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The Modern View of the Atom Chemistry Fall 2013 History Democritus: atoms as indivisible units Thomson: discovery of electrons Rutherford: small, massive, and positivelycharged nucleus Bohr: special orbits for electrons Quantum Mechanical Atom Nucleus: protons and neutrons Positively (+) charged Heavy Small Dense Question: What are the properties of the nucleus? Quantum Mechanical Atom Electron cloud: where an electron could be Heisenberg Uncertainty Principle: can’t be sure where an electron is Cloud is much bigger than nucleus Orbitals: various shapes of electron clouds Question: Why do we represent electrons as clouds instead of dots? Subatomic Particles Particle Charge Mass Location Proton +1 Heavy Nucleus Neutron 0 Heavy Nucleus Electron -1 Light Electron cloud Question: What are the properties of the three subatomic particles? Atomic Number (Z) Number of protons Identifies which element an atom is Example: A carbon atom has 6 protons while an iron atom has 26 protons Carbon Iron Question: Which subatomic particle determines the identity of an element? Mass Number (A) Mass number = #protons + #neutrons Example: If a lithium atom has 3 protons and 4 neutrons, its mass number is 7. Question: What two numbers do we add together to get mass number? Checking for Understanding Where do you find the atomic number on the periodic table? What is the atomic number of Oxygen? Where do you find the mass number on the periodic table? What is the mass number of Oxygen? Isotopes Same # of protons but different # of neutrons Same atomic number but different mass number Question: How are isotopes the same? How are they different? Element atomic number 6 C Carbon average atomic mass 12.01 Element symbol Element name Atomic Symbol charge mass number 201 80 atomic number 2+ 2 number of atoms Electrons For neutral atoms (no charge): # electrons = # protons Ions: atoms with charges (i.e. Ca2+ or Br-) Extra electrons for negative (-) charge Fewer electrons for positive (+) charge Question: Why do atoms with negative charges have extra electrons? Summary Questions Why is the proton responsible for the identity of the element? (Hint: what happens when you change the # of protons) What would happen to the atom if you change the number of electrons? Or neutrons