Chapter 8 Covalent Bonding. UEQ What are the unique characteristics of a covalently bonded molecule?. LEQ. What is a covalent bond?. The Covalent Bond. The joining together of two or more elements by the sharing of the valence electrons.
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Chapter 8 Covalent Bonding UEQ What are the unique characteristics of a covalently bonded molecule?
LEQ What is a covalent bond?
The Covalent Bond The joining together of two or more elements by the sharing of the valence electrons. • The Lewis Structure: representation of the electrons in the valance of an atom. * Group A elements = the Group number is the number of valance electrons. For Example: N C P Cl Show the Lewis structure for the following. PH3 CCl4 H2S SiH4
Formation of Covalent Bonds Covalent bonds are formed when valance electrons between two elements are share either by an end-to-end overlap relationship or an side-to-side overlap relationship. • Sigma bond (σ ): single covalent bond with a end-to-end relationship BeCl2 CH4 AlF3 NH3 *sigma bonds are single bonds *bonding length is longer (Table 8.1) *bonding strength is less (Table 8.2)
Formation of a Covalent Bond 2. pi bonds ( π ) : A side-to-side overlap of valance electrons. Forms multiple bonds. CO2 O2 N2 *one sigma + one pi = double bond *one sigma + two pi = triple bond *double bond shorter bond length *double bond stronger bond *triple bond shortest bond length *triple bond strongest bond
Bonds Define Energy The total energy change in a reaction is defined by the energy of the bond broken and formed. *Endothermic reaction: the amount of energy required to break the bonds is greater than is released when a new bond is formed. *Exothermic reaction: the amount of energy required to break an existing bond is less than energy released when a new bond is formed.
LEQ How can you determine rather a covalent bond is single or double or triple bond?
The Octet Rule Shared pairs: electrons being shared Unshared pairs: electrons that are NOT shared N – A = S S : the total number of electrons Shared N : number of valance electrons an atoms Needs to form an octet (eight, 8, for all elements except H and only two,2, for H ). A : number of valance electrons an atom has Available to share in the valance.
Writing the Lewis Formula • Calculate the number of valance electrons Needed for a given molecule. • Calculate the number of valance electrons Available for a given molecule. • Write a reasonable symmetrical ‘skeleton’ structure. A single bond between all elements. • Subtract N – A to find Shared • Counting the skeleton bonded pairs, distribute all electrons for S between elements.
Writing the Lewis Formula 6. After all the shared pairs have been distributed (S), then ensure an octet for each element. 7. Total number of shared and unshared electron must equal A. CS2 C2H4 CO32- CHCl3 NOTE: The central element has smaller EN and is NEVER hydrogen. NOTE: Resonance: More than one configuration for multiple bonds.
LEQ Does the octet rule always apply?
Exceptions to the Octet Rule • Most covalent compounds with Beryllium. • Most covalent compounds of Group IIIA elements. • Compounds in which the central atom must have a share of more than eight electrons to accommodate all constituents. • Compounds containing ‘d’ or ‘f’ transitional elements • Species containing an odd number of electrons. BeCl2 AlF3 SF6 PF5
LEQ What is ‘Molecular Geometry’?
Bonding Theories All molecular bonding follow two theories: 1. VB Theory: ‘Valance Bond’ Theory. *valance electron in a σ and π overlaps 2. VSEPR Theory: ‘Valance Shell Electron Pair Repulsion’ Theory. *defines the molecular geometry of a molecule. *unshared pairs of electrons will maximize the distance between nuclei.
VSEPR Theory • Linear: VB overlap of sp w/ formula AB2 with no unshared pairs. Bonding angle 180o BeCl2 CO2 C2H2 2. Trigonal planar: VB overlap of sp2 w/ formula AB3 with no unshared pairs. Bonding angle 120o BF3 AlBr3 BCl3 3. Tetrahedral: VB overlap of sp3 w/ formula AB4 with no unshared pairs. Bonding angle 109.5o CH4 NH4+ CCl4
4. Pyramidal: VB overlap of sp3 w/ AB3 w/ one unshared pair. Bonding angle 109.5o – 1.5o for each unshared pair on the central element. NH3 PCl3 AsF3 5. Angular or Bent: VB overlap of sp3 w/ AB2 w/ two unshared pairs. Bonding angle 109.5o – 1.5o for each unshared pair on the central element. H2O H2S 6. Linear: VB overlap of sp3 w/ AB w/ three unshared pairs. Bonding angle of 180o HBr HF
7. TrigonalBipyramidal: VB overlap of sp3d w/ AB5 and no unshared pairs. Bonding angle is 90o, 120o and 180o PF5 SbCl5 8. Octehedral: VB overlap of sp3d2 w/ AB6 and no unshared pairs. Bonding angle of 90o and 180o SF6 SeCl6
LEQ What is polarity and how is it applied to molecular bonding?
EN and Polarity Polarity refers to the uneven sharing of valance electrons. Electronegativity refers to the pull on another atoms valance electrons. If the EN values are not the same then there is polarity (uneven pulling). If the EN values are equal, then there is no polarity (even pulling) The closer the EN values the less polar.