Chemical Bonding
Copyright© by Houghton Mifflin Company. All rights reserved.
Bonding
Intramolecular Bonding Within molecules
Intermolecular BondingBetween molecule
Copyright© by Houghton Mifflin Company. All rights reserved.
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Octet Rule
Atoms tend to gain, lose, or share electrons in order to acquire a full set of valence electrons
A very important ruleAn Octet consist of eight electrons
This refers to the outermost eight electronsCompounds form to satisfy the Octet Rule
Ionic or Covalent Bonds
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Types of IntramolecularBondsIonic
Formed by transfer of electronsHeld together by electrostatic attraction
CovalentFormed by sharing of electronsHeld together by shared electrons
MetallicFormed when electron(s) become detached from
metal atomsSea of electrons among + ions metal of atomsHeld together by electrostatic attraction
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Electronegativity values for selected elements.
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Electronegativity Difference and Bond Type
Electronegativity Electronegativity Difference Difference
(Approximate)(Approximate)
Bond TypeBond Type ExampleExample
0-0.40-0.4 Nonpolar Nonpolar CovalentCovalent
H-HH-H
0.4-1.00.4-1.0 Moderately Moderately Polar CovalentPolar Covalent
H-ClH-Cl
1.0-21.0-2 Very Polar Very Polar CovalentCovalent
H-FH-F
≥≥2.02.0 IonicIonic NaClNaCl7
Ions as packed spheres.
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Ionic bonding occurs when one atomIonic bonding occurs when one atomtransfers an electron to another atomtransfers an electron to another atom
Both atoms involved become chargedBoth atoms involved become chargedOne is negatively chargedOne is negatively chargedOne is positively chargedOne is positively charged
This occurs when a metal reacts with a This occurs when a metal reacts with a nonmetalnonmetal
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Na+1 + Cl-1 NaCl (table salt)
Ionic Bonds
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Ions and ChargesCations Positive IonsAnions Negative Ions
Group 1, 2, 3Form ions with charges equal to their group
numberGroup 5, 6, 7
Form ions with charges equal to group number minus eight
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Ionic Compounds and FormulasIonic Compounds and FormulasThe formula of a compound describes The formula of a compound describes
what elements are in the compound and what elements are in the compound and in what proportions.in what proportions.
Compounds that are held together by Compounds that are held together by ionic bonds are called ionic bonds are called ionic compoundsionic compounds..
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Common polyatomic ion names
FormulaFormula NameName
NHNH44++ Ammonium ion Ammonium ion
COCO332-2- Carbonate ionCarbonate ion
POPO443-3- Phosphate ionPhosphate ion
SOSO442-2- Sulfate ionSulfate ion
OHOH-- Hydroxide ionHydroxide ion
NONO33-- Nitrate ionNitrate ion
Formation of Ionic Compounds
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AlAl3+ 3+ SOSO442- 2-
AlAl22(SO(SO44))33Ionic compounds forms such that total of ionic charges is zero.
For above case:
2 x (+3) = 6 3 x (-2) = -6
Total 0
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The Lewis dot structure for Oxygen
OOxygen is in group VIA so it has 6 valence electrons
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The Lewis dot structure for Chlorine
Clchlorine is in group VIIA so it has 7 valence electrons
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The Lewis dot structure for calcium
Cacalcium is in group IIA so it has 2 valence electrons
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Making calcium chloride
+Ca Cl Cl
CaCl2
• Covalent BondsCovalent Bonds• A covalent bond is a chemical bond that is A covalent bond is a chemical bond that is
formed when two atoms formed when two atoms share a pairshare a pair of of electrons.electrons.
• HH.. + H + H.. H H::HH• Covalent Compounds and FormulasCovalent Compounds and Formulas
• In the above example, each hydrogen has a In the above example, each hydrogen has a filled valence shell simulating the electron filled valence shell simulating the electron configuration of helium.configuration of helium.
• Compounds that are held together by covalent Compounds that are held together by covalent bonds are called covalent compounds.bonds are called covalent compounds.
• Covalent compounds form from atoms on the Covalent compounds form from atoms on the right side of the periodic tableright side of the periodic table
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Multiple BondsMultiple Bonds..• In electron dot notations, a pair of electrons can be In electron dot notations, a pair of electrons can be
represented by a pair of dots represented by a pair of dots :: . .• This can be a bonding pair or a lone pair (non-This can be a bonding pair or a lone pair (non-
bonding pair).bonding pair).• Bonding pairs can also be represented by lines Bonding pairs can also be represented by lines
connecting atoms.connecting atoms.• HH::H = HH = H--HH
• When one pair of electrons is shared, it is called a When one pair of electrons is shared, it is called a single bondsingle bond..
• H-HH-H
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• When When two pairstwo pairs of electrons are shared it is of electrons are shared it is called a called a double bond.double bond.
• When When three pairsthree pairs of electrons are shared it is of electrons are shared it is called a called a triple bondtriple bond..
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Bond Length• Bond Length varies for covalent bonds
between different elements• See p. 187
• Two important trends• As one moves down a group, bond length
between atoms increases• F—F 0.128 nm• Cl—Cl 0.198 nm• Br—Br 0.228 nm
• Multiple bonds are shorter than single bonds• Example: two carbon atoms bonded together
• Single covalent bond 0.154 nm• Double covalent bond 0.134 nm• Triple covalent bond 0.120 nm
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Exceptions to the Octet RuleExceptions to the octet rule include those for atoms that
cannot fit eight electrons, and for those that can fit more than eight electrons, into their outermost orbital.
Hydrogen forms bonds in which it is surrounded by only two electrons.
Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons.
Main-group elements in Periods 3 and up can form bonds with expanded valence, involving more than eight electrons, e.g. PF5 and SF6
Electron-Dot Notation
To keep track of valence electrons, it is helpful to use electron-dot notation.
Electron-dot notation is an electron-configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol. The inner-shell electrons are not shown.
Lewis Structures
Electron-dot notation can also be used to represent molecules.
F F
H H
Chapter 6
The pair of dots between the two symbols represents the shared electron pair of the hydrogen-hydrogen covalent bond.
For a molecule of fluorine, F2, the electron-dot notations of two fluorine atoms are combined.
Lewis Structures
The pair of dots between the two symbols represents the shared pair of a covalent bond.
F F
F F
Chapter 6
In addition, each fluorine atom is surrounded by three pairs of electrons that are not shared in bonds.
An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom.
See Example, p. 185
Multiple Covalent Bonds
Double and triple bonds are referred to as multiple bonds, or multiple covalent bonds. (See Table 2, p. 187)
In general, double bonds have greater bond energies and are shorter than single bonds.
Triple bonds are even stronger and shorter than double bonds.
When writing Lewis structures for molecules that contain carbon, nitrogen, or oxygen, remember that multiple bonds between pairs of these atoms are possible.
Chapter 6
Molecular Shape
• Arises because electrons repulse one another• Called VSEPR• Valence Shell Electron Pair Repulsion• VSEPR states that in a small molecule, the
pairs of valence electrons are arranged as far apart from each other as possible
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Summary of Molecular ShapesType Bond
Angle
Unshared Pairs
Example
Linear 180° Balanced(2 each side)
CO2
Trigonal
Planar
120° none BCl3
Tetrahedral 109.5° none CH4
Pyramidal 107° one NH3
Bent 105° two H2O
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Bent molecular structure of the water molecule.
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A closer look at a water molecule (bent)
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Molecular structure of methane (Tetrahedral).
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Pyramidal
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Molecular OrbitalsWhen two atoms combine, the molecular orbital model assumes that their atomic orbitals overlap to produce molecular orbitals, or orbitals that apply to the entire molecule.Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole.A molecular orbital that can be occupied by two electrons of a covalent bond is called a bonding orbital.
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8.3
Molecular Orbitals
Sigma BondsWhen two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei, a sigma bond is formed.
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8.3
Molecular Orbitals
When two fluorine atoms combine, the p orbitals overlap to produce a bonding molecular orbital. The F—F bond is a sigma bond.
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8.3
Molecular OrbitalsPi Bonds
In a pi bond (symbolized by the Greek letter ), the bonding electrons are most likely to be found in sausage-shaped regions above and below the bond axis of the bonded atoms.
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8.3
PolarityBonds can be polar or nonpolar
Depends on electronegativity differenceMolecules can be polar or nonpolarPolar molecules are called dipoles
One end of a polar molecule has “+” charge; other end has “-” charge
Will align in electric field Will be attracted or deflected by magnetic field
Polarity of a molecule is determined by shape and the type of bonds between its atoms
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The chlorine atom attracts the electron cloud more The chlorine atom attracts the electron cloud more than the hydrogen atom does. Why?than the hydrogen atom does. Why?
Polar MoleculesA hydrogen chloride molecule is a dipole.
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8.4
Intermolecular Forces
• Attraction between molecules• Holds groups of molecules together• Intermolecular forces are known as van
der Waals forces• Dipole-Dipole attraction• Dispersion Forces• Hydrogen Bonds
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van der Waals Forces• Dipole-Dipole attraction
• Electrostatic attraction between polar molecules• Molecules line up like magnets
• Dispersion Forces• Polarity arises due to electron imbalance• Weak electrostatic forces arise
• Hydrogen Bonds• Strong dipoles arising from covalent bonds of
hydrogen atom to a very electronegative atom• H electronegativity = 2.1 F=4.0
• Molecules line up like magnets• Results in liquids with high boiling points
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Probability representations of the electron sharing in HF.
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Charge distribution in the water molecule.
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Water molecule behaves as if it had a positive and negative end.
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Polarity
Formaldehyde CH2O Forms dipole due to high electronegativity of O
Carbon Dioxide CO2Has polar bond but they cancel outCO2 is nonpolar
Water H2OWater is a bent molecule due to unshared pairsunlike CO2 bond polarities do not cancel outWater is polar and forms dipole
Why is water liquid and Carbon Dioxide a gas?
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Large Molecules
Examples: protein, Subunits linked together in a chainOften bend and twist to form 3D shape
Chains, rings, balls
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The three states of water:
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Table 12.3
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Relative sizes of some ions and their parent atoms.
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The three possible types of bonds.
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Figure 12.1: The formation of a bond between two atoms.
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Tetrahedral arrangement of electron pairs.
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109.5°
Tetrahedral arrangement of four electron pairs around oxygen.
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Table 12.1
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Figure 12.6: Polar water molecules are strongly attracted to negative ions by their positive ends.
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Figure 12.6: Polar water molecules are strongly attracted to positive ions
by their negative ends.
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The NH3 molecule pyramidal structure.
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Bond Energies and Bond Lengths for Single Bonds
Section 2 Covalent Bonding and Molecular CompoundsChapter 6
Hybrid Orbitals
When atoms bond, their outer orbitals become distortedOrbitals do not look like those of unbonded atomHybrid orbitals are formed which are a cross
between the bonding orbitalsspsp2
sp3
Orbital hybridization contributes to shape of molecule
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Hybrid Orbitals
