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rd 3 ESO-D. IES AMES 06-07 EUROPEAN SECTION- CHEMISTRY & PHYSICS ATOMIC STRUCTURE DISCOVERIES/EXPERIMENTS CONCLUSIONS Early Chemical Discoveries: Law of Conservation of Mass (1744, Antoine Lavoisier): “The total mass of substances present after a chemical reaction is the same as the total mass of substances before the reaction” Law of Constant Composition (1799, Joseph Proust): “All samples of a compound have the same composition-the same proportions by mass of the constituent elements” Dalton’s Atomic Theory (1803, John Dalton) Dalton proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. 1. Each chemical element is composed of minute, indestructible particles called atoms. Atoms can neither be created nor destroyed during a chemical reaction. 2. All atoms of an element are alike in mass and other properties, but the atoms of one element are different from those of all other elements. 3. In each of their compounds, different elements combine in a simple numerical ratio. Electrons and other discoveries: Electric charges: static electricity Electrolysis: Faraday's work on the chemical reaction produced when an electric current passes through a liquid resulted in the laws of electrolysis. The discovery of Electrons: Cathode ray tube (Thomson, 1897) On April 30, 1897, Joseph John Thomson announced that cathode rays were negatively charged particles which he called 'corpuscles.' He also announced that they had a mass about 1000 times smaller than a hydrogen atom, and he claimed that these corpuscles were the things from which atoms were built up. The “Plum pudding” model (1904, J.J. Thomson) In this model, the atom is composed of electrons (which Thomson still called "corpuscles," though Stoney had proposed that atoms of electricity be called electrons in 1894), surrounded by a soup of positive charge to balance the electron's negative charge, like plums surrounded by pudding. http://www.sciencemuseum.org.uk/on-line/electron/section2/shockwave2.asp rd 3 ESO-D. IES AMES 06-07 EUROPEAN SECTION- CHEMISTRY & PHYSICS X-rays and Radioactivity 9 Wilhelm Roetgen (1895) noticed that when cathode ray tubes were operating, certain materials outside the tubes would glow or fluoresce. Because of the unknown nature of this radiation, Roentgen coined the term X-ray. X-ray are a form of high-energy electromagnetic radiation. 9 By the early 1900s Henry Becquerel, Marie and Pierre Curie discovered radioactivity. 9 Ernest Rutherford identified three types of radiation from radioactive materials: Rutherford`s atomic model: The nuclear atom (1909, E. Rutherford) 1. Most of the mass and all of the positive charge of an atom are centred in a very small region called the nucleus. The atom is mostly empty space. 2. There exist as many electrons outside the nucleus as there are units of positive charge on the nucleus. The atom as a whole is electrically neutral. 3. The protons and neutrons in the nucleus take up very little volume but contain essentially all the atom’s mass. A number of electrons equal to the number of protons move around the nucleus and account for most of the atom’s volume. Rutherford's gold foil experiment The Rutherford scattering experiment. (a) When a beam of alpha particles is directed at a thin gold foil, most particles pass through the foil undeflected, but a small number are deflected at large angles and a few bounce back toward the particle source. (b) A closeup view shows how most of an atom is empty space and only the alpha particles that strike a nucleus are deflected. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf Protons and neutrons Rutherford discovered the protons in 1919, and James Chadwick discovered the neutrons in 1932. ATOMIC NUMBER (Z) The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic number also describes the number of electrons around the nucleus. MASS NUMBER (A) The mass number is the number of neutrons added to the number of protons. A= Z + N ISOTOPES Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. ATOMIC MASS The atomic mass of a chemical element is the mass of an atom at rest, most often expressed in unified atomic mass units. The atomic mass is often synonymous with relative atomic mass (RAM), average atomic mass and atomic weight; however, it is subtly different in that it can either be the abundance-weighted average of isotope masses of an element or the mass of a single isotope. rd 3 ESO-D. IES AMES 06-07 EUROPEAN SECTION- CHEMISTRY & PHYSICS Quantum theory (Max Planck, 1900) Electromagnetic spectrum Black body radiation Niels Bohr (1913) applies quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits defined by their angular momentum. This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit. Photoelectric effect (Einstein, 1905) Uncertainty Principle- Heisemberg(1927) Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Uncertainty- you can not know both the position and velocity of a particle. Wave Mechanics- Schrödinger(1930) viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom. Particle/Wave duality- De Broglie(1923) Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein. Quantum Mechanical Model for an atom According to the Principles of Quantum Mechanics electrons are distributed around the nucleus in "probability regions". These probability regions are called "atomic orbitals".