes — showing that these particles were universal components of all matter. Through careful measurements, Thomson concluded that these negatively charged particles were much smaller than atoms, overturning the belief that atoms were indivisible. 

He initially called them corpuscles, but they were later renamed electrons.

However, one can observe that in this model of atom the negatively charged particles which we call the electrons are embedded in a pool or sphere of positive charge. It should be noted that the positive and negative charges are equal in magnitude so that the atom becomes electrically neutral which in turn makes it stable.

But unfortunately, Thomson’s Model was discarded, as it failed to explain the experimental results obtained by many other scientists — especially the results of one man, a man who contributed wholeheartedly to the development of atomic structure, a man who would later be known as ‘The Father of Nuclear Physics’.

RUTHERFORD'S MODEL OF AN ATOM

I must say, Rutherford’s α-particle scattering experiment, its postulates, and his observations still resonate in my mind even today. In fact, this experiment was a guaranteed question in our 9th-grade final exam! Believe it or not, I wrote about this experiment as if I was the one performing it myself — and that’s probably why I love explaining this model more than any other atomic model. There’s something about Rutherford’s sheer brilliance and the simplicity of the experiment that makes it unforgettable.

                                                 

Figure 4: Sir Ernest Rutherford

THE 𝛂-PARTICLE  SCATTERING EXPERIMENT

Rutherford took a gold foil that was about 1000 atoms thick. Now, why gold foil, you ask? Simple — gold could be beaten into extremely thin sheets, and Rutherford needed just that for his experiment!

The α-particles he used had considerable energy. To directly quote my textbook, α-particles are doubly charged helium ions (He²⁺) with a mass of 4u, and as they moved, they carried kinetic energy along with them.

And now, Rutherford was ready to begin his historic experiment… perfect silence in his lab… all eyes on the gold foil…

Suddenly, Rutherford screamed with joy!

He was truly astonished by his observations — something so unexpected that it completely changed our understanding of the atom. His excitement is clear from one of his famous quotes that followed this legendary experiment:

'' This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits at you!'' 

                                          

His observations were truly remarkable.

1. He found that most of the alpha particles passed straight through the gold foil without getting deflected.

2. Only some alpha particles were deflected from their original path.

3. Some alpha particles rebounded after getting deflected.

Rutherford in turn concluded that,

1. The positively charged region inside the atom was small.

Explanation: Alpha particles are positively charged and we already know that the nucleus is also positively charged. From our earlier lessons in Physics we already learnt that like charged repel and unlike charges attract one another. 

Therefore if the positively charged nucleus occupies a larger volume within the atom then the most of alpha particles will be deflected as like charges repel, however Rutherford found that most of the alpha particles passed straight through the gold foil without any deflection which suggested that nucleus occupies a small volume within the nucleus compared to the size the atom.

2. Most of the region inside an atom is empty, as most of the alpha particles went straight through the gold foil without getting deflected.

                             

Figure 5: Rutherford's Nuclear Model of an Atom

                                       

Rutherford published his famous paper on the Nuclear Model of the Atom, where he postulated that the nucleus occupies the center of the atom, and nearly all of the atom’s mass is concentrated in this tiny nucleus. The reason for this, he suggested, is that the nucleus contains both protons and neutrons, which together account for almost the entire mass of the atom — much larger than the mass of the electrons moving around the nucleus in circular orbits.

However, Rutherford made one big theoretical mistake — he believed that electrons revolve around the nucleus in circular orbits.

Though most of his conclusions were groundbreaking and accurate, this particular idea about circular orbits turned out to be incorrect.

Here’s the problem — any object moving in a circular orbit around a fixed mass (in this case, the nucleus) experiences centripetal acceleration. Now, electrons are charged particles, and according to classical electromagnetic theory, a charged particle under acceleration should continuously radiate energy.

So, if Rutherford’s picture were true, the electron would gradually lose energy, spiraling inward until it collapsed into the nucleus.

Well, we already know that atoms are stable. If Rutherford’s "circular orbit" theory was correct, it would directly contradict the very fact that atoms are stable structures. This raised a huge question — how can atoms exist at all if electrons are supposed to collapse into the nucleus? 

One can find an interesting analogy here — Rutherford was Thomson’s student, and now Rutherford’s model would soon be refined and improved by his own student, Niels Bohr!

REFERENCES:

1. Referred the NCERT textbook Class 9 for some clarifications.

2. All the images shown in this blog were taken directly from Wikimedia Commons

*All the media published in this blog belongs to their original creators and 'Knowledge Through Science' does not claim any right over it.