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• Atomic structure and interatomic bonding


  Important reason to understand is that the type of interatomic bond can explain a material's properties
• Carbon


  • Graphite is relatively soft and has a "greasy" feel, diamond is one of the hardest known materials
  • Diamond is a poor conductor of electricity, graphite is a reasonably good conductor
• Disparities in properties of diamond and graphite are directly attributable to a type of interatomic bonding found in graphite that does not exist in diamond
• Each atom consists of a very small nucleus composed of protons and neutrons and is encircled by moving electrons
• Atomic number (Z)


  The number of protons in the nucleus
• Atomic mass (A)


  The sum of the masses of protons and neutrons within the nucleus
• Isotopes


  Atoms of some elements that have two or more different atomic masses
• Atomic weight


  The weighted average of the atomic masses of the atom's naturally occurring isotopes
• Atomic mass unit (amu)


  A scale where 1 amu is defined as 1/12 of the atomic mass of the most common isotope of carbon, carbon 12 (12C)
• Mole


  In one mole of a substance, there are 6.022 x 10^23 (Avogadro's number) atoms or molecules
• Atomic weight of an element or molecular weight of a compound may be specified on the basis of amu per atom (molecule) or mass per mole of material
• 1 amu/atom (or molecule) = 1 g/mol
• Quantum mechanics


  The principles and laws that govern systems of atomic and subatomic entities
• Bohr atomic model


  Electrons are assumed to revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less well defined in terms of its orbital
• Wave-mechanical model


  The electron is considered to exhibit both wave-like and particle-like characteristics, and position is described by a probability distribution or electron cloud
• Quantum number


  Parameters that specify the size, shape, and spatial orientation of an electron's probability density (or orbital)
• Quantum numbers


  • Principal quantum number (n)
  • Azimuthal quantum number (l)
  • Magnetic quantum number (ml)
  • Spin quantum number (ms)
• Principal quantum number (n)


  Specifies the shell, can take on integral values beginning with unity
• Azimuthal quantum number (l)

  Designates the subshell, values range from 0 to (n-1)
• Subshell designations


  s, p, d, f
• Electron orbital shapes depend on the azimuthal quantum number (l)
• Orbital shapes


  • s orbitals are spherical
  • p orbitals have a dumbbell shape
• Probability


  Measure of the likelihood of an event occurring
• Distance from nucleus


  Measure of how far an electron is from the nucleus of an atom
• Comparison of Bohr and wave-mechanical atom models

  • Figure 2.3
• s electron orbital


  • Spherical shape
• Electron orbital shapes


  • s
  • p
  • d
  • f
• s orbital


  Spherical and centered on the nucleus
• p orbital


  Has a nodal surface in the shape of a dumbbell
• p orbitals are labeled px, py, and pz
• Orbital configurations for d subshells are more complex and are not discussed here
• Magnetic quantum number (ml)

  Determines the number of electron orbitals for each subshell
• When l = 0, ml can only have a value of 0 corresponding to an s subshell with one orbital
• For l = 1, ml can take on values of -1, 0, and +1, and three p orbitals are possible
• d subshells have five orbitals, and f subshells have seven
• In the absence of an external magnetic field, all orbitals within each subshell are identical in energy
• When a magnetic field is applied, these subshell states split, with each orbital assuming a slightly different energy
• Quantum numbers


  • n (principal)
  • l (angular momentum)
  • ml (magnetic)
  • ms (spin)
• Associated with each electron is a spin moment, which must be oriented either up or down
• The fourth quantum number, ms, can have two values: +1/2 (for spin up) and -1/2 (for spin down)