Bonding & Molecular Shapes Dr. Ron Rusay Spring 2003 © Copyright 2003 R.J. Rusay

Chemical Bonds Definition: Attractive forces which hold atoms together and provide a particular molecular arrangement of atoms with new chemical properties.

Ionic vs. Covalent Bonding

Electron Configurations Noble Gases and The Rule of Eight  A nonmetal and a metal react to form an ionic compound: Valence electrons of the metal are lost and the nonmetal gains these electrons. (Ionic Bonding)  When two nonmetals react: They share electrons to achieve a Noble Gas Configuration. (Covalent Bonding)

Octet Rule: General Comments  2nd row elements C, N, O, F observe the octet rule.  2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive.  3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.  When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

Lewis Electron-Dot Symbols for Elements in Periods 2 & 3

Isoelectronic Ions  Ions containing the the same number of electrons are isoelectronic with a Noble Gas  (O 2 , F , Na +, Mg 2+, Al 3+ )  But the ion sizes are not all the same O 2  > F  > Na + > Mg 2+ > Al 3+

Ionic Bonds  Result from electrostatic attractions of closely packed, oppositely charged ions.  Form when an atom which can easily lose electrons reacts with one which has a high electron affinity, that is, it can easily gain electrons.  Mg and Cl; K and O

Covalent Bond Lengths  Interatomic distance.  It is the distance where the bond energy is at a minimum value, and which is the most stable atomic form.

Covalent Bonding & Bond Length

Bond Lengths and Covalent Radius

The Periodic Table Bond Lengths : Bond Strengths

Actual vs. Expected Bond Length Actual = nm Expected = nm

Electronegativity

Electronegativity  The ability of an atom in a molecule to attract shared electrons to itself.  = (H  X) actual  (H  X) expected

Periodic Trend

Electronegativity Differences & Polar Covalent Bonds  A molecule with a relatively high difference in electronegativities, such as HF, has a center of positive charge and a center of negative charge. It is polar, having an experimentally measureable dipole moment.

The spectrum of bond differences: Ionic at one extreme : Non-polar covalent at the other.

Bond Shapes

Lewis Structure  Shows how valence electrons are arranged among atoms in a molecule.  Reflects central idea that stability of a compound relates to noble gas electron configuration.

Lewis Structure of Water

Fundamental Bonding Patterns  Carbon has a total of four bonds: 4 single bonds4 single bonds 2 single bonds plus 1 double bond2 single bonds plus 1 double bond 1 single bond plus 1 triple bond1 single bond plus 1 triple bond  Oxygen has a total of 2 bonds plus 2 “free pairs of electrons”: 2 single bonds2 single bonds 1 double bond1 double bond

Fundamental Bonding Patterns  Nitrogen has a total of three bonds plus 1 “free pair of electrons”: 3 single bonds3 single bonds 1 single bonds plus 1 double bond1 single bonds plus 1 double bond 1 triple bond1 triple bond  Hydrogen only has 1 single bond.  This is the total of almost all bonding arrangements in organic molecules.

Lewis Structures of Simple Molecules N H H H. Ammonia C NN O HH H H. Urea

Carbonate Ion CO 3 2- : The carbonate polyatomic ion has two coordinate bonds, resonance forms and two electrons from a cation!

The Lewis Structure for Sulfur trioxide

Where to draw the sulfur- oxygen double bond? There are three options. Simplified drawings without free pairs of electrons: Experimental data shows that each of the three sulfur-oxygen bonds are the same length.

Resonance  Occurs when more than one valid Lewis structure can be written for a particular molecule.  These are resonance structures. The actual structure is an average of the resonance structures.

Resonance: Delocalized Electron-Pairs OzoneOzone : O 3... OO O OO O I II O O O.. Resonance Hybrid Structure One pair of electron’s resonates between the two locations!!

Ozone and CFCs Ozone and CFCs CCl 2 F 2 Freon 12.. C Cl FF.. O O O

VSEPR Model Valence Shell Electron Pair Repulsion

VSEPR Model  The molecular structure which surrounds a given atom is determined principally by minimizing electron pair repulsions through maximizing separations.

Molecular Models  Computer Generated Models Ball and stick models of ammonia, water and methane. For many others see: