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Our Journey To The Centre Of The Atom - An Introduction

Meghna K K, Senior Researcher, Department of Physics, Prayoga, Bengaluru

 
“What is matter?”

This has long been one of science's fundamental questions. The search for an answer to this question led to many groundbreaking discoveries. Let us first attempt to answer this question before delving into the discoveries that have altered our understanding of the universe. Matter is defined as “anything that occupies space and has a mass” in a high school science textbook. A straightforward but abstract definition. But what exactly is matter made of?

What is matter made of?

The idea that matter is composed of indivisible building blocks has been around for a long time. The Greeks referred to this smallest unit of matter as Atomos, which means "indivisible." This word gave rise to the modern term "atom". According to recorded history, the Greek philosopher Democritus proposed that matter was made up of indestructible, distinct units called atoms.

Other ideas about the nature of matter were also present. They were all philosophical discourses. None of these theories were supported by scientific or experimental evidence. The human eye was the first detector that assisted in understanding the nature of matter. The eyes detect visible light reflected from objects, revealing their different natures and states of matter. The wavelength of visible light ranges from 400 to 700 nm. Even with strong lenses, we couldn't see anything smaller than that. To see matter on a smaller scale, we needed some kind of "rays" with shorter wavelengths and a sensor to detect them. We will see how humans solved this problem and probed matter on a very small scale.

What does an atom look like?

  • Atomic models and other discoveries

In 1803 the English chemist John Dalton developed a more scientific theory of atoms and compounds. He coined the ancient Greek concept of the atom as an indivisible solid sphere that could not be created or destroyed. According to Dalton’s model, atoms of a given element were similar and a chemical compound is a combination of atoms of different types [1]. But his theory could say very little about the nature of atoms.

The discovery of cathode rays has led to a series of discoveries that would help unravel the mysteries of the atom. The conduction of electricity through gas was thoroughly studied by scientists in the mid-18th century. In 1878, Sir William Crookes discovered the flow of particles emitted from the cathode which could cause glow in gas at low pressure. The discovery of X-rays by Roentgen in 1895 opened the door to new ways of studying the structure of matter. He found that X-rays, unlike visible light, can penetrate thick layers of finely powdered rock salt, human flesh, and some other materials. While investigating the properties of x rays, Henri Becquerel discovered radioactivity (named by Marie Curie) in 1896. Unlike X-rays, the radiation he found was charged and hence got deflected in the magnetic field. Another kind of radiation was also found which was neutral but more penetrating than X-rays. At the beginning of 1900, Rutherford, who studied radioactivity in detail, named these radiations as Alpha (α, positively charged), Beta (β, negatively charged), and Gamma (γ, Neutral) [2].

In experiments with cathode rays, British physicist J J Thomson discovered that cathode rays were attracted to positively charged metals and repelled by negatively charged metals. He also found that the particles in these rays are much smaller than hydrogen, and these particles were also found in many atoms. It was the discovery of the first subatomic particles, later called electrons. This finding proved for the first time that an atom was not indivisible and had smaller components. J J Thomson won the 1906 Nobel Prize for this discovery.

Following the discovery of electrons, J J Thomson put forward his atomic model, later known as the Plum Pudding model, in 1904. According to his atomic model, negatively charged particles were in atoms like plums in the pudding and positive charge was continuously distributed throughout the atom [1,4].

So, the fundamental building blocks of matter at this time of the history were atoms and electrons!

  • Rutherford’s Experiment

In 1911, another breakthrough occurred. Thomson's student, Rutherford, devised an experiment to investigate the effect of alpha particles on matter [1,3]. A radioactive source that emitted α particles, a thin gold foil, and a screen that can detect α particles were used in the experiment (see Figure 1). Rutherford was aware that α particles were very fast, and were heavier than electrons. And, according to the Plum Pudding model, which was the accepted model at the time, most of the α particles should pass through the gold foil with no or little deflection. Only a few of them would be scattered off by atoms at extremely small angles. Rutherford’s assistant Geiger and a research student Ernest Marsden carried out the experiment.

Figure 1: A Schematic of Rutherford’s α scattering experiment [credit: Wikimedia Commons].


The majority of the alpha particles were able to penetrate the gold foil, as expected, and a few of them were scattered by the atom at small angles. Unexpectedly, a small number of alpha particles were deflected at large angles or scattered back towards the source. Rutherford devised a new atomic model to explain this result.

According to this new model, the significant portion of the atom was empty, all of the positive charge of the atom and the majority of the mass of the atom were concentrated at the centre of the atom. This part of the atom where all the positive charge was accumulated and was infinitesimally small in comparison to the total volume of the atom, was called the nucleus of the atom by Rutherford. Rutherford’s atomic model was known as the nuclear model (see Figure 2).

Rutherford's model was a significant step toward a better understanding of the structure of the atom. It could not, however, explain how electrons are arranged in the empty space surrounding the nucleus.

Figure 2: Evolution of atomic models


We'll take a break from the story of atomic models to investigate the nucleus in greater depth.

The nucleus

The new discovery raised more questions (as always happens in science!). Rutherford's model was modified by Bohr, who explained how electrons were arranged in an atom. These models were successful in explaining chemical compounds, and radioactivity to some extent. However, they were unable to answer questions such as, what is the structure of the nucleus? Is the nucleus made up of constituents? If so, how are they bonded? The experimental methods for investigating the "building blocks" became more challenging as the dimensions of the "building blocks" got smaller.

At the same time, our understanding of radioactivity was rapidly improving. By 1909, Rutherford had demonstrated convincingly that the alpha particles were nothing but the helium nucleus. His calculations also showed that the dimension of the nucleus was of the order of 10^-14 m. It was discovered that emitting alpha and beta particles caused the nucleus of one atom to change to that of another. This implied that the nucleus had an internal structure that could change during radioactive processes.

Because radioactive nuclei emitted alpha and beta particles, a nuclear model containing alpha and beta particles was proposed. This model, however, was rejected because there were non-radioactive nuclei and nuclei smaller than alpha particles. To explain the observations, a particle with a single positive charge was required. The hydrogen nucleus was the best candidate particle. It was referred to as a 'proton' at that time. By then, beta particles had been identified as electrons. This was the proton-electron nuclear model [1,4].

According to this model, the nucleus's mass number A should be nearly equal to the nuclear charge Z. However, with the exception of hydrogen, the nuclear charge Z was always equal to or less than (1/2)A. It was proposed that the nuclei had (A-Z) electrons to cancel the extra charge of protons. Only a small fraction of nuclear mass would be contributed by electrons. There would be A number of protons and (A-Z) number of electrons inside the nucleus and A number of electrons outside the nucleus, resulting in a neutral atom. However, as the mass of the nucleus was more precisely measured, this model failed to explain the results and was rejected.

Rutherford observed a process known as nuclear disintegration in 1917. A proton from the nitrogen nucleus could be knocked out by an alpha particle. However, the probability of this process was so low that when one million alpha particles were passed through nitrogen gas, only one proton was produced. This was the first instance of artificial disintegration. Rutherford also proposed that higher-energy alpha particles could be used to break down the nuclear structure of lighter atoms. He predicted that the tightly packed proton and electron inside the nucleus could be a new particle called a ‘neutron’.

Scientists discovered that when bombarded with extremely fast alpha particles, light elements such as beryllium emitted radiation that was more penetrating than a proton. James Chadwick, a student of Rutherford, began researching these radiations in 1932. He discovered that this was a particle with a mass similar to that of a proton. This was the discovery of the neutron, another constituent of the nucleus [1,4].

Protons and neutrons

Protons and neutrons are collectively known as nucleons. The discovery of nucleons raised a new set of questions. What is the force that kept them together? Since the neutron is electrically neutral, it cannot be the electromagnetic force. Gravity cannot play a role in these particles' attraction because their mass is so small. This attractive force must also be significantly stronger than the repulsive force between protons. This force which holds nucleons together is known as strong nuclear force [5].

Other particles created by strong force had been observed in high energy cosmic ray interactions by the end of the 1940s. It was discovered in the early 1960s that nucleons were particles with a finite size of about 10^-15 m. Many other particles were discovered as a result of the research on cosmic rays.

These and other discoveries like magnetic properties of protons and neutrons led to the hypothesis that nucleons were not 'elementary' particles. In 1964, physicists Gell-Mann and Zweig independently proposed that nucleons and the newly found particles formed by strong forces were composed of elementary particles with 'fractional' charges equal to ‘(1/3)e’ or ‘(2/3)e’ where ‘e’ is the electric charge of an electron or proton. Gell-Mann called them quarks (see Figure 3) [4,5,6].

Figure 3: Graphical representation of the nature of matter (taken from [8])


The quark model and the discovery of quarks

According to the Gell-Mann-Zweig model, there are three quarks known as the 'up' quark or 'u', the 'down' quark or 'd', the 'strange' quark or 's', and their antiparticles. The proton is made up of two 'up' quarks and one 'down' quark (written as uud), while the neutron is made up of one 'up' quark and two 'down' quarks (written as udd). The electric charge of the up quark is (+2/3)e (where e is the value of the electron charge) and the electric charge of the down quark is (-1/3)e. Gell-Mann received the Nobel Prize for Physics in 1969 for this work [6].

The quark model even predicted new particles which were later discovered. Scientists began searching for quarks. A series of experiments conducted by the MIT-SLAC (Massachusetts Institute of Technology - Stanford Linear Accelerator Center) collaboration from 1967 to 1973 demonstrated the physical existence of quarks [7]. These were collision experiments in which high energy electrons collided with a proton target (remember Rutherford's experiment!). They found that the electrons were scattered off by point-like particles inside protons. In 1990, this discovery was awarded the Nobel Prize in Physics [6].

The quark model could explain many physical phenomena observed by scientists. There are many other types of fundamental particles besides the ones that make up the atom: electrons and quarks. We will discuss them in the next part.

According to current knowledge, these are the fundamental/elementary particles that have no internal structure (that is, they are not made up of smaller particles). These particles make up all matter, including us. Our understanding of these particles and their interactions is represented in The Standard Model of Elementary Particles [5].

References

  1. Holton, Gerald James. Introduction to Concepts and Theories in Physical Science. Addison-Wesley Press, India, 1952.

  2. Pierre Radvanyi, Jacques Villain. The discovery of radioactivity, Comptes Rendus Physique, 18, 2017, 544.

  3. Heilbron, J. L. Ernest Rutherford: And the Explosion of Atoms (Oxford Portraits in Science). United Kingdom, Oxford University Press, USA, 2003.

  4. Weinberg, Steven. The discovery of subatomic particles. Cambridge University Press, United Kingdom, 2003.

  5. Martin, Alan D., and Halzen, Francis. QUARK & LEPTONS: AN INTRODUCTORY COURSE IN MODERN PARTICLE PHYSICS. Wiley India Pvt. Limited, India, 2008.

  6. Michael Riordan. The Discovery of Quarks, SCIENCE, 29 May 1992, 256, 5061, 1287.

  7. SLAC History, https://www6.slac.stanford.edu/about/slac-history, (accessed February 2022).

  8. Quark Search, http://www.tsgc.utexas.edu/quarks/quark-page.html, (accessed February 2022).

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