Covalent Bonds

 Octet rule – all atoms want to have 8 electrons in their outer shell  Exception – helium (2)  Noble gases – very stable  Binary ionic compounds – cations and anions lose or gain e -  Resulting ions have noble gas configuration

 Chemical bond where elements share valence electrons to gain noble gas configuration  Usually occurs between elements that are close to each other on the periodic table  Majority are nonmetallic elements  CO 2  H 2 O  SiO 2

 Molecule – forms when two or more atoms are bonded covalently  Carbohydrates, proteins, fats, DNA, wool, cotton, synthetic fibers  Diatomic molecules – elements not found in nature as single atom because more stable this way  H 2, N 2,O 2, F 2,Cl 2,Br 2,I 2  Share one pair of valence electrons

 Formed when a single pair of e - is shared between two bonded atoms  Shared pair - bonding pair  Lewis structures – use electron dot diagrams to show how electrons are arranged in molecules  Single bond is represented by two dots or a single line  Example: H 2  H: H or H—H

 Form when more than more than one pair of e- is shared between two bonded atoms  # valence electrons of an element is associated with the number of shared electron pairs needed to complete the octet  Double bond = 2 e - pairs being shared (2 lines)  Triple bond = 3 e - pairs being shared (3 lines)

 Sigma bond (σ) - single covalent bonds  Results if atomic orbitals overlap end to end concentrating the e- in a bonding orbital between the two atoms  Bonding orbital - Localized regions where bonding electrons most likely found  Form when:  S orbital overlaps another s orbital  S orbital with p orbital  P orbital with p orbital

 Pi bonds (π) form when parallel orbitals overlap to share electrons  Double covalent bond:  1 sigma bond and 1 pi bond  Triple covalent bond:  1 sigma bond and 2 pi bonds

 Bond length – distance between two bonding nuclei at the position of maximum attraction  Determined by size of atom and how many electron pairs are shared  Inverse relationship between # if electron pairs shared and bond length  single bonds weaker than double…. Etc.  F 2 bonds weaker than N 2

 Bond dissociation energy – amount of energy required to break a specific covalent bond  Direct relationship between bond energy and bond length  Bonded closer together – takes more energy to separate