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Understanding Molecular Structures: Formulas, Lewis Structures, and VSEPR Models

Molecular formulas indicate the types and quantities of atoms in a compound, exemplified by C2H5OH. Structural formulas visually represent atom connections using symbols and bonds. To create a Lewis structure, identify the central atom, count valence electrons, form bonds, distribute remaining electrons, and ensure all atoms satisfy the octet rule unless exceptions apply. This guide also introduces VSEPR theory for predicting molecular shapes based on electron pair repulsion, revealing how shapes influence chemical properties.

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Understanding Molecular Structures: Formulas, Lewis Structures, and VSEPR Models

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  1. Chapter 8 Section 3 Molecular Structures

  2. 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.

  3. 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.

  4. 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.

  5. 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).

  6. 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

  7. Lewis Structure Example • You try…H2O

  8. 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?

  9. 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

  10. Lewis Structure Example • You try CO

  11. 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.

  12. Polyatomic Example • PO4-3

  13. Polyatomic Example • You try NH4+

  14. 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

  15. Chapter 8 Section 4 Molecular Shapes

  16. 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

  17. 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.

  18. Molecular Shapes

  19. Molecular Shapes Linear Example: BeCl2 Trigonal Planar Example: AlCl3

  20. Molecular Shapes Tetrahedral Example: CH4 Trigonal Pyramidal Example: PH3

  21. Molecular Shapes Bent Example: H2O TrigonalBipyramidal Example: NbBr3

  22. Molecular Shapes Octahedral Example: SF6

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