1 chapter 8 covalent bonding 2 i. octet rule l what is the octet rule? l the octet rule states that...
TRANSCRIPT
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Chapter 8
Covalent bonding
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I. Octet Rule What is the Octet Rule?
The octet rule states that atoms lose gain or share electrons in to acquire a full set of 8 valence electrons
Create a drawing of Mg and Cl, Al and Cl
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II. Covalent BondsA Chemical Bond occurs when electron
are shared.
Hydrogen and Hydrogen
A molecules are formed from the overlap of orbitals and sharing of electrons
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Covalent bondsa. Nonmetals hold onto their valence
electrons.
b. They can’t give away electrons to bond.
Still want noble gas configuration.
c. Get it by sharing valence electrons with each other.
d. By sharing both atoms get to count the electrons toward noble gas configuration.
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Its all in the distancea. Too far no bond
b. Too close electrons repel
c. Just right and a molecule is born
A molecules are formed from the overlap of orbitals and sharing of electrons
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Covalent clip
Covalent_Bonds.asf
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III. Molecule A. a covalently bonded compound.
– 1. Tend to occur between non metals that are close together on the periodic table.
– a. diatomic molecules – occur naturally in nature
a. this is a more stable arrangement.
H H F F Br Br Cl Cl N N
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I. Single Covalent BondA. A sharing of two valence electrons.
1.Only nonmetals and Hydrogen.
2.Different from an ionic bond because they actually form molecules.
3. Two specific atoms are joined.
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How does H2 form? The nuclei repel
++
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When Atoms Combine to make Molecules
Fig 8-1
Atoms contain both positive and negative charges. When they come Together they arrange themselves so that the attractive forges of oppositeCharges is greater than the repulsive forces of like charges
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How does H2 form?
++
The nuclei repel But they are attracted to electrons They share the electrons
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How to show how they formed It’s like a jigsaw puzzle. I have to tell you what the final formula
is. You put the pieces together to end up
with the right formula. For example- show how water is formed
with covalent bonds.
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Water
H
O
Each hydrogen has 1 valence electron
Each hydrogen wants 1 more
The oxygen has 6 valence electrons
The oxygen wants 2 more
They share to make each other happy
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Water Put the pieces together The first hydrogen is happy The oxygen still wants one more
H O
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Water The second hydrogen attaches Every atom has full energy levels
H OH
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use acetate sheets to work on page 244 1-5 to create lewis structures
Use a square of acetate to show each atom (use vis a vis pens only)
Show the overlap of orbitals Draw the outcome in your notebook-use
structural formula (line to represent pair) Circle the shared pairs
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Lewis structure 1. PH3
2. H2S
3. HCl
4. CCl4
5. SiH4
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Lewis structure Use molecular model kit to build 1. PH3
2. H2S
3. HCl
4. CCl4
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Covalent Bond Formation
Covalent bond forms by overlap of orbitals. Two types of bonds
Sigma bond: all single bonds are sigma bonds
Pi bond: in multiple bonds: the first one is sigma, all other bonds are pi.
There areSingle bondsMultiple bonds (double and triple only)
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Two types of Bonds Sigma bonds from
overlap of orbitals along the axis connecting the nuclei between the atoms
Pi bond ( bond): perpendicular overlap of p-orbitals above and below the axis connecting the atoms
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Sigma bond: s-s Orbital Overlap
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Pg 247- # 12 a-e
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III. Multiple BondsA. Sometimes atoms share more than
one pair of valence electrons.
B. A double bond is when atoms share two pair (4) of electrons.
C. A triple bond is when atoms share three pair (6) of electrons.
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Carbon dioxide CO2 - Carbon is central
atom ( I have to tell you)
Carbon has 4 valence electrons
Wants 4 more Oxygen has 6 valence
electrons Wants 2 more
O
C
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Carbon dioxide Attaching 1 oxygen leaves the oxygen 1
short and the carbon 3 short
OC
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Carbon dioxide Attaching the second oxygen leaves
both oxygen 1 short and the carbon 2 short
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more
OCO
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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the
bond
OCO
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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the
bond
OCO8 valence electrons
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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the
bond
OCO8 valence electrons
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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the
bond
OCO
8 valence electrons
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How to draw them Add up all the valence electrons. Count up the total number of electrons
to make all atoms complete octet rule. Subtract. Divide by 2 Tells you how many bonds - draw them. Fill in the rest of the valence electrons
to fill atoms up.
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Examples NH3
N - has 5 valence electrons wants 8
H - has 1 valence electrons wants 2
NH3 has 5+3(1) = 8
NH3 wants 8+3(2) = 14
(14-8)/2= 3 bonds 4 atoms with 3 bonds
N
H
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N HHH
Examples Draw in the bonds All 8 electrons are accounted for Everything is full
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Examples HCN C is central atom N - has 5 valence electrons wants 8 C - has 4 valence electrons wants 8 H - has 1 valence electrons wants 2 HCN has 5+4+1 = 10
HCN wants 8+8+2 = 18
(18-10)/2= 4 bonds 3 atoms with 4 bonds -will require multiple
bonds - not to H
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HCN Put in single bonds Need 2 more bonds Must go between C and N
NH C
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HCN Put in single bonds Need 2 more bonds Must go between C and N Uses 8 electrons - 2 more to add
NH C
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HCN Put in single bonds Need 2 more bonds Must go between C and N Uses 8 electrons - 2 more to add Must go on N to fill octet
NH C
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Another way of indicating bonds
Often use a line to indicate a bond Called a structural formula Each line is 2 valence electrons
H HO =H HO
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Structural Examples
H C N
C OH
H
C has 8 electrons because each line is 2 electrons
Ditto for N
Ditto for C here Ditto for O
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Build the molecules using the molecular model kit.
HCN H2O NH3
CH4
C2H2
PH3
H2S http://www.youtube.com/watch?v=NYFE5usl
aNo
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http://www.youtube.com/watch?v=NYFE5uslaNo
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IV. Bond strengthA. bond strength is the energy needed to
break a covalent bond1. Bond length and atom size help
determine bond strengtha. The shorter the bond length The greater the bond strength
i. single bond – longest length ii. Double bond – medium lengthiii triple bond – shortest length
WHICH HAS THE GREATEST BOND STRENGTH?
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B. Energy is needed to create and break a covalent bond.
1.Energy is released when a covalent bond forms
2. Bond dissociation energy- is needed to break a bond
a. always a positive number
i. It takes 159KJ/mol to break F2
Would it take more or less to break N2 - why?
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V. Chemical reaction energyA. Endothermic reaction – energy is
needed1. More energy is needed to break the bond
than is needed to create new bond.
AB + energy A + B
NH4SCN + Ba(OH) + energy ( freezes to wood because heat is pulled from the reaction
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B. Exothermic reaction – energy is released
1. more energy is released when bonds form than is needed to break bonds
A + B AB + energy
CaCl + baking soda and water energy is released as heat
Hw: pg 247 1-12
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I. Naming compoundsA. Two types
1. Ionic - metal and non metal or polyatomics.
2. Covalent- we will just learn the rules for 2 non-metals.
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Covalent compounds Two words, with prefixes. Prefixes tell you how many. mono, di, tri, tetra, penta, hexa, Hepta,
octa, nona, deca First element whole name with the
appropriate prefix, except mono. Second element, -ide ending with
appropriate prefix. Practice
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Writing Formulas Two sets of rules, ionic and covalent To decide which to use, decide what the
first word is. If is a metal or polyatomic use ionic. If it is a non-metal use covalent.
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PREFIXESMono - one di - twoTri- threeTetra- four penta- five hexa- six Hepta- sevenOcta - eight nona - nine deca - ten
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CO2
CO CCl4
N2O4
XeF6
N4O4
P2O10
Naming Covalent Compounds
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Covalent compounds The name tells you how to write the
formula
Sulfur dioxide diflourine monoxide nitrogen trichloride diphosphorus pentoxide
Work on naming ditto
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I. Acids Substances that produce H+ ions when
dissolved in water. All acids begin with H. Two types of acids: Oxyacids Non Oxyacids- Binary Acids
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A. Binary Acids 1. Binary Acids( Hydrogen and one
other element
– A. hydro + element name + IC + acid
– Examples
– 1. HCl-
– 2. HF-
3. HBr-
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B. Oxyacids 1. oxyacids- (hydrogen + oxyanions)
– a. name anion + (ic or ous) + acid• i. use of ic or ous depends on the number of oxygen
atoms in the oxyanion
Examples:HNO3- nitric acid HNO2 –Nitrous acid
H3 PO4-phosphoric acid
H2PO3 phosphorous acid
HC2H3O2 –acetic acid
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H2SO4
sulfuric acid
H2SO3
sulfurous acid
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Formulas for acids hydrofluoric acid- HF
carbonic acid- H2CO3
hydrosulfuric acid
phosphorous acid
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Hydrates Some salts trap water crystals when
they form crystals. These are hydrates. Both the name and the formula needs to
indicate how many water molecules are trapped.
In the name we add the word hydrate with a prefix that tells us how many water molecules.
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Hydrates In the formula you put a dot and then
write the number of molecules. Calcium chloride dihydrate =
CaCl22 Chromium (III) nitrate hexahydrate =
Cr(NO3)3 6H2O
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I. ResonanceA. When more than one dot diagram with
the same connections are possible.
1. SO2
Which one is it? Does it go back and forth. It is a mixture of both, like a mule.
7020 Oct 97 Bonding and structure (2) 70
Sulfur Dioxide, SO2
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
These equivalent structuresare called:
RESONANCE STRUCTURES. The proper Lewis structure
is a HYBRID of the two.
Each atom has OCTET . . . . . BUT there is a +1 and -1 formal charge
••O OS
••
••
••
••••
••O OS••
••
••
••
••
+— —+
Rules 1-3 O—S —O
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Draw the resonance structure
O3
SO3
SO2
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Coordinate Covalent Bond When one atom donates both electrons
in a covalent bond. Carbon monoxide CO
OC
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Coordinate Covalent Bond When one atom donates both electrons
in a covalent bond. Carbon monoxide CO
OC
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Coordinate Covalent Bond When one atom donates both electrons
in a covalent bond. Carbon monoxide CO
OC
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How do we know Have to draw the diagram and see what
happens. Often happens with polyatomic ions and
acids.
Work on drawing lewis structures from packet
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