l-9 chemical bonding

7/29/2019 L-9 Chemical Bonding

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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

l-9 chemical bonding

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