Class notes for Chapter 5: Chemical Bonding I in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include electronegativity, bond polarity, Lewis structures, resonance, octet rule and exceptions, and VESPR theor...
Electronegativity and Bond Polarity
●Representing electrons with dots in the Lewis model is effective but oversimplifies the concept
○Lewis model implies that shared electrons are always equally shared
●Ex: Hydrogen fluoride
○ looks equally shared
○However in lab, when put in an electric field, H orients itself towards the negative side and F orients towards positive side
■Means that H has slight positive charge, F slight negative charge
■Represented by:
●
●δ(delta) = partial charge
○Does NOT make ionic bond – electron is still shared, just unequally
●Polar Covalent Bond: covalent bond in which atoms have different electronegativities → uneven distribution of electron density
Electronegativity
●Electronegativity: the ability of an atom to attract shared electrons toward itself in a chemical bond
●Periodic trends:
○Electronegativity increases left to right
○Electronegativity increases up a group
○Generally inversely related to atomic size
○Keep in mind from B to F, increase of 0.5 starting from 2.0; H is an outlier and has an electronegativity between B and C; biggest decrease from F to Cl and the
decrease in electronegativity between rows 1 and 2 decreases as you move left ●Linus Pauling: American chemist who discovered electronegativity and developed electronegativity values for the periodic table
●F is the most electronegative element with a value of 4.0
○Has to do with how much it PULLS electrons, not how many it actually has
Bond Polarity, Dipole Moment, and Percent Ionic Character
●Degree of polarity in a chemical bond depends on the electronegativity difference (abb. EN) 𝚫
○Greater EN = more polar bond 𝚫
○If 2 atoms with identical electronegativities form a covalent bond, they share the electrons equally in a purely covalent ( nonpolar) bond
○If 2 atoms have a large enough EN difference, the electron is transferred completely and becomes an ionic bond ●
●Dipole Moment: measure of the separation of positive and negative charge in a molecule
○Use the dipole moment to quantify the polarity of a bond
○Magnitude of the dipole moment is given by:
■
■q = magnitude
■r = distance
○Debye (D): unit used for reporting dipole moments
■1D = 3.34x10-39 C x m
■Will not be asked to calculate, but important to understand concept
○The smaller the magnitude and the smaller the distance, the smaller the dipole moment
○The larger the dipole moment, the more polar the bond
●Percent Ionic Character: ratio of a bond’s actual dipole moment to the dipole moment it
would have if the electron transferred completely from one atom to the other, multiplied by 100%
○EX: diatomic molecule w bond length of 130pm has dipole moment of 3.5D
■q = 1.6x10-19 C, r=130 pm (the approx. length of a short chemical bond)
■(1.6x10-19 C)(130x10-12m) → 2.1x10-29C x m → 6.2D
■Percent ionic character = 3.5D / 6.2D x 100% → 56%
○A bond in which an electron is completely transferred from one atom to another would have 100% ionic character
■Even the most ionic bonds don’t reach 100%
■Bonds with greater than 50% ionic character are referred to as ionic bonds
■* no need to calculate PIC but know the 50% rule – will be told if PIC is more or less than 50% and that it increases as electronegativity increases
○Generally increases as electronegativity increases
Bond Energies and Lengths
●Formation of covalent bonds release energy
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