Chapter 8 Section 3 Molecular Structures

Structural Formulas Molecular formulas tell you the type and number of each atom in a molecule. Example: C2H5OH There a different models that can be used to represent a molecule.

Structural Formulas One of the most useful is the structural formula, which uses letter symbols and bonds to show the relative positions of atoms. You can predict the structural formula of a molecule by drawing the Lewis structure of it.

Rules for drawing Lewis Structures 1. Predict the central atom – usually the atom in the molecule closest to the left side of the periodic table. 2. Determine the number of electrons available for bonding – This is equal to the sum of the valence electrons for all atoms present. 3. Bond each atom – use two electrons to form a single bond between each atom present. 4. Distribute remaining electrons.

Rules for drawing Lewis Structures 5. Check that the octet rule is satisfied for all atoms (except hydrogen) If not, begin making double and triple bonds until the octet rule is obeyed by all atoms (except hydrogen).

Lewis Structure Example NH3 Predict central atom – N Sum electrons – 5 + 1 + 1 + 1 = 8 Bond each atom using a pair of electrons Distribute remaining electrons Check octet rule

Lewis Structure Example You try…H2O

Lewis Structure Example CO2 Predict central atom – C Sum up electrons – 4 + 6 + 6 = 16 Bond each atom Distribute remaining electrons Check for octet rule…is it satisfied?

Lewis Structure example CO2 continued The octet rule was not satisfied for each element, so we now start over using a double bond instead of a single bond and repeat the process

Lewis Structure Example You try CO

Lewis Structures for Polyatomic Ions Only difference is that these molecules have charges. If the charge is negative, add that number to the total number of valence electrons. If the charge is positive, subtract that number from the total number of valence electrons.

Polyatomic Example PO4-3

Polyatomic Example You try NH4+

Exceptions to the Octet Rule Suboctets: A few elements can form bonds and have less than an octet. An example is Boron BH3 Expanded Octets: Usually can occur in elements found in periods 3 or higher because of its d energy levels. These have more than an octet. Example: Phosphorous PCl5

Chapter 8 Section 4 Molecular Shapes

VSEPR Model The shape of a molecule determines many of its physical and chemical properties. The molecular geometry or shape of a molecule can be determined once a Lewis structure is drawn. The used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion Model or VSEPR Model

VSEPR Model This model is based on the arrangement that minimizes the repulsion of shared and unshared electron pairs around the central atom. The electron pairs in a molecule repel each other and this force causes the atoms in the molecule to be positioned at fixed angles relative to one another. The angle formed by two terminal atoms and the central atom is the bond angle.

Molecular Shapes Molecular Shape Total Electron Pairs Shared Pairs Lone Pairs Bond Angles Linear 2 180 Trigonal Planar 3 120 Tetrahedral 4 109.5 Trigonal Pyramidal 1 107.3 Bent 104.5 Trigonal Bipyramidal 5 90/120 Octahedral 6 90

Molecular Shapes Linear Example: BeCl2 Trigonal Planar Example: AlCl3

Molecular Shapes Tetrahedral Example: CH4 Trigonal Pyramidal Example: PH3

Molecular Shapes Bent Example: H2O Trigonal Bipyramidal Example: NbBr3

Molecular Shapes Octahedral Example: SF6