The Modern View of the Atom Chemistry Fall 2013

History  Democritus: atoms as indivisible units  Thomson: discovery of electrons  Rutherford: small, massive, and positively- charged nucleus  Bohr: special orbits for electrons

Quantum Mechanical Atom  Nucleus: protons and neutrons Positively (+) charged Positively (+) charged Heavy Heavy Small Small Dense 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 Heisenberg Uncertainty Principle: can’t be sure where an electron is Cloud is much bigger than nucleus Cloud is much bigger than nucleus Orbitals: various shapes of electron clouds Orbitals: various shapes of electron clouds Question: Why do we represent electrons as clouds instead of dots?

Subatomic Particles ParticleChargeMassLocation Proton+1HeavyNucleus Neutron0HeavyNucleus ElectronLight 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 CarbonIron 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 C Carbon atomic number average atomic mass Element symbol Element name 66

Atomic Symbol Hg atomic number mass number charge number of atoms

Electrons  For neutral atoms (no charge): # electrons = # protons  Ions: atoms with charges (i.e. Ca 2+ or Br - ) Extra electrons for negative (-) charge Extra electrons for negative (-) charge Fewer electrons for positive (+) 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