Chapter Six Representing Molecules

Section 6.1 The Octet Rule

The Octet Rule Recall: atoms want noble gas configurations Octet Rule: atoms will gain, lose, or share electrons to achieve a noble gas configuration Typically 8 valence electrons Atoms will bond with each other to achieve a full octet

Lewis Structures A pair of shared electrons can be represented by either with 2 dots or with a dash Unshared electrons are called lone pairs F F F

Lewis Structures Types of bonds: Single bonds: bond containing only 2 electrons Multiple bonds: bond containing more than 2 electrons Double bond: 4 electrons (or 2 pairs of electrons) Triple bond: 6 electrons (or 3 pairs of electrons)

Bond Strength In a particular pair of elements Triple bonds are the shortest Double bonds are in the middle Single bonds are the longest Bond energy is the energy required to BREAK bonds between atoms

Section 6.2 Electronegativity and Polarity

Electronegativity Electronegativity is a periodic trend Ability of an atom to attract electrons to itself when bonded to another atom Quantified by the Pauling Scale

Electronegativity

Electronegativity

Electronegativity

Categories of bonds Let’s consider three molecules: H2, HF, NaF

Ionic, Polar Covalent, Nonpolar Take the difference in electronegativities of two atoms bonded together If the difference is 0.5 or lower, the bond is nonpolar covalent If the difference is between 0.5 and 2.0, the bond is polar covalent If the difference is greater than 2.0, the bond is ionic

Determine if the bond is ionic, polar covalent, or nonpolar covalent The bond in ClF (chlorine and fluorine) The bond in CsBr The carbon-carbon double bond in C2H4 In which of the following molecules are the bonds most polar: H2O, BCl3, PCl5

Section 6.3 Drawing Lewis Structures

Drawing Lewis Dot Diagrams 1) Determine the central atom and place terminal (“outside”) atoms around central atom Central atom typically is the least electronegative element in compound, the element with only 1 atom, and/or the element written first in compound 2) Count total # of v.e. 3) Bond all terminal atoms to central using single bond Each bond is 2 electrons; subtract from total # of v.e.

Drawing Lewis Dot Diagrams 4) Complete the octets of terminal atoms w/ remaining v.e. 5) If any electrons left over, put on central atom 6) Use multiple bonds to complete octet of any elements where necessary

Examples of Lewis Dot Structures CH4 H2O O2 CO2 CN-

Group Quiz #1 Draw the Lewis Dot Structures for the following: CS2 NF3 ClO3-

Section 6.4 Lewis Structures & Formal Charge

Formal Charge Another way of keeping track of electrons in a molecule Formal Charge = (# of v.e.) – (# of associated electrons) Ex: Ozone (O3) Now you try: NO3-

Using Formal Charge Formal Charge can help us determine the best Lewis Structure when there are options Consider the following two skeletal structures for CH2O. Which one is preferred?

Formal Charge Rules Lewis structures where all formal charges are zero is preferred Small formal charges (0 and +/-1) are preferred to big formal charges (+/-2, +/-3, etc.) The best arrangements are where the more electronegative atoms have the more negative formal charge

Group Quiz #2 Draw the Lewis Structures for the following compounds and determine the formal charge on EACH atom SO32- CO32-

Section 6.5 Resonance

Resonance Structures Consider the molecule NO3- and its Lewis Structure

Section 6.6 Exceptions to the Octet Rule

Exceptions to the Octet Rule Central atom has fewer than 8 v.e. due to electron shortage Ex: Boron (happy w/ 6); Beryllium (happy w/ 2) Central atom has fewer than 8 v.e. due to odd # of electrons (known as radicals) Ex: Nitrogen (NO2) Central atom has more than 8 v.e. Ex: Sulfur (SF6) and Xenon (XeF4) See pp. 201—204 for examples/explanations

Formal Charges, Resonance Structures, AND Exceptions! Consider the polyatomic ion: SO42-. What would be the BEST Lewis dot structure? What about PO43-?

Group Quiz #3 Draw the Lewis Structure for antimony pentafluoride (SbF5) Draw the Lewis Structure for Borane (BH3) Draw the Lewis Structure for Nitrogen Disulfide (NS2)