History of Atomic Structure

Kevin Li
Mrs. Baldwin
Honors Chemistry
3 November 2012
Historical Development of Atomic Structure Over the many years of the world’s history, there have been numerous developments and discoveries pertaining to atomic structure. Each of these breakthroughs gradually led us—step by step—to a deeper understanding of what makes up each and every atom. Because of this, our perception of an atom’s structure today is vastly different from the first idea of an atom’s structure from many centuries ago. From Dalton’s theory that all atoms were spherical in shape to the quantum atomic model, we owe our current knowledge of the atomic structure to the many great minds and thinkers that have carried the idea to the through and detailed concept that we are familiar with today—without them, we would cease to understand everything within our world that surrounds us. The idea that the things that we encounter in our lives are actually made up of smaller subunits arose numerous years—millennia—in Ancient Greece. A man named Leucippus hypothesized that all matter around us is made tiny subatomic units called atom—the word atom originates from the Greek word atomos, which in Greek virtually means “indivisible” (Stanford Encyclopedia of Philosophy: Ancient Atomism). This idea was actually inherited by Democritus is widely given credit for this idea (Boorse 1). However, his theory would be rivaled by famous Greek philosopher Aristotle, who claimed that all matter consisted of four elements: earth, air, water and fire. [See figure 1.1] Aristotle also believed that these elements possessed certain qualities: dryness, coldness, moistness, and hotness—where earth is dry and cold, water is cold and moist, air is moist and hot, and fire is hot and dry (A Timeline on Atomic Structure). He claimed, in addition, that the atomic theory of Democritus was nonsense. His misguided and incorrect theory would be spurred by the support of figures of members of the Catholic Church, who spurned the valid theory of Democritus as “Godlessness” (The Greek Concept of Atomos: The Indivisible Atom). For this reason, religion would play a momentous role in the direction of which science and innovation was headed; in fact, it suspended and delayed the further discoveries in atomic structure. It would not be for a long time until scientists finally took another step forward in the development of atomic structure. Very many years after Democritus and Aristotle, a physicist named Isaac Newton made another contribution toward the grasp of atomic structure. In the early 1700s, he suggested that atoms were held together with attractions, otherwise known as forces. In addition, he stated that matter is formed of “solid, massy impenetrable particles” (Timeline of Atomic Theory). His explanation of the “forces” would be extremely significant in the physical science—in fact, numerous laws, such as his laws of motion and thermodynamics, were devised by him then still stand very adamantly and prominently in our world of physics today. A century later, a scientist—John Dalton—emerged in the world of atomic structure as a significant pioneer and innovator, revitalizing the previously dormant world of atomic structure. The son of a poor English weaver, he did not let money stand in his way of knowledge: he became a teacher and meteorologist. His interest in gases and their properties eventually led him to the world of chemistry and atomism. In 1808, he would publish a book called New System of Chemical Philosophy, where he explored an atomic theory that picked up from where Democritus had left off (Boorse 38). In this book, he would explore chemistry in even greater depth than ever before, and reasoning from a large number of observations, concluded with a few groundbreaking postulates. All of them built off the theory of Democritus by stating that each element is composed of extremely small particles called atoms, atoms that comprised elements. All atoms of a given element were the same and atoms of different elements had different properties and masses. Dalton was the first to explore atomic theory by investigating the weight of atoms (John Dalton—Biography), and also stated that atoms of an element are not changed or modified during chemical reactions; they cannot be created nor destroyed either (Brown 36). He also proved in his experiments that atoms combine in simple whole number ratios. He eventually crafted a model of an atom as simple spheres, using large wooden balls [Figure 1.2] to demonstrate their shape and how they interacted with each other. These numerous contributions to the chemistry world earned him respect and reverence from the scientific world, and his theories were widely accepted. Most of his atomic theory has remained to this day in chemistry textbooks, with only a few corrections and objections. Along with the atomic theory, there were also advancements in technology, as well. Cathode ray tubes, vacuum tubes that produced images when its phosphorescent surfaces were struck by electron beams [See Figure 1.3] began to be developed in the mid-1800s (Timeline of Cathode Ray Tube). First explored and built by German physicists Heinrich Geissler and Julius Plucker, the cathode ray tube was examined heavily by the Englishman Sir William Crookes. Addressing the issues with cathode rays such the difficulty of producing the law pressures necessary to study them, Crookes began his researches, which were reported in his famous paper, “On the Illumination of Lines of Electrical Pressure and the Trajectory Molecules” (Boorse 91). Although he did not have a scientific background, similar to most scientists, he attended the Royal College of Chemistry and eventually went on to become in charge of the meteorological department at Radcliffe Observatory in Oxford University (Encyclopedia Britannica: Sir William Crookes). His extensive research on the cathode ray tubes led him to numerous discoveries, such as the dark space around the cathode (now called Crookes’ dark space) and that cathode rays traveled in straight lines and produced both phosphorescence and heat when they struck certain materials (Atomic Structure Timeline). Additionally, with the introduction of spectrum analysis by R.W. Bunsen and G.R. Kirchhoff, he investigated compounds that contained selenium and discovered thallium. His thoroughly developed cathode ray tube that was an enormous improvement over previous models, however, would be a doorway into further discoveries in the future, and he was knighted for his scientific achievements. Taking the discoveries of Crookes on the cathode ray tube, Joseph John Thomson, a professor of experimental physics at Cambridge University, discovered that cathode rays were actually negatively charged particles, which he called “corpuscles” (J.J. Thomson and the Discovery of the Electron). The “corpuscles” that he discovered were actually what we know as electrons. By adding an electric-magnetic field, he discovered that the particles actually curved towards the positive side, so he concluded that they possessed a negative charge—since things that are negatively charge are naturally attracted to those with positive charges, and that the hydrogen had a mass that was equal to 2000 of these particles, although the particles had a very large charge-to-mass ratio. From this unearthing of information, he determined that atoms were made up of negatively charged particles that were the electrons. He devised a visual model to accompany his theory called his Plum Pudding Model (Chemical Heritage: Joseph John Thomson). This model consisted of a sphere that represented the atom with smaller spheres which represented electrons embedded inside and on its surface [See Figure 1.4]. Thomson determined that the regions with the small spheres possessed a negative charge while the areas without the electrons possessed a positive charge. For his work, he eventually won a Noble Prize in physics in 1906 and a knighthood in 1908. After the monumental discovery of the negatively charged particles that were electrons, positively charged particles had been investigated and discovered as well. Eugene Goldstein was a German physicist and like Thomson, experimented with the cathode ray tube, as a physicist. In 1886, he discovered that in addition to the cathode rays in the tubes, there was also another ray that traveled in the opposite direction—anode rays. (Atomic Structure Timeline). Calling them “canal rays”, he discovered that there were also positively charged particles the same way that Thomson had discovered electrons. Goldstein had discovered protons although because he was not a very proverbial scientists and his research was considered somewhat incomplete, he is not credited of discovering the proton today. A couple of decades after Thomson’s discovery of the electron, an English physicist named James Chadwick noticed a discrepancy between the atomic numbers of an element and its consequent atomic mass—for example, although helium has two protons, it has an atomic mass of 4 amu. He knew that electrons were out of the question because they possessed so little mass, so he hypothesized that there were two extra particles in the helium atom. Because during his time, the discovery of new subatomic particles had been dependent on magnetic-electric forces (and the eventually discovered neutron has no electrical charge), discovering the unknown particle proved to be a challenge. He immersed himself in the world of radioactive research and after copious amounts of research and experiments, managed to discover the neutron by bombarding alpha rays coming from natural radioactive material onto a beryllium atom, observing that when bombarded, beryllium emitted material rays that contained particles with no electrical charge and possessed around the same mass as protons (Chadwick’s Discovery of the Neutron). Because of his work in the radioactive science field, he was chosen as one of the physicists in the Manhattan Project the brought about the monumental creation and eventually the unfortunate detonation of the atom bomb during World War II. He went on to win the Nobel Prize in physics and was knighted in the mid 1900s. In 1909, an American from the University of Chicago named Robert Millikan accomplished the feat of measuring the charge of an electron by performing an experiment, which is known today as the “Millikan oil-drop experiment” (Brown 38). His apparatus was large and barrel-shaped; and electrons floated in between the top of the apparatus and a positively charged plate. Using an oil spray, small drops of oil picked up the electrons and were allowed to fall between two electrically charged plates. Then he monitored the drops, measuring how the voltage on the plates affected their rate of fall. From the observations and data, he calculated the charges on the drops—his experiment showed that the charges were always integral multiples of 1.60x10-19 coulombs, which he concluded was the charge of one electron. This was a significant discovery in atomic theory because it provided actual numbers and solid fact instead of nebulous theories. Later in his career, Millikan would be awarded the Noble Prize in Physics for his scientific contributions to atomic theory. Ernest Rutherford, a New Zealand-born British physicist and chemist, and a student of J.J. Thomson at Cambridge University, is considered to be one of the most important figures in the formation of atomic theory and regarded as the father of nuclear physics. He was particularly interested and active in nuclear physics and chemistry early in his career: his work with his mentor J.J. Thomson gave him a strong scientific background, as they worked together on the study of electrical conduction of gases (Rutherford—A Brief Biography). After many years of research of ingeniously utilizing x-rays and radioactive rays to initiate electrical conduction in gases, he found himself being increasingly drawn to the world of radioactivity itself and made substantial gains in that field. In 1896, he would design what would be the world’s most sensitive electromagnetic wave detector, which could identify waves from several hundred meters away (Ernest Rutherford: A Biography). In 1899, after an intensive study on the rays released by radioactive material, he discovered that there were two types of emissions that were released from radioactive atoms, in particular the element uranium: he named them alpha and beta rays, which consisted of alpha and beta particles, respectively. The alpha particles, Rutherford discovered, were much larger and had a positive charge while the beta particles were smaller, fast-moving, and negatively charged (Brown 39). These types of particle rays, along with a third type, called a gamma ray, have can penetrate through numerous substances (See Figure 1.5]. He would use these newly-discovered concepts to take the knowledge of atomic structure even further in an experiment famously known as his Gold Foil Experiment. In 1910, Rutherford was studying the angles at which the alpha particles scattered as they passed through a thin gold foil a few thousand atomic layers in thickness. He and his coworkers saw that a very large percentage of the alpha particles passed directly though the foil without deflection. However, a few were found to have slightly deflected off the gold foil—around 1 percent [See Figure 1.6]. One of Rutherford’s assistants, Ernest Marsden, was asked to look for evidence of the scattering at large angles of the alpha particle. The shockingly discovered that a small amount of scattering at considerably large angles had occurred—this evinced that Thomson’s plum pudding model was incorrect (Brown 40). In 1911, Rutherford was able to postulate that most of the mass of the atom and all of its positively charged material are in a very small and extremely dense region called the nucleus. Most of the total volume of the atom actually consisted of the space where electrons moved around the nucleus (Brown 40). This explained why some of the particles deflected off the foil and scattered: because they came very close to the small gold nucleus—the repulsion between the highly charged gold nucleus and the alpha particle was strong enough to deflect the less massive alpha particle. With the eventual discovery of the protons and neutrons, Rutherford was able to render a new atomic model with the new knowledge of atomic theory which was called the “planetary model” because in his model, the tiny electrons orbited around the nucleus very similarly to how planets orbit around the sun (Rutherford’s Planetary Model of the Atom). In his model, the protons and neutrons were tightly packed into the nucleus while around it, small negatively charged electrons orbited around [See Figure 1.7]. With his various noteworthy achievements, Rutherford received a Noble Prize in Chemistry, was knighted in 1914, and has an element named after him: the synthetic radioactive element called rutherfordium [See Figure 1.8]. A Danish physicist who was acquainted with all the great minds of science such as Thomson, Rutherford, and Einstein, Niels Bohr was also another prominent member in the world of physics and atomic theory. Having closely worked with both Thomson and Rutherford in their endeavors toward uncovering more of the mystery of the atomic structure, Bohr considered the atomic structure as well, eventually adding to Rutherford’s atomic model that electrons revolve in stable orbits and do not radiate energy while doing so—but when energy is introduced, electrons can momentarily leave their orbits and emit energy in the form of electromagnetic waves—this is called the excited state (Niels Bohr’s Contributions to Chemistry). This theory was just a precursor to the world of quantum physics, and provided as a passage to another field of exploration in science. Since Bohr’s model was a quantum physics-based model of Rutherford’s atomic model, it is usually referred to today as the Rutherford-Bohr model. Bohr would receive much acclaim later in his life, being awarded a Noble Prize in Physics and an element named after him—the synthetic radioactive element known as bohrium [See Figure 1.9]. Thanks to the innumerable great minds of physics and chemistry, we not have a very concrete knowledge of how atoms are structured. From Democritus to Bohr, the initial enigma of the atomic structure has been revealed—little by little—over the years so that we no longer need to guess about the atoms that make up everything around us. The countless hours of research and experimentation of the scientists and philosophers have been incredibly rewarding: not only has the atom been in detail explained, but also new doorways have been opened that have led us to even more discoveries. While exploring the atom, we have also stepped into the world of radioactive and nuclear physics, which has led to the development of weapons that have immeasurable and fearsome power. We have also seen a glimpse of quantum physics, which is still being uncovered and examined to this day—the greatest thing about science is that no matter what we understand, there will always be things out there that remain undiscovered.

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