Evolution of Atomism PDF

Evolution of Atomism Democritus and Leucippus 5th Early Atomists (philosophers who believed in...

Evolution of Atomism Democritus and Leucippus 5th Early Atomists (philosophers who believed in Century atomism) believed B.C. that particles have innate physical properties- size, shape, order, and position These physical properties are common to all materials and do not change over time. Physical properties of an atoms affected the Figure 1: Proponents of Atomism: (left) overall characteristic and properties of the materials they made up to. Democritus and his teacher Leucippus (right). © Materials undergo physical and chemical as the http: classicalwisdom.com atoms inside them hit or collide with each other all matter is composed of very small, These ideas, however, were rejected since does indivisible particles which he called not support the Creationism (belief that atomos (Greek- “indivisible”) everything is created by a divine being) with his teacher Leucippus, started the philosophical belief called atomism (ancient and modern concept of atom) Empedocles postulated that all particles are composed of fire, earth, air and water If not atoms, “What are the primordial substances these particles are later called Empedocledean from which everything is made up of?” elements Anaximenes believed that all materials are made up of air; Heraclitus supposed it was fire; Thales thought it was water; and Xenophanes assumed it was earth Figure 2 : Empedoclean Elements Plato Figure 3: Plato strongly rejected the atomists’ ideas. Source: www.britannica.com Treated these Empedocledean elements as geometrically, named Platonic solids platonic solids were formed by combining several triangles in 3- dimensional space as: -Fire- tetrahedron -Air- octahedron -Water- icosahedron -Earth- cube He also added one, a dodecahedron- the shape of the universe  John Dalton Aristotle Disagreed to the idea of atom and believed it to be a blaspheme to God as divine being he supported the idea of Empedocledean elements in nature: air, fire, water and earth these elements existed in balance with two qualities -fire- hot and dry -earth- dry and cold -water- wet and cold -air- wet and hot Figure 6 : John Dalton (Left) and his Solid Sphere Model (Right). © he identified a fifth element called Aether (air present www.fineartamerica.com in upper sky with which the Greeks’ Gods and Goddesses breathe) Reintroduced the idea of Democritus and Leucippus Matter are made up of extremely small hard sphere, called “atom” Each substance or element has unique atomic composition Atoms of the same element have the same mass, size, density, volume, and other physical properties and undergo chemical and physical changes However, his works was not supported by experimentation 19th Century Figure 4: Aristotelian Elements Ernest Rutherford Joseph John Thomson Figure 6: Rutherford, with his partners and students Geiger and Marsden Discovered the protons using Gold Foil experiment Asserted that protons are found at the center of the atom, called nucleus, opposite to Thomson’s claim Figure 5: J.J. Thomson (Left) and his Plum Pudding Model (Right). © In the experiment, his team shot a thin gold foil with https://www.toppr.com positively charged particle (alpha particles) Discovered the electrons through cathode ray tube They found that most of these particles went straight on experiment and claimed that electrons are evenly the foil and a fraction of the particles rebounded from the distributed in an atom foil. In his experiment, cathode rays (ray of light emitted from His team argued that there were repulsive forces that cathode ray tube) were deflected towards the positive caused the particles to bounce back plate This led to the conclusion that positively charged He then concluded that cathode rays are composed of particles are present, and is at the center of the atom negatively charged particles called “electrons” which contained most of the mass of an atom Calculated the charge-mass ration of negatively and He then proposed a Nuclear Model of an atom positively charged particles, which he found the same This finding led him to claim that negatively charged particles is the same amount with positively charged particles and proposed the Plum Pudding Model 1900s Figure 7: Thomson’s Cathode Ray Experiment ©PearonsEducation Figure 8 : Gold Foil Experiment © Pearson(2008) However, Rutherford’s model cannot explain the 1) energy lost during electric induction, and 2) the ability of an atom to emit light  Niels Bohr 1913 Collective works of James Clerk Maxwell, Max Plank and Albert Einstein proved the dual property of light (wave-like and particle-like) Erwin Schrodinger, along the findings of Werner Karl Heisenberg and Louis de Broglie stated that the exact position and momentum of an electron at any point in time cannot be identified Figure 10: Niels Bohr © Encyclopedia Britannica Bohr sought to sustain the limitations of Rutherford’s 1926 model of an atom and claimed that electrons move around the nucleus in fixed circular orbits The orbits are specific distances from the nucleus James Chadwick where electrons move He further noted that the orbits signified the energy level which are represented by whole number (1, 2, 3, 4, …) When an atom absorbed energy, the electrons jump from the original orbit (ground state) to higher orbit (exited state) in a process called excitation However, Bohr’s model of an atom could not account for atoms with more than one electrons Figure 9: Chadwick’s experiment on beryllium. © Pearson (2008) Supported Rutherford’s claim regarding the existence of nucleus 1932 Discovered neutrons in his beryllium experiment He found that a stream a penetrating neutral radiation hit the paraffin wax He concluded that the stream of radiation is composed of particles called neutrons. Figure 11: Bohr’s planetary model of an atom where electrons are in different orbits. © Encyclopedia Britannica Present The works of 21st century scientists led to the current model of an atom called Quantum Mechanical Model. In this model, the following concepts are described: Atom with nucleus containing the protons (positively charged particles) and neutrons (no charge) Electrons are spread-out in the entire atom Electrons do not occupy fixed position and no specific moment at any point in time Figure 12: Representation of the Quantum Mechanical Model of an Atom. Source: https://www.123rf.com