9.5-9.6 Lewis Structures and Covalent Bonding

H O Lewis Diagrams Provide models for covalent bonding Shows bonded atoms sharing their valence electrons Also depict single atoms and their valence electrons H O

Single Covalent Bonds Atoms share valence electrons to obtain an octet (or duet in case of Hydrogen/Helium) and become stable In single covalent bonds, only one pair of electrons is shared This pair of shared electrons is called a bonding pair Pair that belongs only to one atom is known as the lone pair

Single Bonds H O One Valence Electron Six Valence Electrons

Single Bonds Duet Duet Octet H H O H

Representing Bonding Pairs Bonding pairs are usually shown as a line H O _

Halogens Lewis structures show why halogens are diatomic Cl –

O =O Double Covalent Bonds Two atoms can share two electron pairs to achieve an octet O =O Double bonds are shorter and stronger than single bonds

N Triple Covalent Bonds Atoms can also share three electron pairs Triple bonds are even shorter and stronger than double bonds

Covalent Bonding: Models and Reality Lewis model predicts properties of molecular compounds Accounts for why certain combinations of atoms form molecules while others do not Also shows why covalent bonds are highly directional

Directional vs. Non Directional Covalently bonded atoms prefer specific orientations in space relative to one another As a result, covalently bonded molecules have definite shape

Directional vs. Non Directional Continued Ionic bonds have no electron sharing and the number of anions surrounding a cation is limited Ionic bonds are limited by a number of factors: Charges of ions, sizes and efficiency of the lattice packing

Attraction Between Covalently Bonded Atoms Attraction between atoms is due to the sharing of electron pairs Each bond links one pair of atoms UNLIKE ionic bonds Fundamental units of covalent bonds are molecules

Intermolecular Forces vs. Intramolecular Forces Covalently bonded molecules have weaker intermolecular forces Covalently bonded molecules have stronger intramolecular forces

Melting/Boiling Points of Molecular Compounds Molecular compounds tend to have lower melting and boiling points than ionic compounds Why? Molecules remain intact when the melt/boil, only INTERACTIONS between separate molecules are overcome and since molecular compounds have weaker intermolecular forces, they tend to boil/melt at lower temperatures.

Electronegativity and Bond Polarity There are limitations to the Lewis theory Shared electrons always seem to be shared equally H F

(shown without valence electrons) F ẟ+ ẟ- H F _ Electron Density is greater on the fluorine atom

H F Polar Bonds The hydrogen-fluorine bond is said to be polar This means it has a positive (hydrogen) and negative (fluorine) pole

Polar Covalent Bonds Intermediate between pure covalent bonds and ionic bonds Most common covalent bonds between dissimilar atoms are polar covalent

Practice Problem Explain the distribution of charge in the following molecule - + N H + +

Definitions Homonuclear Molecules- Molecules composed of one type of element Heteronuclear Molecules- Molecules composed of more than one type of element

Electronegativity Quantified by American Chemist Linus Pauling He compared bond energy of a heteronuclear diatomic molecule with the bond energies of its homonuclear counterpart

Electronegativity Continued H₂ and F₂ bond energies =436 kJ/mol Pauling reasoned that if HF bond was purely covalent, the bond energy of HF would be an average of the bond energies 256 kJ/mol Bond energy turned out to be 565 kJ/mol Pauling suggested that the additional bond energy was do to the ionic character of the bond

Electronegativity Trends Electronegativity generally increases across a period and up a column Fluorine is the most electronegative element Facium is the least electronegative element Electronegativity is inversely related to atomic size The larger the atom the less ability it has to attract to electrons to itself

Bond Polarity, Dipole Moment and Percent Ionic Character Degree of polarity in a chemical bond depends on the electronegativity difference (∆EN) between the two bonding atoms The greater the difference the more polar the bond

Electronegativity Differences The Effect of Electronegativity Difference on Bond Type Electronegativity Difference Bond Type Example Small (0-0.4) Pure Covalent Cl₂ Intermediate (0.41-1.99) Polar Covalent HCl Large (2.0+) Ionic NaCl

Practice Problem Determine whether the bond formed between each pair of atoms is covalent, polar covalent or ionic Electronegativities Sr= 1.0 F=4.0 N=3.0 Cl= 3.0 O=3.5 Sr and F N and Cl N and O Ionic Pure Covalent Polar Covalent

Continuum of Bond Types

Dipole Moment Polarity of a bond is quantified by the size of its dipole moment Dipole moment- the mathematical product of the separation of the ends of a dipole and the magnitude of the charges Dipole Moment (µ) occurs anytime there is a seperation of a positive and negative charge

Dipole Moment Continued The magnitude of a dipole moment is created by separating two particles of equal but opposite charges of magnitude (q) by a distance ( r) µ=qr Dipole moment of a completely ionic bond can be understood by calculating the dipole moment that results from separating a proton and electron (q=1.6 X 10^-19 C) by a distance of r=130 pm you end up with 6.2 D

Dipole Moment Continued Debye (D) is the unit commonly used for reporting dipole moments 1D= 3.34 X 10 ^ -30 C*m The smaller the magnitude of the charge separation and the smaller the distance between the charges the smaller the dipole moment

Dipole Moments of Different Molecules Dipole Moments of Several Molecules in the Gas Phase Molecule ∆EN Dipole Moment Cl₂ 0 0 CIF 1.0 0.88 HF 1.9 1.82 LiF 3.0 6.33

Percent Ionic Character Comparing the actual dipole moment of a bond to what the dipole moment would be if the electron were completely transferred from one atom to the other allows a sense of the degree to which the electron is transferred This quantity is called percent ionic character

Percent Ionic Character Continued Measured dipole moment of bond Dipole moment if electron were completely transferred X 100% A bond in which an electron is completely transferred from one atom to another would have a 100% ionic character ALTHOUGH even the most ionic bonds do not reach this ideal

Practice Problem In the gas phase, silver chloride (AgCl) has a dipole moment of 6.08. What percent ionic character is closest to that of silver chloride? 10% 30% 50% 90%

In general, bonds greater than 50% ionic character are referred to as ionic bonds

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