Unit 9: Covalent Bonding Chapters 8 & 9 Chemistry 1K Cypress Creek High School

Lewis Structures Count up total number of valence electrons Connect all atoms with single bonds “multiple” atoms usually on outside “single” atoms usually in center; C always in center, H always on outside. Complete octets on exterior atoms (not H, though) no unpaired electrons 4) Check valence electrons match with Step 1 all atoms (except H) have an octet; if not, try multiple bonds any extra electrons? Put on central atom

Lewis Structure Practice Carbon tetrachloride Total valence electrons 32 Central atom carbon Bond to each of the chlorines Fill up the octet of each of the chlorines Check number of valence electrons Check octet of each atom Cl Cl C Cl Cl

Lewis Structure Practice Methane (CH4) Ethyne (C2H2) Water (H2O) H H C H H C H H O H

Diatomic Elements Some nonmetal elements on the periodic table exist in nature only as pairs called diatomic molecules; bonded covalently H, O, F, Br, I, N, Cl Hydrogen Molecule (H2) http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Molecular Shapes Applies to covalent compounds only. Use VSEPR steps (valence shell electron pair repulsion): 1) Identify the central atom as the element that can form the most bonds 2) Draw the Lewis dot structure for the molecule 3) Count total # of electron pairs around the central atom 4) Count # of bonding pairs of electrons around the central atom 5) Count # of lone pairs of electrons around the central atom 6) Look at summary chart, identify shape

# of e- pairs around central atom Molecular Shapes # of e- pairs around central atom # of bonding pairs of e- # of lone pairs of e- Name Shape 2 linear 3 trigonal planar 4 tetrahedral 1 trigonal pyramidal angular (bent) 5 trigonal bipyramidal 6 octahedral

Exceptions to Octet Rule Beryllium Has 2 valence electrons Full with 4 valence electrons Ex: BeI2 Aluminum 3 valence electrons Full with 6 valence electrons Ex: AlCl3 Boron Ex: BH3

Expanded Octets Some atoms bond so they have more than 8e- in the outer level Occur only around a central nonmetallic atom from period 3 or higher PCl5 SF6

Molecular Shape Example What is the shape of water (H2O)? 1) Central atom? oxygen 2) Draw Lewis Dot… 3) # of total electron pairs around central atom? 4 4) # of bonding pairs around central atom? 2 5) # of lone pairs around central atom? 2 6) Refer to chart to identify shape…angular (bent)

Molecular Shape Prediction Try to predict the shape using VSEPR and your shape chart: 1) BCl3 2) CH4 3) NH3 Lewis Dot Central Atom # e- pairs # bonded pairs # lone pairs Shape