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The History of the Atom A Timeline of Thousands of Years of Thinking about Matter Leucippus and Democritus Democritus’s universe: theorized that the two fundamental and oppositely characterized constituents of the natural world are indivisible bodies—atoms—and void or space atoms move about in the void (empty space), collide, attach to others to form compounds. He believed atoms contained… NO OPEN SPACE within them, but were surrounded by open space Democritus’s teacher was Leucippus. Read a paragraph about him, here. Atom as solid matter History of the Atom • 430 B.C. – Leucippus of Miletus and Democritus of Abdera • They debated whether a substance could be split indefinitely (continuous matter), or if eventually there would be a point when you could no longer break it down (discontinuous matter). • They theorized that the universe was made up of void (space) and small indivisible ATOMS. • This is the start of ATOMISM. •400 B.C. – Aristotle and Plato • Rejected Atomism – believed in Fire, Earth, Air, and Water as the 4 elements • This belief dominated science for over 2000 years! The link below is an experiment. You may do it for a learning plan activity, or you can just watch it as part of the Webquest: ~1700’s From Philosophy to the Age of Reason: Newton and the Birth of the Scientific Method 1770’s – Lavoisier proposed that in ordinary chemical reactions, matter cannot be created or destroyed (law of conservation of mass). He also changed the phlogiston theory to a modern theory of combustion. – What is the phlogiston theory? – Describe Antoine and Marie-Anne Paulze Lavoisier’s discovery: 1799 – Proust’s observation of the fact that specific substances always contain elements in the same ratio by mass led to the law of definite proportions (law of constant composition). Law of Definite Proportions/Law of Multiple Proportions: Another explanation: History of the Atom • 1803 – Dalton, known as the “Father of Modern Atomic Theory,” gathered experimental evidence supporting and explaining the theories of Lavoisier and Proust; his was the first atomic theory based on experimental evidence. – Dalton’s atomic theory: – (1) All matter is composed of atoms that are indivisible. – (2) Atoms of the same element are identical. – (3) Atoms of different elements are dissimilar. – (4) Atoms of the same element can unite in more than one ratio with another element to form more than one compound. Atoms can unite with other atoms in simple numerical ratios to form compounds (law of multiple proportions). History of the Atom John Dalton used simple symbols to represent elements and because he was colorblind, he chose to use black and white. Element table to the right, graphic representation of Dalton’s atomic theory below History of the Atom 1811 Avogadro hypothesized that equal volumes of gases, at the same temperature and pressure, have the same number of molecules. 1865 Mendeleev arranged elements into 7 groups with similar properties. He discovered that the properties of elements “were periodic functions of their atomic weights.” This became known as the Periodic Law. History of the Atom History of the Atom Mendeleev's Periodic Table (1871) One more time, because it’s that good... History of the Atom Notice that large numbers of discoveries of elements coincide with other scientific discoverieis! Why was the electron the first subatomic particle that was discovered? *Try some experiments with static electricity! Static Electricity Experiments *Learn about the Crookes tube! History of the Atom • 1870’s – Crookes experiments were some of the first evidence that electrons exist. He discovered “Cathode rays” had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass. His experiments were a model for other scientists to use in advancing the knowledge about electrons. History of the Atom • 1897-1904 – J.J. Thomson, Thomson discovered the electron, the first known particle that is smaller than the atom, and experimentally determined its charge-mass ratio by deflecting them by magnetic and electric fields. He also showed that canal rays consist of positively-charged particles and calculated the mass of the proton to be at least a thousand times that of the electron. He also discovered isotopes by using a type of mass spectrometer. Thomson developed the plum-pudding model of atoms. History of the Atom • 1896 – Becquerel found that matter containing uranium exposes sealed photographic film. This led to the discovery that rays are given off by uranium and radium. This links to a story of Becquerel’s discovery: • 1898 – Marie and Pierre Curie discovered radium and its ability to give off rays. They called the spontaneous decay of these elements “radioactivity.” Radioactivity Alpha Particles: consist of 2 protons and 2 neutrons (same as the helium nucleus) and are emitted by a radioactive source. They are large, heavy and positively charged. Beta Particles: high-energy, high-speed electrons or positrons emitted by a radioactive source. They are much smaller and lighter than Alpha Particles and are negatively charged. Gamma Radiation: electro magnetic radiation of very high frequency produced by sub-atomic particle interactions. It is energy, so it has no mass or charge. History of the Atom 1900 - Planck introduced the idea that (electromagnetic) energy is radiated in small packets called quanta. • 1911 – Millikan used an oil drop experiment to determine the charge on an electron (1.602 x 10-19 coulomb) and then used Thomson’s e/m value to calculate the mass of an electron (9.11 x 10-28 gram). • 1911 Rutherford predicted the existence of the neutron and demonstrated that the nucleus is a small, positively charged heavy core in an atom, which consists mostly of space. (Gold foil experiments) Suggested the name proton for the fundamental positively charged particle, the nucleus of the hydrogen atom. History of the Atom What was wrong with Rutherford's model? *The evidence that different elements burn to produce different colored flames, along with the work of Planck and others into a smallest particle of energy, led to a revision in Rutherford's model. History of the Atom •1922 •Bohr used and modified Rutherford’s model of the atom. Bohr originated the idea that electrons travel in a definite energy level around the nucleus with no loss of energy from their motion. He applied Planck’s idea of small packets or quanta of energy to his electron model. •Electrons in energy levels further from the nucleus have greater energy. •An electron can give off a photon of energy equivalent to the difference in energy levels if it falls from one energy level to a lower one. •An electron cannot fall to a lower energy level if it is full (an energy level can hold only a certain number of electron.) •Normally every atom is in its ground state, meaning the lowest energy levels are full and no electron can lose energy. •Atoms can absorb energy from an outside source and an electron can jump to a higher energy level, an excited state. It rapidly falls back and energy is emitted. History of the Atom The Hydrogen Atom History of the Atom • Schrodinger, 1930 – Viewed electrons as continuous clouds and introduced “wave mechanics” as a mathematical model of the atom. • Chadwick, 1932 – Using alpha particles Chadwick discovered a neutral atomic particle with a mass close to a proton. He is credited with the discovery of the neutron. Whose atomic models are depicted below? What are the key characteristics of each? What discoveries led to each new model?