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.
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.
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
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: http://ep.llnl.gov/msds/pdb/ http://ep.llnl.gov/msds/orgchem/Chem226/Smell-Stereochem.html