chemical bonds a dog’s tale about an element’s search for happiness (chemical stability)
TRANSCRIPT
CHEMICAL BONDSCHEMICAL BONDS
A Dog’s Tale about an A Dog’s Tale about an
Element’s Search for Element’s Search for HappinessHappiness
(chemical stability)(chemical stability)
CHEMICAL BONDSCHEMICAL BONDS
The forces that hold atoms together…The forces that hold atoms together…often driven by the Octet Rule and often driven by the Octet Rule and
stability.stability.
(Mutual electrical attraction between (Mutual electrical attraction between nuclei and valence electrons of nuclei and valence electrons of
different atoms that binds them.)different atoms that binds them.)
Chemical BondsChemical Bonds
Sometimes it helps to Sometimes it helps to think of bonds (which think of bonds (which
you can't see) in you can't see) in terms of familiar terms of familiar
things you can see. things you can see. This is called an This is called an
analogyanalogy. .
Let's use the natural Let's use the natural attraction of dogs to attraction of dogs to bones as an analogy bones as an analogy
to the attractions that to the attractions that cause chemical bonds.cause chemical bonds.
Chemical BondsChemical Bonds
The negatively charged electrons The negatively charged electrons determine how two or more determine how two or more
atoms will interact when they are atoms will interact when they are brought near each other. In a brought near each other. In a
sense, the atoms fight over the sense, the atoms fight over the available electrons in much the available electrons in much the same way two or more dogs will same way two or more dogs will
fight over bones. fight over bones. The The Dog Bone AnalogyDog Bone Analogy works works quite well for several types of quite well for several types of
atomic bonds. atomic bonds. DogsDogs are the are the atoms.atoms.
BonesBones are the are the electronselectrons..
Octet RuleAtoms tend to gain, lose, or share electrons in order to acquire a full
set of valence electrons.
8 (most of the time) 2 (1st energy level)or
Happy dogs!
IONIC BONDSIONIC BONDS
One big greedy thief dog!One big greedy thief dog!Ionic bonding can be best imagined as one Ionic bonding can be best imagined as one
big greedy dog stealing the other dog's big greedy dog stealing the other dog's bone. bone.
IONIC BONDSIONIC BONDSThe bone represents the electron that is up for grabs. The bone represents the electron that is up for grabs.
When the big dog gains an electron he becomes negatively When the big dog gains an electron he becomes negatively charged, and the little dog who lost the electron becomes charged, and the little dog who lost the electron becomes
positively charged. positively charged. These two ions (that's where the name ionic comes from) are These two ions (that's where the name ionic comes from) are
attracted very strongly to each other as a result of the opposite attracted very strongly to each other as a result of the opposite charges.charges.
IONIC BONDSIONIC BONDSChemical bonding that results from transfer of Chemical bonding that results from transfer of
electrons and electrical attraction between cations electrons and electrical attraction between cations and anions.and anions.
Form between metals Form between metals (on the left side of the (on the left side of the Periodic Table)Periodic Table) and non-metals and non-metals (on the right side of (on the right side of Periodic Table)Periodic Table)
Metal loses electron(s) and becomes positively Metal loses electron(s) and becomes positively charged…a cation.charged…a cation.
Non-metal gains electron(s) and becomes Non-metal gains electron(s) and becomes negatively charged…an anion.negatively charged…an anion.
The oppositely charged ions are held together The oppositely charged ions are held together because they are strongly attracted to each other.because they are strongly attracted to each other.
Determine difference in electronegativity Determine difference in electronegativity – See page 161See page 161– Difference of greater than 1.7 is often considered ionicDifference of greater than 1.7 is often considered ionic– Ionic character greater than 50%Ionic character greater than 50%
IONIC BOND?IONIC BOND?
Cs and F? Cs and F?
Page 161 electronegativity chartPage 161 electronegativity chart
F en = 4.0F en = 4.0
Cs en = 0.7Cs en = 0.7
----------------------------
3.3 is the electronegativity difference3.3 is the electronegativity difference A bond between Cs and F is ionic, greater than 1.7A bond between Cs and F is ionic, greater than 1.7
IONIC COMPOUNDSIONIC COMPOUNDS Expressed as “formula units” rather than Expressed as “formula units” rather than
molecules because they form networks of ions molecules because they form networks of ions attracted to each other…not independent neutral attracted to each other…not independent neutral units.units.
Characteristics:Characteristics:– Crystalline structureCrystalline structure– High melting pointsHigh melting points– BrittleBrittle– Dissolve in waterDissolve in water– This separates the ions by breaking the ionic This separates the ions by breaking the ionic
bonds…separated ions move freely making solution of bonds…separated ions move freely making solution of ionic compounds good conductors of electricity.ionic compounds good conductors of electricity.
– Also conductive when melted.Also conductive when melted.
Formula unit is NaCl, 1 sodium and 1 Formula unit is NaCl, 1 sodium and 1 choride, the simplest ratiochoride, the simplest ratioIonic compounds form orderly arrangements of the ions , Ionic compounds form orderly arrangements of the ions , called crystal lattices.called crystal lattices.
COVALENT BONDSThe dogs share the bones.
There are two types of sharing:
UNEQUAL EQUAL
COVALENT BONDSBonds formed when electrons are
shared
Covalent bonds form between non-metals(on the right side of the Periodic Table)
The electrons are shared by the atoms. Electrons may be shared equally or
unequally. Molecules are formed because atoms
sharing electrons must be near each other
COVALENT BONDS
Enormous variety of size, shape, physical, Enormous variety of size, shape, physical, and chemical properties.and chemical properties. often lower melting point, often lower melting point, not necessarily crystalline not necessarily crystalline do not conduct electricity, do not conduct electricity, may or may not dissolve in watermay or may not dissolve in water
POLAR COVALENT BONDSPOLAR COVALENT BONDS
Unevenly matched dogs that are Unevenly matched dogs that are willing to share.willing to share.
These bonds can be thought of as two or These bonds can be thought of as two or more dogs that have different desire for more dogs that have different desire for
bones. They share unequally.bones. They share unequally.
POLAR COVALENT BONDSPOLAR COVALENT BONDS
The bigger dog has more strength to possess a The bigger dog has more strength to possess a larger portion of the bones. Sharing still takes larger portion of the bones. Sharing still takes place but is an uneven sharing. place but is an uneven sharing.
POLAR COVALENT BONDSPOLAR COVALENT BONDS
In a covalent bond, the electrons are shared In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule for both.between atoms to fulfill the Octet Rule for both.
In a polar covalent bond, the electrons are In a polar covalent bond, the electrons are shared unequally. They are attracted more to the shared unequally. They are attracted more to the more electronegative element.more electronegative element.
The electrons spend more time near the more The electrons spend more time near the more electronegative element, making it seem more electronegative element, making it seem more negative and the other end of the molecule seem negative and the other end of the molecule seem more positive…or “polar”more positive…or “polar”
Determine difference in electronegativity Determine difference in electronegativity – See page 161See page 161– Difference of .3-1.7 is often considered polar covalentDifference of .3-1.7 is often considered polar covalent– Ionic character 5-50%Ionic character 5-50%
POLAR COVALENT BOND?POLAR COVALENT BOND?
Unequal sharing of electronsUnequal sharing of electrons
Partial negativechargePartial
positive charge
POLAR COVALENT BOND?POLAR COVALENT BOND?
H and S?H and S?
Page 161Page 161
S en = 2.5S en = 2.5
H en = 2.1H en = 2.1
----------------------------
0.4 is the electronegativity 0.4 is the electronegativity differencedifference
A bond between H and S is polar covalent, .3-1.7A bond between H and S is polar covalent, .3-1.7
Polar molecules and polar Polar molecules and polar bondsbonds
Partial positive charge towards less electronegative element
Partial negative charge towards element with greater electronegativity.
Two sides to the molecules/bonds
S
Polar bonds lead to polar Polar bonds lead to polar moleculesmolecules
Polamolecules
Calm, quiet, humble on the sideline Fearless, crazed, maniacal on the
field
Two sides to Troy Polamalu
Polar molecules and polar Polar molecules and polar bondsbonds
Partial positive charge towards less electronegative element
Partial negative charge towards element with greater electronegativity.
Two sides to the molecules/bonds
S
NON-POLAR COVALENT NON-POLAR COVALENT BONDSBONDS
Covalent Bonds: Dogs of equal strength Covalent Bonds: Dogs of equal strength share equally.share equally.
Covalent bonds can be thought of as two or Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. more dogs with equal attraction to the bones.
They share equally.They share equally.
NON-POLAR COVALENT NON-POLAR COVALENT BONDSBONDS
Since the dogs are identical, then the dogs share Since the dogs are identical, then the dogs share the bones evenly. Since one dog does not have the bones evenly. Since one dog does not have more of the bone than the other dog, the bone is more of the bone than the other dog, the bone is equally shared between both dogs. One dog does equally shared between both dogs. One dog does not have more than the other.not have more than the other.
NON-POLAR COVALENT NON-POLAR COVALENT BONDSBONDS
In a covalent bond, the electrons are In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule shared between atoms to fulfill the Octet Rule for both.for both.
In a non-polar covalent bond, the In a non-polar covalent bond, the electrons are shared equally.electrons are shared equally.
The charge is evenly distributed between The charge is evenly distributed between both atoms. The molecule is non-polar both atoms. The molecule is non-polar meaning one side does not have more charge meaning one side does not have more charge than the other. than the other.
Determine difference in electronegativity Determine difference in electronegativity – See page 161See page 161– Difference of less than .3 is often considered nonpolar covalentDifference of less than .3 is often considered nonpolar covalent– Ionic character less than 5 %Ionic character less than 5 %
NON-POLAR COVALENT NON-POLAR COVALENT BOND?BOND?
Cl and Br?Cl and Br?
Page 161 for enPage 161 for en
Cl en = 3.0Cl en = 3.0
Br en = 2.8Br en = 2.8
----------------------------
0.2 is the electronegativity difference0.2 is the electronegativity difference A bond between Cl and Br is non=polar covalent, A bond between Cl and Br is non=polar covalent,
less than 0.3less than 0.3
NON-POLAR COVALENT NON-POLAR COVALENT BOND?BOND?
Equal sharing of electronsEqual sharing of electrons
Equaldistribution of charge
Equaldistribution of charge
Diatomic ElementsDiatomic ElementsThey pair up!They pair up!
BrBr22
II22
NN22
ClCl22 HH22
OO22
FF22
The diabolical DIATOMIC BrINClHOF TWINS!
METALLIC BONDSMETALLIC BONDS
Mellow dogs with plenty of bones to go Mellow dogs with plenty of bones to go around. They don’t have to worry about around. They don’t have to worry about
sharing!sharing!
METALLIC BONDSMETALLIC BONDS
These bonds are best imagined as a room These bonds are best imagined as a room full of puppies who have plenty of bones to full of puppies who have plenty of bones to go around and are not possessive of any go around and are not possessive of any
one particular bone. This allows the one particular bone. This allows the electrons to move through the substance electrons to move through the substance with little restriction. The model is often with little restriction. The model is often
described as the "kernels of atoms in a sea described as the "kernels of atoms in a sea of electrons.”of electrons.”
METALLIC BONDsMETALLIC BONDs
Electrons are “delocalized”Electrons are “delocalized” Electrons move freely around and Electrons move freely around and
between atoms in the network of between atoms in the network of empty orbitals. The electrons move empty orbitals. The electrons move through the substance with little through the substance with little restriction. The model is often restriction. The model is often described as the "kernels of atoms in described as the "kernels of atoms in a sea of electrons.”a sea of electrons.”
METALLIC BONDSMETALLIC BONDS
The free movement of electrons The free movement of electrons explains the properties of metals…explains the properties of metals… Conductivity: free movement of Conductivity: free movement of
electrons electrons Malleability & ductility: bonding is the Malleability & ductility: bonding is the
same in all directionssame in all directions Luster: energy released as electrons Luster: energy released as electrons
move easily between orbitalsmove easily between orbitals
We will look more closely at the different types of bonds
to understand how millions of different compounds form
from only about 100 different elements!
Lewis Dot of Ionic Lewis Dot of Ionic Compounds (electrons Compounds (electrons
taken, not shared)taken, not shared) Write the dot structures of the neutral Write the dot structures of the neutral
atomsatoms Write the ions created by the loss or Write the ions created by the loss or
gain of electrons, in brackets. The gain of electrons, in brackets. The metals have no dots. The non-metals metals have no dots. The non-metals have a full outer shell. have a full outer shell.
Show the ion charges as superscripts.Show the ion charges as superscripts. Show the ratio of ions needed to create Show the ratio of ions needed to create
a net charge of zero.a net charge of zero.
Criss-Cross Method to Criss-Cross Method to Determine Ratio of IonsDetermine Ratio of Ions
Write the ion symbols and charges Write the ion symbols and charges (superscripts). (superscripts). Get this info from periodic Get this info from periodic table for elements, polyatomic ion list for table for elements, polyatomic ion list for polyatomic ions.polyatomic ions.
Criss-cross the charge (numbers Criss-cross the charge (numbers only) to subscripts. only) to subscripts. This shows the ratio of This shows the ratio of ions required for a net charge of zero.ions required for a net charge of zero.
Simplify.Simplify. Mg Mg2+2+ N N3-3-
MgMg33NN22
Charges of ions from PTCharges of ions from PTgrou
p1 2 13 14 15 16 17 18
ve 1 2 3 4 5 6 7 2 or 8
charge
1+ 2+ 3+ 4+/- 3- 2- 1- 0
Why does it work?Why does it work?
The goal is a net charge of 0. The The goal is a net charge of 0. The charges, or oxidation numbers, must charges, or oxidation numbers, must
add up to zero.add up to zero.
MgMg33NN22
3 x 23 x 2++ = 6= 6++
2 x 32 x 3--= 6= 6--
0 net charge0 net charge
Lewis Dot Structures of Lewis Dot Structures of Covalent Bonds(shared Covalent Bonds(shared
electrons)electrons) Lewis dot structures show atoms near each Lewis dot structures show atoms near each
other sharing pairs of electrons to create other sharing pairs of electrons to create bonds and satisfy octet rule for both atoms.bonds and satisfy octet rule for both atoms.
They may share up to 3 pairs of electronsThey may share up to 3 pairs of electrons– 1 SHARED PAIR :SINGLE BOND1 SHARED PAIR :SINGLE BOND– 2 SHARED PAIRS ::DOUBLE BOND2 SHARED PAIRS ::DOUBLE BOND– 3SHARED PAIRS :::TRIPLE BOND3SHARED PAIRS :::TRIPLE BOND
It does not matter which atom the electrons It does not matter which atom the electrons come from because they are shared.come from because they are shared.
Covalent bonds Covalent bonds usually usually follow the octet follow the octet rulerule
Put all of the valence electrons in a Put all of the valence electrons in a pot and distribute them between the pot and distribute them between the atoms so each atom has a full octet atoms so each atom has a full octet by sharing up to 3 pairs of electrons.by sharing up to 3 pairs of electrons.
: single bond
:: double bond
:: triple bond