AP Chem 2.5a

Introduction to Lewis Electron Dot Diagrams

  • Instructor: Jeremy Krug

  • Unit Overview: Discussion on simulating the shape of molecules and drawing their representations.

Methodology

  • Lewis Electron Dot Diagrams: A method to represent molecules using valence electrons placed around atom symbols.

  • Valence Electrons: Electrons in the outermost shell of an atom that are involved in forming connections (bonds) with other atoms.

Example of Lewis Structure

Step-by-step Drawing Process

  1. Identify Valence Electrons:

    • Fluorine: Located in Group 17, has seven valence electrons.

    • Chlorine: Also in Group 17, has seven valence electrons as well.

  2. Drawing Dots:

    • Fluorine: Represent the seven valence electrons as dots around the symbol in the following way:

    • Chlorine: Represents its seven valence electrons similarly.

    • Importance of pairing dots where feasible (up to four pairs).

  3. Forming Covalent Bonds:

    • A pair of shared electrons between fluorine and chlorine constitutes a single covalent bond, represented by a line connecting atoms (bond).

  4. Final Structure: Includes unshared pairs around the outside and shared pairs represented as lines for a clear visualization of the bond.

The Octet Rule

  • Definition: Most atoms become stable at eight valence electrons; they will gain, lose, or share electrons to achieve this state.

  • Significance of Octet: Greater stability associated with having eight electrons compared to fewer.

  • Covalent vs. Ionic Bonds:

    • Covalent bonds generally involve electron sharing.

    • Ionic bonds involve the donation or stealing of electrons.

Exceptions to the Octet Rule

Key Exceptions

  1. Hydrogen:

    • Hydrogen only holds up to two electrons because it has one occupied energy level (first shell).

  2. Boron:

    • Typically stable with six valence electrons instead of eight due to bonding configurations; occasionally it may achieve eight but typically defaults to six.

  3. Expanded Octets:

    • Certain non-metals from Period 3 and beyond (e.g., phosphorus, sulfur, chlorine) can have more than eight electrons (10 or 12) in their valence shells, leading to the term "expanded octet."

Example Applications

Example 1: Carbon Tetrachloride (CCl₄)

  1. Molecule Configuration: Carbon is the central atom, surrounded by four chlorine atoms.

  2. Valence Electrons:

    • Chlorine: Four atoms, each with seven valence electrons (7 × 4 = 28 dots total).

    • Carbon: In Group 14, has four valence electrons.

  3. Drawing Process: Place carbon in the center and add chlorine atom dot representations around it:

  4. Resulting Bonds: Each chlorine shares one pair of electrons with carbon resulting to:

    • Four single bonds represented by four connecting lines.

Example 2: Carbon Dioxide (CO₂)

  1. Molecule Configuration: Carbon as the central atom flanked by two oxygen atoms.

  2. Valence Electrons:

    • Oxygen: Group 16, six valence electrons for each oxygen (6×2 = 12 dots total).

    • Carbon: Group 14, four valence electrons.

  3. Adjusting Bonds: Carbon must achieve eight valence electrons, requiring the sharing of electrons:

  4. Final Structure: Two shared pairs between carbon and each oxygen represent double bonds, indicated as:

    • Two pairs of lines between carbon and each oxygen atom.

Example 3: Ammonia (NH₃)

  1. Molecule Configuration: Nitrogen central to three hydrogen atoms.

  2. Valence Electrons:

    • Hydrogen (group 1): Each hydrogen has one valence electron (3 × 1 = 3 electrons).

    • Nitrogen (group 15): has five valence electrons.

  3. Drawing Process: Three hydrogen atoms arranged around nitrogen with nitrogen achieving eight electrons (two per hydrogen, hydrogen is an exception).

  4. Bonds Representation:

    • Three single shared pairs indicating that nitrogen has one lone pair (unshared pair) represented above.

Conclusion

  • Emphasized learning to draw Lewis electron dot diagrams and understanding molecular shapes using the octet rule and its exceptions.

  • Next steps will include delving deeper into exceptions of the octet rule in future discussions.