Covalent Bonds ZBoth atoms involved (typically nonmetal) “want” to gain e - to become stable ZElectrons are shared in order to allow this to happen ZThe number of e - shared depends on the element ZWhen atoms share two, or more, electron(s), they form a molecule

Compounds formed with covalent bonds are neutral and, generally, follow the OCTET rule! ZAll atoms are most stable when they have the electron configuration of a noble gases (THIS MEANS HAVING EIGHT* VALENCE ELECTRONS) *For nonmetals the most common exception to the octet rule (when forming bonds) is HYDROGEN.

Highly Stable = Noble gas configuration ZHydrogen - will share 1 e - ZOxygen - will share 2 e - ZNitrogen - will share 3 e - ZChlorine - will share 1 e - ZCarbon - will share 4 e -

Covalent Bonding in Hydrogen Electron sharing can occur only when electron orbitals from two different atoms overlap.

Formation of a Covalent Bond FG08_006.JPG

The number of covalent bonds formed by a nonmetallic element is often directly related with the number of electrons it must share (commonly equivalent to the number of lone pairs of e - the atom has) in order to obtain an octet of electrons.

Types of Covalent Bonds ZSingle - ONE pair of e - shared ZDouble - TWO pair of e - shared ZTriple - THREE pair of e - shared ZCoordinate - Both electrons being shared originate from a single atom

(a) A “regular” covalent single bond is the result of overlap of two half-filled orbitals. (b) A coordinate covalent single bond is the result of overlap of a filled and a vacant orbital. - atoms participating in cc bonding generally do not form their normal # of covalent bonds Ex.: HO 2 Cl, CO

Types of Covalent Bonds

Electron Dot Structures of HCl FG08_009.JPG

Lewis Structures ZStructures which represent in a drawing the arrangement of the atoms and the types of covalent bonds ZThere are FIVE basic steps to follow. ZWe’ll use water as an example:).

Step 1: ZArrange the atoms! ZRemember, Hydrogens always on the periphery ZUse the expected bonding patterns to arrange the atoms the atom that forms the most bonds is typically in the middle of the structure.

Step 2: ZCount the electrons ZObtain a total to work from

Step 3: ZAdd the bonds & lone pairs ZRemember to give each atom (except H) an “octet” of e - (but, don’t exceed the number of e - available)

Step 4: ZUse multiple bonds to fill octets when needed ZConvert one lone pair to a bonding pair for each pair of e - needed to complete an octet

Step 5: ZExceptions to the octet rule ZLess than an octet ZHydrogen & Boron ZMore than an octet ZPhosphorus & Sulfur, Noble gases

Lewis Structures - examples ZCCl 4 ZPBr 3 ZF 2 ZH 2 S ZNH 4 + ZSO 4 -2 ZO 3

Double Bonds ZO2ZO2 ZC2H4ZC2H4 ZCO 2 ZCH 2 CHCHCH 2 ZNO 3 - ZCO 3 -2

Triple bonds ZN2ZN2 ZC2H2ZC2H2 ZHCN

Exceptions (more than an octet) - Only elements in rows 3 and beyond Why? ZPCl 3 and NCl 3 ZPCl 5 but not NCl 5 ZXeF 4 ZSF 6

Resonance ZSome molecules have measured values of bond lengths which do not support the Lewis structure drawn for the molecule ZExample: Ozone, O 3 ZTo adequately represent such molecules with Lewis structures, you should draw all possible arrangements of ELECTRONS.

Naming Binary Molecular Compounds ZUsed only with the second element ZEx. Dihydrogen monoxide ZEx. Tetraphosphorus decoxide

Common vs. Chemical Names Z Chemical Name

3-D arrangements of electron pairs Arrangement of valence electron pairs about a central atom that minimize repulsions between the pairs. Since double & triple bonds are multiple electron pairs in the same location, they act like a single pair when determining the geometry of the molecule

3-D Models of Molecules (a) Acetylene molecule. (b) Hydrogen peroxide molecule. (c) Hydrogen azide molecule.

Bonding

Molecular Geometry - VSEPR

Bond Polarity & Electronegativity ZThe difference in the electronegativities (ability of an atom to attract electrons) of the two bonded atoms can be used to define the “polarity” of the bond.

Abbreviated periodic table showing Pauling electronegativity values for selected representative elements.

Bond Polarity (a) In the nonpolar covalent bond present, there is a symmetrical distribution of electron density. (b) In the polar covalent bond present, electron density is displaced because of its electronegativity.

Bond Lengths ZBond lengths are measured using nucleus-nucleus distances. ZFor bonds between the same two atoms: ZSingle > Double > Triple ZExample: C-O

Molecular Polarity (a) Methane is a nonpolar tetrahedral molecule. (b) Methyl chloride is a polar tetrahedral molecule. Hint: Is the molecule SYMMETRICAL?

Determining Molecular Geometry & Polarity - a Shortcut! ZGeometry ZPolarity center atom 1.All bonds non-polar = NON- POLAR molecule 2.Bonds are Polar a)Only VSEPR bonding groups - go to B. b)Has VSEPR nonbonding groups = POLAR molecule attached atoms 1.All attached atoms the same = NON-POLAR molecule 2.One or more different element’s atoms attached = POLAR molecule

Examples - use Lewis structures to guide you ZDetermine the geometry & polarity (look for symmetry) of these “molecules”: 1.HF 2.H 2 O 3.SF 2 4.NI 3 5.SiBr 4 6.SeO 3 7.CO 2 8.CO Na 2 SO 4