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The Structure of Atoms Bohr-Rutherford Diagrams Bohr-Rutherford diagrams are one model that describes what an atom looks like. Consider the atom of lithium. What does the BohrRutherford diagram look like? Step 1: Using the periodic table, calculate the number of protons, neutrons and electrons. Atomic number 3 Li Lithium 6.941 Atomic mass For lithium: atomic number = 3 atomic mass = 6.941 7 (round to closest whole number) Therefore: # protons = atomic number = 3 # neutrons = n = atomic mass – atomic number = 4 # electrons = # protons = 3 Step 2: Draw the nucleus by first writing the symbol of the element and indicating the number of protons (p) and neutrons (n). Step 3: Draw the electrons in their orbits. Only a certain number of electrons can be held in each orbit: - fill the lower orbits (or energy levels) first - the first orbit will hold up to two electrons. - the other orbits can hold up to eight electrons. - these rules hold up to calcium (element 20) Examples of Bohr-Rutherford Diagrams: Carbon: # protons = atomic number = # neutrons = atomic mass – atomic number = # electrons = # protons = Aluminum: # protons = atomic number = # neutrons = atomic mass – atomic number = # electrons = # protons = Lewis Dot Diagrams - a Lewis dot diagram is a short form diagram which shows only the “relevant” electrons (the VALENCE eletrons) - the number of valence electrons corresponds to the group to which the element belongs - the element symbol is drawn and “dots” around the symbol are drawn to represent the number of valence electrons Examples: Sodium Carbon Aluminum Barium Bromine Argon BOHR-RUTHERFORD/LEWIS DIAGRAMS 1 H 3 Li 4 11 Na 12 Mg 19 K 20 Ca Be 5 13 B 6 Al 14 C 7 N 8 O 9 Si 15 P 16 S 17 2 He F 10 Ne Cl 18 Ar