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Atomic Structure Model Timeline Dalton 1803 Thomson 1897 Rutherford 1911 Bohr 1913 Cloud 1926 Dalton’s Model (p.161) Dalton proposed the first model of atoms; he thought atoms were a solid sphere Dalton’s Atomic Theory 1) All elements consist of atoms that cannot be divided 2) All atoms of the same element are exactly alike. Atoms of different elements are different. 3) An atom of one element cannot be changed into an atom of a different element. 4) Compounds form when atoms of more than one element combine in a specific ratio. Only points (2) and (4) are still true today! Thomson’s Model (p. 162) Thomson suggested that atoms had negatively charged electrons set in a soft positive sphere. Each electron is represented by the symbol e.−. Think of Thompson’s model like a ball of chocolate chip cookie dough. It is also called the “plum pudding” model. Can you think of any other analogies for Thomson’s model? Rutherford’s Gold Foil Experiment (p.162) If atoms were a soft positive sphere, alpha particles fired at it should have punched straight through. Rutherford was surprised that a few particles were deflected strongly. Which are the paths of the particles that were not predicted by Thomson’s atomic model? Rutherford’s Gold Foil Experiment (p.162) Use the diagrams to compare the expected and observed results of Rutherford’s gold foil experiment. Rutherford’s Model (p. 163) The gold foil experiment resulted in the discovery of the nucleus. According to Rutherford’s model, an atom was mostly open space. The “6+” in the model means that there are six protons in the nucleus. Bohr’s Model (p. 163) Niels Bohr tested Rutherford’s model by observing the light emitted by the element hydrogen. Bohr’s Model (p. 163) His results suggested that electrons move in specific orbitals around the nucleus of an atom like planets orbiting the sun. Modern Model of the Atom Now we understand that orbitals are not “set in stone” as Bohr thought. Rather, electrons move in cloudlike regions referred to as energy levels. We also know that atoms are made up of protons, neutrons, and electrons. . Electron Cloud This is the closest we have been able to come to visualizing the parts of the atom. This image is a mathematical projection of the electron cloud(s) around an atom, based off experimental mass and charge measurements. The nucleus is too small and dense to visualize. Particles in an Atom An atom is made up of protons, neutrons, and electrons. What is the correct charge for each atomic particle? So…what about QUARKS? Well, the charges of protons, neutrons, and electrons have to come from SOMEWHERE… https://www.youtube.com/watch?v=nlv06lSAC7c Believe it or not, the parts of the atom CAN be split! QUARKS are the fundamental particle from which protons, neutrons, and electrons are made. Interpreting Atomic Structure Each block on the periodic table contains a chemical symbol, element name, atomic number, and mass number. For example, Potassium has the symbol “K,” (2) an atomic number of 19 (1) and an atomic mass of 39.098 (3) amu. Bananas are rich in potassium!. Interpreting Atomic Structure Atomic Number - number of protons in an atom - tells you which element you have Mass Number - total number of protons and neutrons - number of neutrons equals the mass number minus the atomic number Electrons - number of electrons in an atom is the same as the atomic number (number of protons) Isotopes An isotope is an atom with fewer or more neutrons than usual. The mass number on the periodic table is actually a weighted average of the masses of the isotopes of that element. What is the missing information for each isotope, below? 14 6 7 Ions An ion is an atom with fewer or more electrons than usual. Ions with a positive charge have fewer electrons than protons. Ions with a negative charge have more electrons than protons. How would you find the number of electrons if you knew the charge and atomic number of an ion? Let’s Practice! Example Video https://www.youtube.com/watch?v=G3ImfVYSxoc Remember! Atomic Number = #Protons OR #Electrons Atomic Mass = #Protons + #Neutrons Now complete the worksheet “atomic structure basics”