Chapter 6 Chemical Bonding Covalent molecules
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Transcript of Chapter 6 Chemical Bonding Covalent molecules
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CHAPTER 6CHEMICAL BONDING
COVALENT MOLECULES
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PART 1:CHEMICAL BONDS
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CHEMICAL BONDS Mutual electrical attraction between the
nuclei and valence electrons of different atoms that binds the atoms together
Atoms are usually less stable by themselves than combined with other atoms
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TYPES OF BONDING Ionic Bonding
Chemical bonding resulting from the attractions of cations and anions In pure ionic bonding, atoms give up
electrons completely to other atoms, which accept them completely to form ions which attract one another
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TYPES OF BONDING Covalent Bonding
Chemical bond that shares the electrons between two atoms
A pure covalent bond results in the electrons being “owned” equally by two atoms Both nuclei have these electrons around
them an equal amount of timeExample of purely covalent bond : F2
Both atoms attract each others’ electrons equally (both have an electronegativity of 4.0, so the pull on the electrons is exactly the same from both atoms)
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CLASSIFYING BONDS:IONIC OR COVALENT Most compounds do not have purely
covalent or ionic bondsThe bonds fall somewhere on a continuum
Look at the bond’s percentage of ionic characterBonds with 0 to 50% ionic character are classified as covalentBonds with > 50% ionic character are classifiedas ionic
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IONIC VS. POLAR COVALENT VS. NONPOLAR COVALENT BONDS Ionic Bonds
Electrons get transferred between atoms forming ions that attract one another
Polar Covalent BondElectrons shared between atoms unequally
Nonpolar Covalent BondElectrons shared between atoms equally
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PART 2:CLASSIFYING BONDS USING
ELECTRONEGATIVITY
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CAN USE ELECTRONEGATIVITY DIFFERENCE AS A GUIDE We can predict the bond type between
two atoms by calculating the absolute value of the electronegativity difference between them.
Electronegativity Difference
Bond Type
0- 0.3 Nonpolar Covalent
>0.3 – 1.7 Polar Covalent
>1.7 Ionic
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IONIC BONDING If the absolute value of the
electronegativity difference between two atoms is > 1.7, then the bond they form is probably ionic
Example #1: NaClNa – electronegativity of 0.9Cl – electronegativity of 3.0Electronegativity difference = | 0.9 – 3.0 | = 2.1
An electron moves from the Na atom to the Cl atom causing Na to have a +1 charge and Cl to have a -1 charge
The Na + ion is then attracted to the Cl- ion forming a very strong bond
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POLAR COVALENT BONDS If the absolute value of the electronegativity
difference between two atoms is >0.3 - 1.7, then the bond they form is probably polar covalent
Covalent because the electrons are shared between the atoms
Polar because the electrons are shared unequally between the atoms (spend more time around one nucleus)
Example: BF3 Boron – electronegativity of 2.5 Fluorine – electronegativity of 4.0 Electronegativity difference of | 4.0-2.5|= 1.5
Polar covalent - electrons spend more time around the F nucleus due to greater electronegativity
Electron cloud is distorted towards the Fluorine atoms (more electron density) resulting in a partial negative charge on the fluorine atoms and a partial positive charge on the boron atom.
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NONPOLAR COVALENT BONDS If the absolute value of the
electronegativity difference between two atoms is 0.0- 0.3, then the bond they form is probably nonpolar covalent
Electrons are shared equally between the nuclei of the bonded atoms, resulting in a balanced distribution of electrical charge
Ionic character of 0 to 5% ionic character
Example: F2
Electronegativity difference of |4.0 – 4.0| = 0.0
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PART 3:COVALENT
COMPOUNDS
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COVALENT COMPOUNDS Molecules
Neutral group of atoms held together by covalent bonds
Can exist on its own as an individual unit Molecular formula
Shows the relative numbers of atoms of each kind in a chemical compound by using element symbols and subscripts to show the number of atoms Examples:
H2O - water
O2 - oxygen
C12H22O11 - sucrose
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BOND ENERGY Energy required to break a chemical
bond and form neutral isolated atomsUnits kj/moleEnergy required to break one mole of bonds
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OCTET RULE Chemical compounds tend to form so
that each atom by gaining, losing or sharing electrons has an octet of electrons in its highest occupied energy level
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EXCEPTIONS TO THE OCTET RULE Hydrogen – needs total of 2 electrons Beryllium – needs total of 4 electrons Boron – needs 6 electrons Aluminum – needs 6 electrons Atoms of elements in groups 15, 16, 17,
and 18 can sometimes form expanded octetsCan have more than 8 electrons
surrounding the nucleusUsually not N, O, or F
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LEWIS STRUCTURES AKA ELECTRON DOT NOTATION Symbols represent the nucleus and
inner shell electrons Dot pairs between the symbol represent
pairs of shared electrons Dots adjacent to symbol represent
unshared pairs of electrons
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STEPS FOR DRAWING LEWIS DOT STRUCTURES1. Count up the number of valence
electrons. E.g. in CCl4 there is 1 C which has 4 valence
electrons and 4 Cl atoms which each have 7 valence electrons Therefore for C 1 x 4 = 4 and for Cl 4 x 7 = 28 Adding 4 and 28 together gives you 32 electrons
2. Identify the central atom and put the other atoms around the central atom.
If C is present, put C in the center. Otherwise, the least electronegative atom goes in
the center. H never goes in the center. Halogens don’t usually go in the center unless they
are bonded to other halogens
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STEPS FOR DRAWING LEWIS DOT STRUCTURES (CONT’D)3. Put a pair of electrons (dots) between
the central atom and the outside atoms.
4. Start distributing the remaining electrons
Subtract those electrons placed in step 3 from the total in step 1 to find electrons remaining
Put the remaining electrons around the outside atoms first until all atoms either have an octet or are satisfied (Exceptions to the Octet Rule for the exceptions).
5. If there are electrons left over after satisfying the octet rule, put them as unshared pairs on the central atom.
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STEPS FOR DRAWING LEWIS DOT STRUCTURES (CONT’D)6. If there are too few electrons
for all atoms (including the central atom) to have an octet (or be satisfied if it is an exception), then double and or triple bonds should be considered.
C, N, and O are most atoms likely to form double or triple bonds.
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DRAWING LEWIS DOT STRUCTURES FOR POLYATOMIC IONS To draw the Lewis dot structure for a
polyatomic ion (a covalently bonded molecule that has a charge), you follow the same steps except the following:When counting up the valence electrons in
step 1, add the absolute value of the charge if the charge is negative to the total of the valence electrons. If the charge is positive, subtract the absolute value of the charge from the total valence electrons.
After completing the Lewis dot structure, put the entire structure in brackets with the charge on the right upper corner on the outside of the right bracket.
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MULTIPLE COVALENT BONDS Some elements, especially C, N, and O
can share more than one electron pair.Double bond – 2 pairs of electrons are
sharedTriple bond – 3 pairs of electrons are shared
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MULTIPLE BONDS ENERGIES AND BOND LENGTHS Double bonds have a greater bond
energy and shorter bond lengths than single bonds
Triple bonds have a greater bond energy and shorter bond lengths than single bonds
Triple bonds stronger than double Double bonds stronger than single
Bond Avg. Bond Length (pm)
Avg bond energy (kj/mole)
C-C 154 346
C=C 134 612
C= C 120 835
C-N 147 305
C=N 132 615
C=N 116 887
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RESONANCE STRUCTURES Bonding in molecules or ions that
cannot be correctly represented by a single Lewis structureDraw possible Lewis structures with double-
headed arrow in between
Bond between oxygens is a hybrid of a single and double bond
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PART 4: CHEMICAL FORMULAS, NAMING, AND
SHAPES
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CHEMICAL FORMULA Chemical formula indicates the relative
number of atoms of each kind in a chemical compound. Molecular formula indicates the number of
atoms of each element contained in a single molecule
E.g. C8H18
Ionic compounds have formula units, not molecules (Remember, these ions are located in a lattice of positively and negatively charged ions – are not found as individual units)
One formula unit is used to designate the simplest ratio of the cations to anions
E.g. in a crystal of NaCl, the formula NaCl is used to represent one unit
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NAMING BINARY MOLECULAR COMPOUNDS (CHAPTER 7 P. 227-229)
Prefix system of naming – must learn prefixesNumber Prefix
1 Mono-
2 Di-
3 Tri-
4 Tetra-
5 Penta-
6 Hexa-
7 Hepta-
8 Octa-
9 Nona-
10 Deca-
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NAMING BINARY MOLECULAR COMPOUNDS Element with the smaller group number is given first
in the name If both elements are in same group, element whose
period number is greater is given first. If first element subscript is one, no prefix is given. If subscript is greater than 1, then the prefix
corresponding to the subscript is added to the beginning of the element name.
The second element is always given a prefix indicating the number of atoms and the element root + ide is used.
The –o or –a at the end of a prefix is usually dropped when the word following the prefix begins with another vowel i.e. monoxide instead of monooxide or pentoxide instead of pentaoxide
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BINARY MOLECULE NAME EXAMPLESFormula Prefix-system name
N2O Dinitrogen monoxide
NO Nitrogen monoxide
NO2 Nitrogen dioxide
N2O3 Dinitrogen trioxide
N2O4 Dinitrogen tetroxide
N2O5 Dinitrogen pentoxide
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VSEPR THEORY Theory used to determine the shape of
moleculesBased on the premise that repulsion
between the sets of valence electrons surrounding an atom causes the electron pairs to be oriented as far apart as possible.
Looks at the bonded pairs and lone pairs on the central atom only Bonded pairs are set as far apart from one
another as possible
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MOLECULAR GEOMETRIES Some
representative molecular geometries – need to learn molecular geometries located on VSEPR Summary sheet in Chapter 6 folder
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HOW TO READ VSEPR SUMMARY SHEET Must determine molecule type from Lewis Dot
structure – must draw this first Let A = central atom (in example below, N) Let B = outside atoms regardless of whether they are the
same element or not (3 outside atoms ) Let E = represent the # of lone pairs on the central atom
(only) (one lone pair on N) Let subscripts for B and E represent the number of outside
atoms and number of lone pairs on the central atom Example: NF3 is AB3E N is central atom or A, the 3 Fs are represented as B3 and there is one lone pair on the
central atom or E Molecular geometry - from VSEPR Summary sheet is trigonal pyramidal
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BOND ANGLES Angles between the bonded pairs of
atoms around the central atom Bond angles between bonded pairs tend
to be larger for those atoms without lone pairs on the central atomLone pairs cause increased repulsion and
force bonded pairs closer together
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POLAR MOLECULES Electrons are unevenly distributed
throughout the moleculeBecause of this there are areas of partial
positive charge and areas of partial negative charge
Forms dipoles Equal but oppositecharges separatedby short distances
More positive more negative
Partial +
Partial -
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FACTORS DETERMINING POLARITY OF MOLECULES Polar Bonds present in molecule Symmetry of molecule
Do dipoles when taken together, cancel out or form a dipole in one direction? If cancel out - then nonpolar If overall dipole is present – polar
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GUIDELINES FOR DETERMINING POLARITY If molecule type is AB2,AB3,AB4, AB5, AB6,
AB2E3, or AB4E2, and the outside atoms are the same element, then the molecule is nonpolar
If molecule type is AB and A and B are the same element, then the molecule is nonpolar
All others are polarE.g. CH4 is AB4 all of the B’s are the same
(H), therefore the molecule is nonpolarCHCl3 is AB4 but not all of the B’s are H, (1 H
and 3 Cls), therefore the molecule is polar
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HYBRIDIZATION When bonding occurs, orbitals from the
bonding atoms become mixed and form new orbitals that are equivalent energies (i.e. hybrid orbitals)Hybrid orbitals
Orbitals of equal energy produced by the combination of two or more orbitals on the same atom# of orbitals produced = # of orbitals that
have combined
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HYBRID ORBITALS AND MULTIPLE BONDS All single bonds are sigma bonds – σ
Atomic orbitals overlap and form hybrid orbitals
Double and Triple bonds have one sigma bond In addition, double bonds also have one π
bond Pi bonds (π) are overlapping atomic p orbitals on
adjacent atomsTriple bonds also have two π bonds
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HYBRID ORBITALS# of bonding pairs and lone pairs around central atom
Hybrid Orbital Formed
# of hybrid orbitals
Geometry of electron Pairs (not geometry of atoms)
2 sp 2 Linear
3 sp2 3 Trigonal planar
4 sp3 4 Tetrahedral
5 dsp3 or sp3d
5 Trigonal bipyramidal
6 d2sp3 or sp3d2
6 octahedral
Count double and triple bonds as one bonding pair since there is only one sigma bond
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PART 5: INTERMOLECULAR FORCES
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INTERMOLECULAR FORCES VS. INTRAMOLECULAR FORCES
Weaker Stronger
Intermolecular forces Between molecules 3 types
Dipole-dipole interaction
Hydrogen Bonding London Dispersion
Forces
Intramolecular Forces Within molecules True bonding
Covalent bonding
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3 TYPES OF INTERMOLECULAR FORCES1. Dipole-dipole interactions
Attraction between molecules that occurs between oppositely charged ends of partially charged molecules
Occurs between polar molecules
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TYPES OF INTERMOLECULAR FORCES2. Hydrogen bonding – special case of
dipole-dipole interactionNOT Bonding!!!!! – intermolecular forceOccurs when highly polarized H is attracted
to the lone pair of a highly electronegative N, O, and F
Occurs with molecules having N, O, or F directly bonded to a H
Strongest intermolecularforce
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TYPES OF INTERMOLECULAR FORCE3. London Dispersion Forces
Weakest intermolecular forceOccurs due to random motion of electrons
in molecules forming temporary dipoles (areas of partial charge) which attract each other (oppositely charged ends attract)
Due to temporary nature of the dipoles, attractions are fairly weak but when added together can be significant
Occurs in all molecules (polar and nonpolar)
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STRENGTH OF INTERMOLECULAR FORCES Hydrogen Bonding > Dipole-Dipole
Interactions> London Dispersion Forces Strength of intermolecular forces
determines whether molecules haveHigh surface tensionHigh boiling pointCapillary action
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DETERMINING THE INTERMOLECULAR FORCES ASSOCIATED WITH A MOLECULE
Is the molecule polar or nonpolar
If polar, is thereAn N,O, or F directlyBonded to an H in the structure?
If yes, then the molecule has hydrogen bonding and London Dispersion Forces
If nonpolar, London Dispersion Forces only
If no, then the molecule has a dipole-dipole interaction and London DispersionForces
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PROPERTIES OF SUBSTANCES ASSOCIATED WITH INTERMOLECULAR FORCES Surface Tension
Force that pulls adjacent parts of a liquid’s surface together, thereby decreasing surface area to the smallest possible size
Molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface forming a surface "film” or “skin”.
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SURFACE TENSION IN MOLECULES The greater the strength of the intermolecular
forces between molecules in a substance the greater the surface tension.
Hexane and Water Which will have the greatest surface tension?
Hexane is nonpolar and only has London dispersion forces whereas water is very polar and has hydrogen bonding as well as London Dispersion Forces Water has higher surface tension
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SURFACE TENSION AND TEMPERATURE The greater the temperature of the
substance, the lower the surface tension.The greater the temperature, the greater
the average kinetic energy of the molecules, the further apart they are from one another and the lower the force of attraction between them.
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BOILING POINTS AND INTERMOLECULAR FORCES Boiling
Change of a liquid to bubbles of vapor that appear throughout the liquid (not just from surface like evaporation)
Boiling points are determined by the strength of the intermolecular forces between moleculesThe greater the strength of the
intermolecular forces between molecules, the higher the boiling point.
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BOILING POINT TRENDS In Groups 15, 16, and
17 there is a more elevated boiling point than expected for compounds with N, O, and F directly bonded to a H due to the very strong hydrogen bonding that occurs. Much larger energy than
normal is required to overcome the attractive force between molecules to allow the liquid to escape into the gas phase.
Higher than expected B.P.
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BOILING POINTS ARE DETERMINED BY STRENGTH OF INTERMOLECULAR FORCES Example 1:
Hexane or Water
Which substance has the higher Boiling PointHexane has only London Dispersion Forces
whereas water has hydrogen bonding and London Dispersion forces
Due to strength of hydrogen bonding, water will have higher boiling point
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BOILING POINTS ARE DETERMINED BY STRENGTH OF INTERMOLECULAR FORCES
Example 2: Isopropyl alcohol vs. waterBoth polar with hydrogen bonding and
London dispersion forcesWhich has the higher boiling point?
Water has stronger intermolecular forces due to compact shape and very polar bonds
Isopropyl alcohol is still polar but due to carbons and hydrogen areas of the molecule that are more nonpolar in nature
Due to stronger intermolecular forces, water has the higher boiling point
Nonpolar areas
Polararea
Highly polar molecule
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Example 3: C8H18 (octane) vs. C4H10 (butane)Which will have a higher boiling point?
Both have only London Dispersion Forces but octane has stronger intermolecular forcesMore randomly moving electrons that
can form temporary dipoles in more places in the molecule
Therefore octane has a higher boiling point than butane
Octane Butane
BOILING POINTS ARE DETERMINED BY STRENGTH OF INTERMOLECULAR FORCES
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CAPILLARY ACTION Attraction of the surface of a liquid to the
surface of a solid Responsible for meniscus seen with water in a
graduated cylinder
Water molecules are attracted to the side of the graduated cylinder and these water molecules attract more water molecules so that water “travels” up the side
If tube is small enough in diameter (capillary tube), then water will rise against gravity due to attractive forces of water molecules
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CAPILLARY ACTION IS DETERMINED BY STRENGTH OF INTERMOLECULAR FORCES
The stronger the intermolecular forces the more capillary action will be seen and the larger the meniscus.Hexane – nonpolar – only weak London
Dispersion Forces will not show a meniscus in a graduated cylinder
Water - very polar – strong hydrogen bonds – there will be a pronounced meniscus in a graduated cylinder and will readily flow up a capillary tube against gravity