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Covalent Bonding. Bonding models for methane, CH 4 . Models are NOT reality. Each has its own strengths and limitations. Covalent Bonds. Polar-Covalent bonds. Electrons are unequally shared Electronegativity difference between .3 and 1.7. Nonpolar -Covalent bonds.
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CovalentBonding Bonding models for methane, CH4. Models are NOT reality. Each has its own strengths and limitations.
Covalent Bonds Polar-Covalent bonds • Electrons are unequally shared • Electronegativity difference between .3 • and 1.7 Nonpolar-Covalent bonds • Electrons are equally shared • Electronegativity difference of • 0 to 0.3
Covalent Bonding Forces • Electron – electron repulsive forces • Proton – proton repulsive forces • Electron – proton attractive forces
Bond Length and Energy Bonds between elements become shorter and stronger as multiplicity increases.
Bond Energy and Enthalpy Energy required Energy released D= Bond energy per mole of bonds Breaking bonds always requires energy Breaking = endothermic Forming bonds always releases energy Forming = exothermic
The Octet Rule Combinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level. Monatomic chlorine Diatomic chlorine
The Octet Rule and Covalent Compounds • Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level. • Covalent compounds involve atoms of nonmetals only. • The term “molecule” is used exclusively for covalent bonding
The OctetRule: The Diatomic Fluorine Molecule F 1s 2s 2p Each has seven valence electrons F 1s 2s 2p F F
The OctetRule: The Diatomic Oxygen Molecule O 1s 2s 2p Each has six valence electrons O 1s 2s 2p O O
The OctetRule: The Diatomic Nitrogen Molecule N 1s 2s 2p Each has five valence electrons N 1s 2s 2p N N
Lewis structures show how valence electrons are arranged among atoms in a molecule. Lewis structures Reflect the central idea that stability of a compound relates to noble gas electron configuration. Shared electrons pairs are covalent bonds and can be represented by two dots (:) or by a single line ( - ) Lewis Structures
2nd row elements C, N, O, F observe the octet rule (HONC rule as well). 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive. 3rd row and heavier elements CAN exceed the octet rule using empty valence dorbitals. When writing Lewis structures, satisfy octets first, then place electrons around elements having available dorbitals. Comments About the Octet Rule
Show how valence electrons are arranged among atoms in a molecule. Reflect the central idea that stability of a compound relates to noble gas electron configuration. Lewis Structures
Hydrogen (and Halogens) form one covalent bond Oxygen (and sulfur) form two covalent bonds One double bond, or two single bonds Nitrogen (and phosphorus) form three covalent bonds One triple bond, or three single bonds, or one double bond and a single bond Carbon (and silicon) form four covalent bonds. Two double bonds, or four single bonds, or a triple and a single, or a double and two singles The HONCRule
Completing a Lewis Structure -CH3Cl • Make the atom wanting the most bonds central • Add up available valence electrons: • C = 4, H = (3)(1), Cl = 7 Total = 14 • Join peripheral atoms • to the central atom • with electron pairs. H .. .. .. H .. • Complete octets on • atoms other than • hydrogen with remaining • electrons C .. Cl .. .. H
Multiple Covalent Bonds:Double bonds Ethene Two pairs of shared electrons
Multiple Covalent Bonds:Triple bonds Ethyne Three pairs of shared electrons
Acetic Acid Two electrons (one bond) per hydrogen Eight electrons (four bonds) per carbon Eight electrons (two bonds, two unshared pairs) per oxygen
