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CHEMICAL
BONDING
Cocaine
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Chemical BondingProblems and questions How is a molecule or
polyatomic ion heldtogether?
Why are atoms distributed atstrange angles?
Why are molecules not flat?
Can we predict the structure?
How is structure related tochemical and physicalproperties?
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Review of Chemical Bonds
There are 3 forms of bonding:
_________complete transferof 1 or more electrons from oneatom to another (one loses, theother gains) forming oppositely
charged ions that attract oneanother
_________some valenceelectrons shared between
atoms _________ holds atoms of a
metal together
Most bonds are
somewhere inbetween ionic
and covalent.
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The type of bond can usually be calculated byfinding the difference in electronegativity of
the two atoms that are going together.
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5Electronegativity Difference
If the difference in electronegativitiesis between:
1.7 to 4.0: Ionic
0.3 to 1.7: Polar Covalent
0.0 to 0.3: Non-Polar Covalent
Example: NaCl
Na = 0.8, Cl = 3.0
Difference is 2.2, sothis is an ionic bond!
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Ionic Bonds
All those ionic compounds were madefrom ionic bonds. Weve beenthrough this in great detail already.Positive cations and the negativeanions are attracted to one another
(remember the Paula AbdulPrinciple of Chemistry: OppositesAttract!)
Therefore, ioniccompounds are usually
between metals andnonmetals (opposite endsof the periodic table).
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Electron
Distribution inMolecules
Electron distribution is
depicted withLewis
(electron dot)structures
This is how you
decide how manyatoms will bondcovalently!
(In ionic bonds, itwas decided withcharges)
G. N. Lewis
1875 - 1946
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Bond and Lone Pairs Valence electrons are distributed
as shared orBOND PAIRS andunshared orLONE PAIRS.
H Cllone pair (LP)
shared or
bond pair
This is called a LEWIS
structure.
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Bond Formation
A bond can result from anoverlapofatomic orbitals on neighboring atoms.
ClH H Cl
+
Overlap of H (1s) and Cl (2p)Note that each atom has a single,
unpaired electron.
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Review of Valence Electrons
Remember from the electron chapterthat valence electrons are theelectrons in the OUTERMOST energylevel thats why we did all thoseelectron configurations!
B is 1s2
2s2
2p1
; so the outer energylevel is 2, and there are 2+1 = 3electrons in level 2. These are thevalence electrons!
Br is [Ar] 4s2 3d10 4p5
How many valence electrons arepresent?
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11Review of Valence ElectronsNumber of valence electrons of a main (A)
group atom = Group number
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12Steps for Building a Dot StructureAmmonia, NH3
1. Decide on the central atom; never H. Why?If there is a choice, the central atom is atom oflowest affinity for electrons. (Most of the time, this is theleast electronegative atomin advanced chemistry we use athing called formal charge to determine the central atom. Butthats another story!)
Therefore, N is central on this one
2. Add up the number of valence electronsthat can be used.
H = 1 and N = 5
Total = (3 x 1) + 5
= 8 electrons / 4 pairs
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3. Form a single bondbetween the central atom andeach surrounding atom (eachbond takes 2 electrons!)
H H
H
N
Building a Dot Structure
H
H
H
N4. Remaining electrons formLONE PAIRS to complete the octet
as needed (or duet in the case of
H).3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8
electrons), while H shares 1 pair.
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5.Check to make sure there are 8electrons around each atomexcept H. H should only have 2electrons. This includes SHAREDpairs.
Building a Dot Structure
6. Also, check the number of electrons in your
drawing with the number of electrons from
step 2. If you have more electrons in the
drawing than in step 2, you must makedouble or triple bonds. If you have less
electrons in the drawing than in step 2, you
made a mistake!
H
H
H
N
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Carbon Dioxide, CO2
1. Central atom =
2. Valence electrons =
3. Form bonds.
4. Place lone pairs on outer atoms.This leaves 12 electrons (6 pair).
5. Check to see that all atoms have 8 electrons
around it except for H, which can have 2.
C 4 e-
O 6 e- X 2 Os = 12 e-
Total: 16 valence electrons
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Carbon Dioxide, CO2
6. There are too many electrons in our drawing. Wemust form DOUBLE BONDS between C and O.
Instead of sharing only 1 pair, a double bond shares 2
pairs. So one pair is taken away from each atom and
replaced with another bond.
C 4 e-O 6 e- X 2 Os = 12 e-
Total: 16 valence electrons
How many are in the drawing?
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17Double andeven triple
bonds arecommonlyobserved for C,N, P, O, and S
H2CO
SO3
C2F4
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Now You Try One!Draw Sulfur Dioxide, SO2
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Violations of the Octet Rule(Honors only)
Usually occurs with B and elements
of higher periods. Commonexceptions are: Be, B, P, S, and Xe.
BF3
SF4
Be: 4
B: 6
P: 8 OR 10
S: 8, 10, OR 12Xe: 8, 10, OR 12
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MOLECULARGEOMETRY
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VSEPR ValenceShell Electron Pair
Repulsion theory. Most important factor in
determining geometry isrelative repulsion between
electron pairs.
Molecule adoptsthe shape that
minimizes the
electron pair
repulsions.
MOLECULAR GEOMETRY
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Some Common Geometries
Linear
Trigonal Planar Tetrahedral
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23VSEPR charts
Use the Lewis structure to determine the
geometry of the molecule
Electron arrangement establishes the bondangles
Molecule takes the shape of that portion ofthe electron arrangement
Charts look at the CENTRAL atom for all data!
Think REGIONS OF ELECTRON DENSITYrather than bonds (for instance, a doublebond would only be 1 region)
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Other VSEPR charts
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Structure Determination by VSEPR
Water, H2OThe electron pair
geometry is
TETRAHEDRAL
The moleculargeometry is
BENT.
2 bond
pairs
2 lone
pairs
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Structure Determination byVSEPR
Ammonia, NH3
The electron pair geometry is tetrahedral.
H
H
H
lone pair of electrons
in tetrahedral positioN
TheMOLECULAR GEOMETRY thepositions of the atoms is TRIGONAL
PYRAMID.
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B nd P l it
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Bond PolarityHCl is POLAR because it
has a positive end and anegative end. (differencein electronegativity)
Cl has a greater share in
bonding electrons than
does H.
Cl has slight negative charge (-d) and H hasslight positive charge (+ d)
H Cl
+d -d
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Bond Polarity
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This is why oil and water will not mix! Oil
is nonpolar, and water is polar.
The two will repel each other, and so youcan not dissolve one in the other
Bond Polarity
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Bond Polarity
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Bond Polarity
Like Dissolves Like
Polar dissolves Polar
Nonpolar dissolvesNonpolar
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Diatomic Elements
These elements do not exist as a single atom;they always appear as pairs
When atoms turn into ions, this NO LONGERHAPPENS!
Hydrogen
Nitrogen
Oxygen
Fluorine
Chlorine
Bromine Iodine
Remember:
BrINClHOF
32Chemical Bonding
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33What is Chemical Bonding?
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gChemical Bonding is two or more atoms combining by sharing
electrons so that a new substance is produced that has different
physical and chemical properties than its component elements
Chemical bonding occurs when atoms share valence electrons
Atoms want to share electrons so that their valence shells are full
and they are chemically stable
-- for most elements, this means having 8 valenceelectrons
-- for hydrogen, helium, lithium, and beryllium (who can
have a full 1st energy level), this means having 2
valence electrons
34Lewis Structures
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Other than the Bohr Model, there is a much simpler way that we
can show the valence electrons of an atom. . .
This is known as either an electron-dot diagram or, more
scientifically, a Lewis Structure
When drawing a Lewis Structure, you place the atomic symbol at
the center and put dots around the symbol to indicate the valence
electrons.
-- DO NOT pair electrons until all four sides have at least
one electron
Examples:
C Na F
35The Octet Rule
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Atoms themselves can gain or lose electrons so that they have a
full valence shell of 8 (or 2) electrons. . .
-- when atoms gain or lose electrons, they are called ions
Depending on whether or not the atom gains or loses an electron, it
can be positively-charged (cation) or negatively-charged (anion)
This need to be stable and have 8 electrons is called the octet rule
In addition to forming ions, atoms can also share their valenceelectrons with other atoms, giving each atom 8 valence electrons
36Types of Chemical Bonding
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yp gThere are two ways in which atoms can share electrons to satisfy
the octet rule:
Ionic Bonding occurs when two or more ions combine to form anelectrically-neutral compound
-- the positive cation loses an electron (or 2 or 3)
-- the negative anion gains the electron (or 2 or 3)
-- the electrons ARE NOT shared between the two
atoms, as the anion steals the electrons from the cation
Covalent Bonding occurs when two or more atoms combine to
form an electrically-neutral compound
-- the electrons are shared between the two atoms
-- neither atom had a charge to begin with, and the
compound remains with zero charge
37Ionic BondingI i b di i th bi i f t i t f
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gIonic bonding is the combining of two or more ions to form an
electrically-neutral compound
How Ionic Bonding Works
1) The giving atom loses a valence electron (or 2 or 3) so that it has
a full valence shell, but a positive charge
2) The gaining atom gains a valence electron (or 2 or 3) so that it
has a full valence shell, but a negative charge
3) The negative and positively-charged ions are attracted to each
other (like a magnet) based on their opposite charge
If you look at your periodic table, you will see that ionic bondingusually occurs in compounds formed between one metal and
one nonmetal
38Metals in Ionic Bonds
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In ionic bonds, metals are always the cation
-- metals are on the left side of the periodic table and only
have 1, 2, or 3 valence electrons
-- it is very easy to lose these electrons and become
positively charged
39Nonmetals in Ionic Bonds
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Nonmetals (with the exception of hydrogen) are always the anion in
ionic bonds
-- nonmetals all have 4 or more valence electrons, so it
is very easy for them to gain electrons to get a full
valence shell
-- when nonmetals gain electrons, they get a negative
charge
40Drawing Ionic Bonds
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When a nonmetal is exposed to a metal, the metal will lose
electrons and the nonmetal will gain electrons, forming a new
compound
-- that new compound is held together by an ionic bond
The individual atoms will have a charge, but the overall compound
will be neutral
When we draw the compound, using Lewis Structures, we show theextra electron around the anion and the cation as having no valence
electrons
-- each ion in the compound is shown as being charged
Example:
Na + Cl Na Cl
41Diagram of Ionic Bonding
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42Polyatomic IonsTh i th t d f th t f
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There are some ions that are made up of more than one type of
atom, these are called polyatomic ions
For example, the polyatomic ion known as ammonium NH4
+ has 4
atoms of hydrogen and one atom of nitrogen, HOWEVER, the
whole ion has an overall charge of +1
-- you will be given the charges of any polyatomic ions
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44Naming Ionic CompoundsI i d ll h t d
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Ionic compounds all have two-word names
The first word in the name is the same as the name of the first ion
(for example, sodium, ammonium, potassium, etc)
The second word in the name is either:
1. If the second ion is polyatomic, it is just the name of
the polyatomic ion
2. If the second ion is an element, the end of theelements name changes to ide
Example: chlorine chloride
oxygen oxide
45Practice Naming Ionic Compounds
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Na2CO3 --
CaSO4 --
KBr --
MgS --
BeCl2 --
NH4F --
46Writing Ionic Compounds From NameIf I h i i d f l t i lf t h d
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If I have an ionic compound, for example, potassium sulfate, how do
I know what the formula is?
REMEMBER THAT THE COMPOUND MUST BE NEUTRALSo, I have Potassium, K, which will lose 1 electron and gain a
positive charge, making it K+
And, I have Sulfate, a polyatomic ion, which has a charge of -2
Therefore, I need 2 potassium ions to balance out my one sulfate
ion, making my formula, K2SO4
-- this is why we have subscripts in chemistry
47What About. . .Wh t if I h t t lti l l t i i i d f
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What if I have to put multiple polyatomic ions in a compound, for
example, magnesium nitrate?
Magnesium has a charge of +2Nitrate is polyatomic and has a charge of -1
Therefore, we need 2 nitrate ions for each magnesium
In this case, we put the entire polyatomic ion in parenthesis and
put the subscript outside the parenthesis
Our answer would be Mg(NO3)2
48Properties of Ionic CompoundsIonic compounds have the following properties:
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Ionic compounds have the following properties:
Crystal Pattern every ion is attracted to all other ions with the
opposite charge
-- this results in a repeating 3-dimensional crystal
pattern
49Properties of Ionic Compounds contd
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High Melting Point the attraction in the crystal pattern leads to
very strong bonds, making it hard to break apart ionic compounds
-- ionic compounds melt at high temperaturesConductivity when dissolved in water, ionic compounds
conduct electricity
-- in water, the bonds dissociate (fall apart), leaving lots
of ions to carry charge
Solid ionic compounds do not conduct electricity very well
-- melted ionic compounds do conduct electricity fairly
well