BondingChapter 8

Types of Chemical Bonds• Ionic Bonds – metals/nonmetals

o Electrons are transferredo Ions paired have lower energy (greater stability) than

separated ionso Electrostatic forces

• Covalent Bonds – nonmetalso Electrons are shared by nucleio Pure covalent – non-polar covalent

• Electrons are shared evenly (F-F)o Polar Covalent

• Electrons are shared unequally • Atoms end up with fractional charges

oδ+ or δ-

Covalent Bond Length• Distance at which the system energy is at a

minimum• Forces at work

o Attractive forces – protons and electronso Repulsive forces – electron-electron and proton-proton

• Energy is given off when two atoms achieve greater stability together than aparto Bond energy

Electronegativity• The ability of atom in a molecule to attract

shared electrons to itself• Trend – increases across and up

Electronegativity & Bonds• Greater electronegativity difference between

two elements means less covalent character and greater ionic character

• Any compound that conducts an electric current when melted is an ionic compound

• If the electronegativity difference < 1.67, then the atoms will share electrons.

Bond Polarity & Dipole Moments

• Dipolar Molecules o Molecules with a somewhat negative and a somewhat

positive end• Dipole moment

o Molecules with preferential orientation in an electric fieldo Slight negative side will be attracted to positive o All diatomic molecules with a polar covalent bond are

dipolar

Bond Polarity & Dipole Moments

• Some molecules have polar bonds, but no dipole momento Linear, radial, or tetrahedral symmetry of charge

distributiono Charge balances/evens out

CO2 CCl4

Bonding & Noble Gas e- Configurations

• Ionic bonds – electrons are transferred until each species attains a noble gas configuration

• Covalent bonds – electrons are shared in order to complete the valence configurations of both atoms

• Predicting Formulas of Ionic Compoundso Based on placement in the periodic tableo Na Na+

• Sizes of ions o Cations are smaller than parent iono Anions are largero Isoelectronic ions – size decreases as the nuclear charge

increases

Binary CompoundsLattice Energy

Binary Ionic Compounds• Lattice energy – change in energy that takes

place when separated gaseous ions are packed together to form an ionic solid

M+(g) + X-

(g) MX(s)

Determining ΔHf°Metal

Nonmetal

• Step 1: Sublimationo Solid Gas

• Step 2: Ionization Energyo Gas Ion

• Step 3: Bond Energyo Eg: Diatomic Single

• Step 4: Electron Affinityo X + e- X-

• Step 5: Formation of solid compound (LE)

• Sum = ΔHf°

Example – formation of LiF

Binary Ionic Compounds• The formation of ionic compounds is

endothermic until the formation of the lattice• The lattice formed by alkali metals and

halogens (1:1 ratio) is cubic except for cesium salts

Lattice Energy Calculations• Lattice Energy = k Q1Q2

ro k = constant dependent on the solid structure and the

electron configurationso Q1 and Q2 = numerical ion chargeso r = shortest distance between centers of the cations and the

anions

• Lattice Energy increases as the ionic charge increases and the distance between anions and cations decreases

• Charge has more impact than distance

Partial Ionic Character of Covalent Bonds

( )• % Ionic Character = measured dipole moment of X-Y x 100% Calculated dipole moment of X+Y-

• Ionic compounds generally have > 50% ionic character• % ionic character is difficult to calculate for

compounds containing polyatomic ions

Calculating Percent Ionic

Covalent Chemical Bond• Strengths of the Bond Model

o Associates the quantities of energy with the formation of bonds between elements

o Allows the drawing of structures showing the spatial relationship between atoms in a molecule

o Provides a visual tool to understanding chemical structure

• Weaknesses of the Bond Modelo Bonds are not actual physical structureso Bonds can not adequately explain some phenomena• Resonance structures

Multiple Bonds• Single bonds – 1 pair of shared electrons• Double bonds – 2 pairs of shared electrons• Triple bonds – 3 pairs of shared electrons

• As the number of shared electrons increases, bond length shortens

• Multiple bonds typically have higher bond energy

Bond Energy & Enthalpy• ΔH = sum of energies required to break old

bonds (endothermic) - sum of the energies released in forming new bonds (exothermic)

• ΔH = ΣD(bonds broken) – ΣD(bonds formed) o D = bond energy per moleo D always has a positive sign

Localized Electron Bonding Model

• Lone electron pairso Electrons localized on an atom

• Bonding electron pairso Electrons found in the space between atomso Shared pairs

• Localized Electron Modelo “A molecule is composed of atoms that are bound together by sharing

pairs of electrons using the atomic orbitals of the bound atoms.”

• Derivations o Valence electron arrangement using Lewis structureso Prediction of molecular geometry using VSEPRo Description of the type of atomic orbitals used to share or hold lone

pairs of electrons