Zumdahl • DeCoste chemistry world of Chapter 12

Figure 12.8: Positions (centers) of the ions.

Figure 12.9: Relative sizes of some ions and their parent atoms.

• Cation – always smaller than parent atom
• Anion – always larger than parent atom

Polyatomic Ions

• Atoms in ion are held together by covalent bonds
• All atoms behave as one unit

Lewis Structures

• Bonding involves just valence electrons
• Lewis Structure: representation of a molecule that shows how the valence electrons are arranged among the atoms in the molecule
• Named after G.N. Lewis – came up with idea while lecturing Chemistry class in 1902

Lewis Structures

• Only include valence electrons
• Use dots to represent electrons
• Hydrogen and Helium follow duet rule
• Octet Rule: eight electrons required – atoms can share
• Bonding pair = shared electrons
• Lone pairs/unshared pairs: electrons not involved in bonding

Steps for Writing Lewis Structures

• Obtain sum of valence electrons from all atoms.
• Use one pair of electrons to form bond between each pair of atoms (can use line to represent 2 bonding electrons instead of dots)
• Arrange remaining electrons to satisfy duet or octet rule (there are exceptions to the octet rule)

Multiple Bonds

• Single bond: 2 atoms sharing one electron pair
• Double bond: 2 atoms sharing two pairs of electrons
• Triple bond: three electron pairs are shared
• Resonance: more than one Lewis structure can be drawn for the molecule
• Insert multiple bonds to satisfy octet rule

Molecular Structure

• aka Geometric Structure
• 3-D arrangement of the atoms in a molecule
• Example: water molecule
• Bent or V-shaped
• Describe precisely using bond angle = 105° for H2O