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Molecular Structures

Topic Review on "Title":

Atoms bond chemically to form molecules.  Lewis structures are a way to represent this bonding on two dimensional paper and determine the molecular geometry of a structure.

Review of bonding
Covalent molecules share electrons while ionic compounds transfer electrons from one atom to another.

Lewis Structures of atoms
The element symbol is drawn to represent the nucleus and core electrons.  The valance electrons are drawn around the symbol—one on each side before doubling up.

Exceptions to the Octet Rule
Most atoms are the most stable with 8 electrons in their valence shell, and will bond until this is reached.  However, hydrogen and helium can only hold 2 electrons in their valence shell.  Boron and Beryllium can be stable with only 6 valence electrons.  Any element in the third row or below can hold more than 8 in the empty d subshells.

Arranging atoms in a Lewis Structure
It is often difficult to know in what order to place the atoms.  There are some general rules that can be followed:

  • For molecules with only 2 elements, arrange the atoms symmetrically
  • “COOH” is a carboxylic acid (both O’s bond to the C and the H goes on one of the O’s)
  • Hydrogen and halogens cannot go in the middle
  • Write the remaining atoms in the order they appear in the formula
  • Write the hydrogen and halogen atoms around the element they are written next to in the formula

Drawing Lewis Structures for covalent compounds
Once the atoms are arranged, a system can be used to complete the Lewis Structure:

  • Arrange the atoms as above
  • Determine the # of valence electrons for each atom
  • Draw the valence electrons—do not double up where a bond is going to form between two atoms
  • Count to see if all atoms have full valences
  • If two atoms adjacent to each other do not have full valences, move in an electron from each to form a double bond.  Repeat for triple bond if necessary.
  • If two atoms that are not adjacent to each other need to double bond, try moving a hydrogen to one of them to cause two atoms adjacent to each other to need the double bond.

Another approach to drawing Lewis Structures
There is a second method that is also commonly used to arrive at the same structure:

  • Arrange the atoms as above.
  • Determine the total # of valence electrons for the whole molecule
  • Put one bonding pair between each set of atoms to be bonded.
  • Place remaining electrons in lone pairs, starting with the most electronegative element.
  • If atoms do not have full valence shells, move a lone pair from an adjacent atom in to double, or triple, bond.

Ionic Structures
Ionic bonds are formed from the transfer of electrons from the metal atom to a non-metal atom or polyatomic ion.  When drawing ionic structures, do not draw the atoms as sharing the electrons.  Rather, remove the electrons from the

Valence Shell Electron Pair Repulsion Theory
Bonds are made of electrons and electrons are negative and therefore repel each other.  Bonds and lone pairs form as far apart from each other as possible.  This theory can be used to determine the electron structure (the 3D shape based upon electron regions—bonding regions and lone pair regions—of the central atom) or molecule structure (the 3D shape based on the electron regions, but named after the bonded atoms only).
A = central atom; X = ligands; E = lone pairs

Electron regions

Molecular Formula

Name

2

AX2

Linear

3

AX3

Trigonal Planar

3

AX2E

Bent

4

AX4

Tetrahedron

4

AX3E

Trigonal pyramidal

4

AX2E2

Bent

5

AX5

Trigonal bipyramidal

5

AX4E

See-saw

5

AX3 E2

T-shaped

5

AX2E3

Linear

6

AX6

Octahedron

6

AX5E

Square pyramidal

6

AX4 E2

Square planar


Rapid Study Kit for "Title":
Flash Movie Flash Game Flash Card
Core Concept Tutorial Problem Solving Drill Review Cheat Sheet

"Title" Tutorial Summary :

Lewis structures are a way to represent molecules in two dimensions.  Lewis structures show the atoms and their valence electrons and in what ways the electrons are shared or transferred to form bonds.  This tutorial shows two methods of drawing Lewis Structures and uses the Valence Shell Electron Pair Repulsion Theory to determine molecular geometry.


Tutorial Features:

Specific Tutorial Features:

  • Electrons are animated as they move to be shared or transferred
  • 3D visualization of molecular geometry
  • Two approaches to drawing Lewis Structures are introduced
  • Animation of electrons moving from atomic orbitals to molecular orbitals
  • How do these questions look on the AP Exam?

Series Features:

  • Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
  • Definition slides introduce terms as they are needed.
  • Visual representation of concepts
  • Animated examples—worked out step by step
  • A concise summary is given at the conclusion of the tutorial.

"Title" Topic List:
  • Review of bonding
    • Covalent versus ionic
  • Lewis Structures
    • Atoms
    • Molecules
      • How to arrange the atoms
      • Drawing structures with multiple bonds
      • A second approach to drawing structures
      • Exceptions to the octet rule
      • Ionic compounds
  • Valence Shell Electron Pair Repulsion Theory
    • Used to determine electron geometry
    • Used to determine molecular geometry


See all 24 lessons in college chemistry, including concept tutorials, problem drills and cheat sheets:
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