Metallic Solids

  • Metallic solid: are held together by the strong forces of attraction between the positively charged metal ions and delocalized electrons (electrons not associated with a single atom or molecule)

           ○ Bonding is equal and nondirectional

Comparing how Metals and Ionic Compounds Conduct Electricity

  • Metals → mobile valence electrons
  •  Ionic compounds → mobile charged particles           

Properties of Metals

  • Tendency to give up one or more electrons to form a positive ion → have low ionization energies
  • Malleable: easy to shape/bend rather than break bcuz adjacent layers of positive metal ions can move relative to one another while remaining in full contact with the electron sea

  • Ductile: pulled into wires
  • Shiny: reflect light bcuz free electrons can bounce back light at the same frequency
  • Solid at room temperature (except mercury which becomes liquid)
  • Metals conduct electricity and heat very efficiently because of the availability of highly mobile electrons

           ○ Metal atoms in a metallic solid generate a “sea” of mobile valence that can easily move from one atom to the next and allow electricity to flow through; Electrons are excited from filled orbitals into the very near empty orbitals

  • If there is a large gap between the filled and the empty levels → electrons cannot be transferred easily to the empty conduction bands = poor conductor

Metal Alloys

  • Alloy: substance that contains a mixture of elements and has metallic properties → two types (difference is size of molecules) 

  • Substitutional alloy: atoms of similar sizes

               ○ Some of the host metal atoms are replaced by another element’s of similar size

  • Interstitial alloy: atoms of different sizes

              ○ Formed when some of the holes in the metal’s structure are occupied by small toms → presence of the interstitial atoms changes the properties of the host metal                               

Bond Polarity and Dipole Moments

  • Polarity: the difference in electronegativity (not the absolute value of electronegativity)

          ○ Ex: N2 and F2 have the same polarity

  • Dipole/having a dipole moment: molecule that has a center of positive charge and a center of negative charge (bcuz of unequal sharing of e-) → polar molecule

         ○ Often has arrow pointing to the (-) charge center

  • Difference in electronegativity between atoms tells us how polar a compound is

         ○ Atoms with high electronegativity with develop a partial negative charge

         ○ Atoms with low electronegativity with develop a partial positive charge

  • Any diatomic molecule that has a polar bond will have a molecular dipole moment
  • With three or more atoms there are two considerations if will have a dipole moment

         ○ There must be a polar bond & geometry cannot cancel it out

  • If any of the terminal elements are different → automatically a polar molecule (polar bonds are asymmetric)

         ○ Unbalanced pull of electrons creates a partial (-) charge on one side of central atom and partial (+) charge on other → separation of charge forms a dipole = polar

Example: HF

  • Red indicates the most electron-rich area (the fluorine atom), and blue indicates the most electron-poor region (the hydrogen atom


Geometry and Dipole Moment

  • Some molecules have polar bonds but do not have a dipole moment (are nonpolar).

           ○ Occurs when bond dipoles are equal in magnitude and opposite in direction, canceling each other out and making the net dipole for the molecule zero

               ■ Ex: CO2 held by polar covalent bonds but is nonpolar

  • A molecule is nonpolar when all of the bond dipoles cancel each other out → net dipole is zero

           ○ Has equal pull of electrons and symmetrical geometry

          ○ Molecules composed of only nonpolar bonds are nonpolar, regardless of shape

  • Molecules in which the central atom is symmetrically surrounded by identical atoms are nonpolar, even if the bonds are polar

          ○ Equal attraction of electrons → net dipole is zero

  • Polar molecules may have some nonpolar bonds
  • Ex: of shapes that do cancel polarity of bonds

          ○ Ex: linear molecules with 2 identical bonds, planar molecules with 3 identical bonds 120 degrees part, tetrahedral molecules

  • Ex: of shapes that do NOT cancel polarity of bonds

          ○ Bent, trigonal pyramidal

  •     → Nonpolar: symmetrical and identical atoms
  • → polar: unequal share of electrons 

 Energy Effects in Binary Ionic Compounds

  • Lattice energy: indicates how strong a bond is (but unlike bond energy is talking about ionic bonds)

              ○ Determines how strongly the ions attract each other in the solid state

  • Lattice energy is stronger in ionic compounds with more highly charged ions and shorter ionic distance (smaller atoms) → the one with the stronger attraction

Example Question: Out of the following pairs of compounds, pick the one w/ greater lattice energy and explain


             ○ (2)(-2) = -4 and (1)(-1) = -1 → MgO is the correct answer

             ○ Explain: “MgO contains the more highly charged ion and thus the ions within that compound will experience a stronger attraction”

          ○ Have same value for charged ions, but Na is smaller than Sr and O is smaller that I → Na2O is the correct answer

          ○ Explain: “although these two compounds contain equally charged ions, sodium oxides have a shorter ionic distance thus the force of attraction is stronger”

  • Process: when given lattice energy question, write out the charges above each ion (ignore subscripts); then, if needed, compare sizes of atoms

The Localized Electron Model

  • Localized electron (LE) model: assumes that bonds and molecules are formed from the overlapping of atomic orbitals on diff. atoms
  • Electron pairs are assumed to be localized on a particular atom or in the space between two atoms 
  • Lone pairs: belong to one atom/nuclei

      ○ Bonding pairs: belong to 2 atoms/nuclei → found in the space between the atoms

               ■ Delocalization: electrons are free to move throughout the entire molecule; can be with either atom