Lewis Dot Structures

Valence Electrons Electrons in the “outer shell” (Bohr) or the highest energy s and p orbitals (Wave Mechanical) of an atom Sulfur 1s22s22p63s23p4 Highest Principal Energy Level is 3 2 electrons in 3s, 4 electrons in 3p = 6 valence electrons

Valence Electron Patterns The number of valence electrons can be easily read from the top of the periodic table.

Lewis Dot Structures Show us the NUMBER and TYPES of bonds in each molecule. We will also be able to tell the SHAPE of the molecules

Drawing Lewis Dot Structures Obtain the TOTAL number of valence electrons in the entire molecules. Use one pair of electrons to form a bond between each pair of bound atoms. Draw a LINE to represent the bonding pair. (In general, if there is a single element, it is the center atom and all others bond to it). Arrange the remaining electrons to satisfy the octet/duet rule in the outer atoms.

Drawing Lewis Dot Structures… If there are any remaining electrons, place them on the center atom. Check to see that there is an octet on EVERY atom (except Hydrogen). If there are not enough electrons to make octets on all atoms, SHARE! Make a DOUBLE BOND If there are still not enough, make a TRIPLE BOND

Practice Problems Draw the Lewis Dot Structures for the following molecules. Make sure: there are no more electrons around the atoms than the number of total valence electrons all atoms have obey the octet rule O2 N2 CO2

Answers O2 N2 CO2 2(6)=12 v.e. 2(5)=10 v.e. 4+2(6)=16

Resonance Structures When two or more correct Lewis Dot Structures can be drawn for a single molecule, they are known as resonance structures. The following molecules have resonance structures. Can you show how many? CO2 NO3-1 SO2 CO3-2 3 2

Exceptions to the Octet Rule Every rule has an exception, eh? BORON Tends to act more like an ionic compound and forms only 3 bonds BF3 makes a molecule with 3 single bonds and only 6 electrons around the B BF3 reacts vigorously with NH3 to make H3NBF3 which further proves the structure

Exceptions to the Octet Rule Any ionic compound Ionic compounds GIVE and TAKE electrons to form their bonds. The ones that GIVE electrons get an octet or duet from their inner electrons. Examples: NaCl MgCl2

Exceptions to the Octet Rule Paramagnetism When an object displays paramagnetism, it reacts to a strong magnet, indicating that it has UNPAIRED ELECTRONS OXYGEN GAS (O2) The LDS that satisfies the octet rule shows that it has a double bond, yet its observed behavior is paramagnetic.

Exceptions to the Octet Rule Odd numbers of electrons NITROGEN NO and NO2 have eleven and fifteen valence electrons, respectively, yet both are observed molecules

Exceptions to the Octet Rule More than an octet Some molecules have more than an octet Only if they have d orbitals that electrons can go into N, O, F CANNOT ever have more than an octet Examples IF5 AsF6 IBr3

Bond Order When referring to bond order, it is usually the type of bond. Single bond = bond order of 1 Double bond = bond order of 2 Higher bond order means: Stronger bonds (harder to break) Shorter bond lengths (atoms closer together)

Bond Order When a molecule has resonance structures, the bond order tends to be an average of the resonant structures. Predict the bond order of each bond in the following molecules that you already wrote resonance structures for: CO2 SO2 NO3-1 CO3-2 2 1.3 1.5