Abstract
Atoms are the building block of the world we live in. However, the understanding about the atoms that we have today has not been given to us just like that. It has been a fascinating journey of humanity in the quest for knowledge by applying his inquisitive mind and experimentation across a range of disciplines that have led to the present-day understanding of the atoms that we have today. Atoms are wonderful as we know them in themselves, in the aggregate forms like molecules, large crystals, and large structures around us. But this wonder crosses all the imaginative boundaries as we try to look within an atom. This paper presents the fascinating journey of humanity that has unraveled this wonderful entity known as atoms and its still more wonderful internal architecture.
Introduction
Today the understanding of atoms is so common that one may overlook the marvelous philosophy and scientific development that has led to the creation of the knowledge about atoms. It is thrilling to know-how from this apparent continuum world and matters the philosophers of ancient time could discern the quantum nature of matter and predict the existence of tiniest matter particle as atoms, without any experimentation to support their philosophy. A philosophy that was scientifically recognized and proved over twenty-four hundred years later.
It is also not less interesting that from the macroscopic observations about chemical reactions in gases and the relationship between macroscopic variables like pressure, temperature and volume; scientists could deduce the existence of such tiny particles like atoms and molecules and provide a scientific basis for atoms – a particle that cannot be dissected. Though atoms were proved to have substructures and therefore could be dissected into constituent particles from a physics point of view; from the point of view of chemistry and chemical reactions, atoms continue to remain what they mean – something that cannot be cut.
The architecture of the atom in itself can be termed the most marvelous structure and hides in itself the mystery of the Creation. The philosophical and experimental – theoretical journey to unravel the internal architecture of atoms has been probably the most interesting and exciting activity in modern science. It has had too many constructive and destructive implications for humanity.
This paper attempts to traverse the journey of conception, theorization, discovery, and deciphering of atoms to create the present-day knowledge about atoms.
Atoms in Philosophy
Humans are the most evolved species and they have a wonderful inquisitive brain. So it was not surprising that they were mesmerized by the marvels of the Creation and tried their best to unravel as much of it as they can. The matter being ubiquitous has attracted significant attention from philosophers and scientists. What is the structure of the matter or what the matter is made up of, was naturally a subject of deliberations among philosophers and scientists since time immemorial.
The debate of continuum versus quantum has always been there. A school of thought felt that matter was continuum like space and this was the dominant school of thought in ancient times because this is something that is supported by what our eyes can see. Looking at the objects around us, it is hard to discard the continuum nature of matter and it is said “seeing is believing”. Aristotle and the Stoic philosophers were in the camp that believed and propagated the continuum theory of matter (Handee W. R. et al 2).
But despite the lure of “seeing is believing”, there have been discerning philosophers even in the ancient days who could see through the apparent continuum the basic building blocks of matter the atoms. That the matter is composed of tiniest particle called “Parmanu”, which combines to form different structures, was pronounced by an Indian sage “Maharshi Kanad” as early as 600 BC, who was the founder of “Vaisesika philosophy” (Deshmukh P. C. et al 5). Greek philosophers like Democritus and Leucippus were also in the camp that believed in the quantum nature of matter and proposed the smallest particle for a matter which remains unchanged and in continuous motion (Serway A. R. et al 98). This article was named by them as “Atom” – something that cannot be cut. This term has become the most important term in the scientific community.
Though I have tried to keep the discussion on philosophy restricted to just one page, this in no way can undermine the importance of this section. Though there was no experimental support that the philosophers provided, it should be seen in the context of the availability of technologies at that time. It is easier to conduct an experiment when the supporting technology is there; further, it is easier to propose a theory, when there is some experimental data and evidence; but is the most difficult thing to propose something radically different, merely based on philosophical understanding, which is proved to be correct over two thousand four hundred years down the line.
Besides, the major significance of atomic theory has lies in the fact that it lays the foundation of quantum theory. Whether the matter is a continuum as appears to our eyes or is quantized as one discerns from the application of logic. It is the answer to this question, for which atomic theory favors quantization of matter over its continuum that is of great significance in all the philosophies.
Modern Atomic Theory
So far all that we have discussed was in the realm of philosophy. Wise men put their brains at work and proposed a model which advocated either continuum or quantized structure of matter. But merely putting a theory is not sufficient in scientific temperament. The theory much support or be supported by either direct or indirect experimental observations. The experiments may be performed by either theorist himself or his peers or his predecessors, but the theory should be validated by experimental observations and should be capable of explaining the experimentally observed results. Scientific supports for the atomic theory came from a whole lot of experiments performed by scientists, and some of these experiments are described in subsequent sections.
Debonair Lavosier’s Conservation of Matter
He was a great chemist and is known as the father of modern chemistry. He made immense contributions to modern chemistry. He was a great experimental chemist. He carried out many experiments about the chemistry of reactions and through his careful measurements of chemical reactions he established conservation of mass in chemical reactions. This was a great supporting idea to atomic theory because based on his findings of conservation of mass in chemical reactions, he could substantiate that all the elements are made of indestructible particles or atoms, which does not get either formed or destroyed in chemical reactions, instead it only changes the way it is combined with atoms of other elements to form new compounds. This was certainly the major experimental support to atomic theory.
Gas Laws
Experimental studies on the behavior of gases also contributed to the atomic theory to a great extent. It is interesting to know how macroscopic measurements like pressure, temperature, volume, and amount of gases and the fact that the same amount of all the gases occupied the same volume was used by the scientist to reason that gases are made of small particles. Kinetic theory of gases was developed with the underlying assumption that gases are made of tiny spherical particles with a lot of space between these particles and these particles move randomly in the space with a kinetic energy distribution (Boltzmann Distribution) and from applying Newtonian mechanics on individual particles and Boltzmann distribution; the macroscopic properties like Pressure, Temperature, etc. were derived. These experiments and the kinetic theory of gases provided the sound scientific background for the development of atomic theory by Dalton.
Daltons Atomic Theory
Dalton is known as the father of atomic theory. He was a great scientist. He was primarily a chemist who made great contributions to other spheres of science as well. Besides, atomic theory his great contribution was in the field of color blindness – which is also known as Daltonism. Dalton studied chemical reactions and focused on the ratios in which different pure substances (elements) combine to form a new compound. He found that different elements always combine in definite integral proportions to form compounds. From this, he elucidated that all the elements must be made of identical non-destructible particles which combine in integral proportions to form a compound. He named these simple particles as Atoms – something that cannot be cut.
Other Illustrative Theories
Though the atomic theory was fully established by the time Albert Einstein came into the picture, any description of atomic theory will remain incomplete without mentioning Einstein’s work on Brownian motion. Albert Einstein made a great theoretical contribution to the zigzag motion of suspended particles in dispersion (known as Brownian motion). This motion is due to molecular impact on small suspended particles. This work provided additional support to atomic–molecular theory and led to improved values of Avogadro’s number.
Internal Architecture of the Atom
Though the atomic theory was sufficient to answer many observations like gas laws, conservation of mass in chemical reactions, laws of chemical combination, etc.; but many interesting observations were still waiting for an answer which atomic theory was not able to provide. Some interesting observations like what is the mechanism by which atoms combine to make larges structures like molecules, crystals, etc.; why negative particles are liberated when a metal is heated, what is the origin of hydrogen spectrum, etc. are some of the questions which could not have been answered as long as we remained firm that atoms are something that cannot be cut or something that has no substructure. Indeed subsequent experimental findings did force people to concede to the fact that atoms have internal structure and a lot of interesting work came out of this quest of humanity to decipher the internal architecture of atoms. Some important experiments relating to the deciphering of atoms are described in the subsequent sections.
Faraday’s Laws of Electrolysis
A brilliant experimental physicist Michael Faraday gave his theory of electrolysis in 1833. He carried out electrolysis of many salts and gave his theory of electrolysis. The essence of his theory of electrolysis is that the Mass of an element deposited on the cathode is
- Directly proportional to the charge transferred.
- Directly proportional to the atomic weight of the element and
- Inversely proportional to the valance of the deposited element
Though it was not realized then, this theory provided strong proof for the theory that molecules are made of atoms, that charge is quantized, and that atoms consisted of parts that are negative and positive. However, the nature of the positive and negative parts of the atom was still unknown.
Discovery of Cathode Rays as Electrons & Plum Pudding Model
While performing experimental studies on electrical conduction through gases in low-pressure discharge tube J. J. Thomson in 1897 discovered that the rays seen in low-pressure discharge were due to negative particles (Thomson J. J., 269). It is worth mentioning here that cathode rays were known even before J. J. Thomson and William Crookes (1832–1919) had already shown with his ‘‘Maltese Cross’’ experiment that that cathode rays move in a straight line (Spear B., 331). But Thomson could correctly predict their nature as a negative constituent of all the atoms and measured their charge by mass (e/m) ratio. It is worth discussing the experiments briefly.
Methods and Materials
The primary equipment used in these experiments was a cathode ray discharge tube (shown in Fig. 1, below) and its variants. This is essentially a sealed glass tube filled with some gas at low pressure and two electrodes – anode (+ve) and cathode (-ve) with the provision to supply high voltage to these electrodes.
Different variants of cathode-ray tubes including those filled with different gases, parallel plate capacitors, phosphor screens, etc. were the principal materials used in these experiments.
When high voltage was applied there was a glow in the discharge tube. It appeared as if something was moving from the cathode towards the anode. This was termed a cathode ray. When an obstruction was placed in the path of the cathode ray, these rays applied force and caused motion in the obstruction implying, these rays have inertia and there was a shadow on the anode implying these rays moved in a straight line. But when an electric field was applied in the transverse direction to their motion by using a parallel plate capacitor, the cathode ray deviated towards +ve plate i.e. in the opposite direction to the electric field. This confirmed that these particles were negative corpuscles or negative particles.
Deviation of the cathode ray in the direction opposite to the direction of the applied field was used to measure the charge to mass ratio (e/m) for the cathode ray. Now the question to be answered was the origin of these –ve particles. Whether this has something to do with the gas that is filled in the discharge tube? To check this, the experiment was repeated with a discharge tube filled with different gases and the nature of the cathode ray remained the same for all the gases, including the e/m ratio. The e/m ratio was that matching with that of the lightest negative charge known so far. This left us with no option but to propose that the cathode ray was nothing but the fundamental constituent of all the atoms.
Now that the atom was no more indivisible, with its –ve constituent was taken out in discharge tube; the bigger question before us was to conceive and propose a model of the atom in light of this finding of the –ve corpuscles. We could not detect any positive particle in our experiment and also the particle was much lighter than the lightest atoms i.e. Hydrogen. Therefore, it appeared as if the atom was a big positively charged particle with uniformly distributed mass into which light –ve electrons were embedded like “Raisins in Pudding” (Fig. 2, below), such that the atom, on the whole, remains neutral and only –ve particles are ejected from it when it is supplied with energy – electrical discharge, heating, etc. Somehow we could not devise an experiment to measure the charge and mass of the atom separately.
Measurements of Electronic Charge
As electron was established as a fundamental constituent of an atom and quantum of –ve charge, it became necessary to know more about it. Thomson experiments could calculate its e/m, but then the value of ‘e’ and ‘m’ could not have been known without measuring either of the two alone. Millikan devised an intelligent experiment to calculate the value of ‘e’. This experiment is known as “Millikan’s Oil Drop Experiment” and is briefly described below.
Methods and Material
A schematic drawing of the experimental setup is shown in Fig. 3, below.
The materials used in the experiment are shown in Fig. 3. The basic principle underlying this experiment is the following.
Oil droplets created in the oil atomizer were made to fall through a parallel plate capacitor. The oil drop was loaded with electrons emitted from zinc when illuminated with UV light or X-ray. The motion of these charged oil drops between the parallel plates of the parallel plate capacitor was monitored using a telescope with and without application of the electric field.
Without the electric field, the force balance will be due to upward drag force on the oil drop and downward gravitational force. When the net force is zero a downward terminal velocity is reached, which can be measured with the telescope. With the electric field on, another force in the picture will be an electrostatic force. Again for an upward terminal velocity, the electrostatic force will be balanced by downward drag force and gravitational force.
Comparing the two balanced equations – one with the electric field on and another with the electric field switched off – for the same oil drop; the charge on the oil drop can be calculated. Similarly, the charge on another particle of the same size, but a different charge was calculated. The ratio of the charge on two oil drops with the same size but a different charge showed that electrical charge is quantized. The experiments confirmed the atomicity (quantization) of charge to approximately within 1% accuracy (Millikan R. A., 349). Millikan’s experiment was pioneering and truly ingenious in light of the technological status of that time.
Lenard’s Model of Atom
Lenard made significant contributions in understanding the cathode ray and made several useful observations about the photoelectric effect. When Lenard bombarded a thin sheet of metal with cathode rays, most of the electrons passed through. Based on this observation he rightly concluded that most of the atom is space and that mass and positive charge of an atom are concentrated in smaller regions. He suggested a model in which an atom is made of light negative particles and heavy +ve particles arranged in a manner that most of the space remains vacant in an atom. However, this left with many questions like what will be the configuration of the +ve and –ve particles and also why +ve particle then does not get liberated from atoms. Many of these questions got answered by the planetary model proposed by Rutherford.
Planetary Model of Atom by Rutherford
Rutherford was working on the scattering of α-particles by metal. He was concerned about the broadening of α-particle beam when it passed through a very thin sheet of metal. He was using a very thin sheet of gold. The experimental setup is shown in Fig. 4, below.
What was observed was that though there was broadening of the incident α-particle beam expectedly by scattering; almost all the particles were able to pass without much deviation. This meant that most of the atom was empty; so where were the mass and positive charge of the atom located. This question was answered by an accidental observation and I must put the description of that accidental discovery in the words of Rutherford himself (Rutherford L, 1936).
“I would like to use this example to show how you often stumble upon facts by accident. In the early days, I had observed that scattering of a-particle and Dr. Geiger in my laboratory had examined it in detail. He found in thin pieces of heavy metal that the scattering was unusually small, of the order of one degree. One day Geiger came to me and said, ‘Don’t you think that young Marsden, whom I have been training in radioactive methods, ought to begin a small research?’ Now, I had thought that, too, so I said, ‘Why not let him see if any α-particle can be scattered through a large angle?’ I may tell you in confidence that I did not believe that they would be, since we knew that the α-particle was a very fast, massive particle, with a great deal of energy, and you could show that if the scattering was due to accumulated effect of several small scatterings the chance of an α-the particle’s being scattered backward was very small.
Then I remember two or three days later Geiger coming to me in great excitement and saying, ‘We have been able to get some of the α-particles coming backward…..’ It was quite the most incredible event that has ever happened to me in my life. It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus.
It was then that I had the idea of an atom with a minute massive center carrying a charge. I worked out mathematically what laws the scattering should obey, and I found that the number of particles scattered through a given angle should be inversely proportional to the thickness of the scattering foil, the square of the nuclear charge, and inversely proportional to the fourth power of the velocity. These deductions were later verified by Geiger and Marsden in a series of beautiful experiments.”
I do not think after putting the words straight from Rutherford, himself I need to write anything more about the nuclear model of the atom with all the positive charge and mass concentrated in the tiny nucleus and the number of electrons sufficient to neutralize an atom revolving around the nucleus like planets around the Sun, keeping almost entire atom empty. However, I must mention some important merits and limitations of this model
- Most, almost all of the experimental work supporting Rutherford’s model was carried out by two co-workers of him namely – H. Geiger and E. Marsden (605). They were able to determine the size of the nucleus of aluminum by this experiment (Keller A., 215).
- The planetary model of the atom with electrons revolving around the heavy and positively charged nucleus was a stable structure as far classical mechanics goes; but the theory of electromagnetism says that a charged particle accelerating / decelerating in an electric field will emit electromagnetic radiation, therefore, the electrons in Rutherford’s atoms must emit radiation all the time, thus losing all their energy in no time and falling into the nucleus killing the atom itself. But it is something that does not happen. So what is the reason?
- What is there to hold so many positive particles (protons) in the nucleus against the huge Columbia repulsion between protons? This was answered by proposing the existence of a strong attractive force between the nucleons, at a later stage by scientists like Yukawa, when nucleons are at a distance smaller than 2 FM. James Chadwick had realized the existence of a much stronger force than an electrostatic force within the nucleus (Chadwick J. and Biele E. S., 923).
- For electrical neutrality, several positive particles (protons) have to be the same as that of the electrons, but then the number of protons required for electrical neutrality of atoms was able to account for only half of the atomic weight. So where from comes the remaining half of the atomic weight. It must be mentioned here that neutrons were not known till then. In 1932 J. Chadwick experimentally proved the existence of neutral particles with mass closer to that of a proton by carrying out an artificial nuclear reaction.
- How to explain the hydrogen spectrum was also not answered by Rutherford’s model. So something more was needed. Something that was proposed by Bohr and the same is discussed in the subsequent section.
Bohr’s Model
Niels Bohr was a great theoretical physicist. He proposed a quantum model of the atom in his three-part paper (Bohr N., 231) by postulating the quantization of energy levels for electrons within an atom. This model was successful in explaining why electrons do not lose energy while revolving around the positively charged nucleus and also the spectrum of the hydrogen atom. The main postulates of Bohr’s quantum model of the atom are the following.
- Electrons revolve around the nucleus like planets in the solar system.
- Energy level of electrons in an atom is quantized i.e. electrons can have only a certain fixed value of energy in an orbit and their energy remains the same as long as electrons remain in that orbit.
- When electrons jump from an orbit with higher energy to the one with lower energy, the difference of the energy is liberated as electromagnetic radiation, conversely when electrons are supplied with energy equal to the difference between the lower level and higher level, they jump to the orbit with higher energy.
- Angular momentum of electrons in an atom is quantized.
This model was radical as it contradicted the well-established theory of electromagnetism by Maxwell. It simply said that for bound electrons this theory will not be applicable as these electrons will be governed by the principles of quantum mechanics.
This theory was able to explain the hydrogen spectrum very well and could calculate different energy levels of electrons, the wavelength of the lines in the hydrogen spectrum, and the value of the Rydberg constant to a great accuracy for the hydrogen atom and hydrogen-like ions i.e. systems having one electron.
On the flip side, one can say that the postulates of this model were arbitrary i.e. all of sudden someone comes and say no rules of electromagnetism will not be applicable for atomic electrons. What was the basis? Nothing, just this hypothesis helped explain the observations existing at that time. So were not these postulates were something like convenient assumptions. While an argument cannot be denied on the face of it, the quantum nature of energy levels for bonded electrons was something that was verified experimentally and also theoretically from the subsequently wave mechanics model of electrons.
In his correspondence principle, Bohr was also able to provide a connection between the quantum level of energy and continuum level of energy at higher quantum numbers and thus he could show that continuum is nothing but a subset of quantum nature of energy levels and that at higher quantum numbers quantum becomes a continuum. Thus it can be said that though it appears arbitrary, the quantum model of the atom proposed by Bohr was a radical and revolutionary theory. Thus far only charge and matter were quantized, but in the aftermath of Bohr’s model, even energy levels became quantized.
Still, Bohr’s model was not able to explain the spectrum of atoms/ions with more than one electron and the presence of hyperfine lines in the hydrogen spectrum. A further refined model of the atom was presented by applying wave mechanics to the electrons. This is briefly discussed below.
Wave Mechanics Model of Atoms
Wave-particle duality was proposed by de Broglie. He said a particle with momentum ‘p’ can be seen as a wave with wavelength λ = h/p; where h is Plank’s constant. Many scientists like Born, Heisenberg, Schrödinger, etc. made significant contributions towards the wave mechanics model of atoms. This is a purely theoretical model. This model considers electrons as wave function ψ(x,y,z,t) and solves this wave function for electrons bonded to an atom. Solution of the wave function for an electron bonded to an atom required three arbitrary integers, which are nothing but quantum numbers. Once ψ(x,y,z,t) is known; ψ is calculated by multiplying ψ(x,y,z,t) with ψ(x,y,z,t). ψ thus calculated is the probability density for finding an electron.
This can be plotted taking nucleus as the origin and a region can be obtained in which the probability of finding that electron is equal to a specified value say 99%. This shape is the shape of the orbital in which that electron is there in that atom. This is a simplistic way to physically visualize the wave mechanics model of the highly mathematical atom. This model remains the most acceptable model of an atom and is capable of explaining most of the observations concerning atoms and molecules.
Other Important Contributions
This discussion about the deciphering of an atom will remain incomplete without mentioning Moseley’s Law, which established atomic number or the number of protons in an atom as the primary determinant of the chemical nature of an atom and the basis for arranging atoms in a periodic table. Hund’s rule, Pauli’s exclusion principle, etc. proved very helpful in deciding the electronic configuration of an atom which determines the valance and chemical characteristics of an atom. The discussion will go endless as the journey to decipher the atom was marvelous, but we have to stop somewhere and this is the right place in my opinion.
Conclusions
Based on the discussions in the preceding sections it can be concluded that the underlying philosophy of the atomic theory and quantization of mass remains great proof of the intellectual capability of the human brain. The experiments designed to give scientific legitimacy to the atomic theory were ingenious. The journey for deciphering the internal architecture of atoms was marvelous and gave birth to many new and radical theories like quantization of charge and energy, wave mechanics model, and an altogether new vision to see the world around us and the Creation. Many useful technologies like CRT tube for television, radio for telecommunication, nuclear energy, laser, etc. owe their birth to this marvelous journey of humanity for deciphering the atom.
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