Chapter 6

Covalent Bonding: atoms that share electrons

Octet Rule

Atoms will lose, gain, or share electrons in order to have the same electron configurations as noble gases

Exception: H atoms attempt to achieve the same as He atoms

Exceptions

Central atom has fewer than eight electrons

B, Be, Al

Central atom hase fewer than eight electrons due to odd number of electrons

central atom has more than eight electrons

expanded octets

Lewis Structures

Resonance

Average of multiple Lewis structures

Differing position of electrons

Formal charge

Used to determine most palusibe Lewis structure

1. Lewis structure in which all formal charges are zero is preferred

2. Small formal charges are preferred to large formal charges

3. Formal charges should be consistent with electronegativities

Equal to valence electrons - associated electrons

Associated electrons: all non-bonding electrons + half the bonding electrons

Drawing Lewis Structures

1. Find sum of valence electrons of all atoms in the molecule or polyatomic ion or molecule

Anion

add one electron for each negative charge

Cation

subtract one electron for each positive charge

2. Draw skeletal structure for the compund

One bond connects each atom to the central atom

The least electron negative atom is typically the central atom

3. For each bond, subtract 2 electrons from the total valence electrons

Shared electrons (bonds) are shown as dashes between atoms

4. Use the remaining electrons to complete the octets of terminal atoms

Complete the octet of the most electronegative atom first

Lone pairs of electrons are represented by a pair of dots

5. Place any remaining electrons on the central atom

6. Complete the octet of the central atom by forming multiple bonds

Bond Length

Multiple bonds are shorter than single bonds

Single Bonds

Held together by one electron pair

Double Bonds

Held together by two electron pairs

Triple Bonds

Held together by three electron pairs

Inversely proportional to bond strength

Ionic Bonds: between metals and nonmetals that involve ions

Electronegativity: ability of an atom in a compound to attract electrons to itself

The greater the difference in electronegativity the more polar the bond

Nonpolar covalent

Electronegativity <0.5

Neutral Atoms held together by equally shared electrons

Polar covalent

Electronegativity 0.5-2.0

Dipole Moment

Distribution of positive and negative charges across molecules

Partially charged atoms held together by unequally shared electrons

Ionic bond

Electronegativity > 2.0

Oppositely charged ions held together by electrostatic attraction