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Atomic Structure The Atom - smallest particle of an element that retains the chemical properties of that element. - Composed of three subatomic particles: protons, neutrons, and electrons. History Democritus ~400BC Said that the nature of things consisted of an infinite number of extremely small particles which he called atoms. (Greek atomos – indivisible). Completely theoretical- no experiments! Which of these does not agree with modern theory? John Dalton ~ Early 1800’s Dalton’s Atomic Theory 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements are not the same. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements can combine to form compounds. 5. In chemical reactions atoms are combined, separated, or rearranged. J.J. Thomson 1897 – Discovered Electron. * Used a cathode ray to identify the electron by measuring its response to both electric and magnetic fields. Interactive Cathode Ray Tube * “Plum Pudding” Model http://www-outreach.phy.cam.ac.uk/camphy/nucleus/side1_nucleus.gif Ernest Rutherford 1911 Gold Foil Experiment: (Geiger and Marsden). * Dense positively charged nucleus. (Most of the mass of the atom.) *Most of the atom is empty space. *Electrons orbited the nucleus. Gold Foil Interactive Model Neils Bohr 1913 Added the idea of electron energy levels to Rutherford’s model. http://csep10.phys.utk.edu/astr162/lect/light/bohr.html James Chadwick 1932 Discovered a particle which increased the mass of an atom without changing the charge. This neutral particle became known as the neutron. Modern Theory Regions of the Atom I. The Nucleus - very small, dense region near the center of the atom containing nearly all the mass of an atom (protons and neutrons). Nucleons Protons – positively charged (+1) with a mass of 1 amu (atomic mass unit) or 1.673x1027 kg (Symbol: p+) Neutrons – - neutrally charged (0) with a mass of 1 amu or 1.675x10-27 kg (Symbol: no) Regions of the Atom II. The Electron Cloud - region surrounding the nucleus occupied by negatively charged subatomic particles called electrons. This region is very large but mostly empty space! Electrons - negatively charged (-1) with a mass of approximately 0 amu or 9.109 x 10-31 kg. Its mass is so small that it practically makes no contribution to the total mass of the atom (Symbol: e-) Overview of the Atom Summary of Subatomic Particles Particle Symbol Charge Mass (amu) proton p+ +1 1 neutron n0 0 1 Electron e- -1 0 Neutrality Atoms are electrically neutral, they do not have a charge, and therefore, the total number of electrons must equal the total number of protons. # p+ = # e - Total Mass An atom’s total mass is primarily composed of only the protons and neutrons (the nucleus). The electron’s mass is so small that it does not contribute! Mass = p+ + no Important Information About the Atom Atomic Number The atomic number is the “fingerprint” of an element (identifies the element). If the number of protons changes then the identity of the element changes with it! Atomic Number (Z) – the number of protons in the nucleus of each atom of that element Z = # of protons Mass Number Mass Number (A) – the total number of protons and neutrons in the nucleus of an atom. A= protons + neutrons Isotopes – atoms of the same element with different numbers of neutrons. This causes them to have different mass numbers. 14 6 C or C - 14 Practice with Particles Give the number of each subatomic particle in the following isotope. 14 6 C Remember: Z = 6 so A = 14 so or C - 14 Z = # of protons and p+=6, and e-=6 14 = p++no 14 = 6 + no no = 8 # p+ = # e- More Practice Isotope silicon-28 silicon-29 silicon-30 copper-65 cesium-133 carbon-14 # p+ # n0 # e- A Z