objectives define chemical bond. explain why most atoms form chemical bonds. describe ionic and...
TRANSCRIPT
ObjectivesDefine chemical bond.
Explain why most atoms form chemical bonds.
Describe ionic and covalent bonding.
Explain why most chemical bonding is neither purely ionic nor purely covalent.
Classify bonding type according to electronegativity differences.
Introduction to Chemical BondingChapter 6
What is bonding?
• Bonding is the “glue” that hold two or more elements together
• This “glue” is most likely formed as a result of a chemical reaction
• Bonding and molecular structure play a central role in determining the course of chemical reactions
What is a bond?
• A bond can be thought of as a force that holds groups of two or more atoms together and makes them function as a unit
• Example : water
OH H
Bonds require energy to break and release energy when made
Bond Energy is the energy required to break chemical bonds-is an endothermic process-requires energy-endothermic process-bond energy values are postive
Bond Formation-is a energy releasing process-exothermic process-the values for bond formation are negative
Ionic Bonds
• Na and Cl– Na is a metal and likes to lose one electron– Cl is a nonmetal and likes to gain one electron– the final ionic compounds is NaCl
Na+ Cl-+ NaClThe electrostatic interaction keeps them together!
Ionic Bonds
• Na looses an electron and chlorine gains it!
• They do this to achieve an octet!
Na Cl
Covalent Bonds
• Covalent Bonds–exist between nonmetals bonded
together–form when atoms of nonmetals
share electrons–electrons can be shared equally or
unequally
Metallic Bonds
• Metallic bonds exist between metals• Occur when two metals, usually the same
metal, are bonded together
The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.
Electronegativity Difference
• If the difference in electronegativities is between:– 1.7 to 4.0: Ionic– 0.3 to 1.7: Polar Covalent– 0.0 to 0.3: Non-Polar Covalent
Example: NaClNa = 0.8, Cl = 3.0Difference is 2.2, sothis is an ionic bond!
Bonds Between Atoms
Covalent
Ionic
Polyatomic Ions
Metallic
Molecular Substance Polar
Nonpolar
Network Solids
What are we going to learn about???
Marriage
Divorce
Forming of a bond is like marriage
• More stable• exothermic
The breaking of a bond relates to a divorce.
• Less stable• Endothermic
PROPERTIES OF IONIC COMPOUNDS
Ionic compound – is composed of positive and negative ions that are equal in charge.
Formula Unit – is the simplest collection of atoms from which an ionic compound’s formula can be established.
Properties of Ionic Bonds
• What is an Ionic Bond?
- An Ionic Bond is a chemical bond resulting from the TRANSFER of electrons from one bonding atom to another
• When is an ionic bond formed?
- An ionic bond is formed when a cation (positive ion) transfers electrons to an anion (negative ion).
What are some characteristics of an ionic bond?
1. Exist as crystalline units at room temperatures
2. Brittle3. Have higher melting
points and boiling points compared to covalent compounds . Melting points are 1000 ˚C
4. Conduct electrical current in molten or solution state but not in the solid state
5. The smallest piece of an ionic compound is a formula unit.
What are some characteristics of an ionic bond?
6. A formula unit is the smallest collection of atoms from which a formula can be established.
7. Usually dissolves in polar solvents such as water. This is called dissociation.
8. The best way to test for an ionic compound is electrical conductivity in solution.
Formula Unitsof salt
Lattice energy- is the energy released when one mole of an ionic crystalline compound is formed from gaseous ions.
Properties of Ionic Compounds
Lattice energy values are negative.
The more negative the value the strongerthe ionic bond.
Properties of Ionic Compounds
Compound Calculated Lattice Energy
NaCl −756 kJ/molLiF −1007 kJ/molCaCl2 −2170 kJ/mol
Covalent Bonds• What is an Covalent Bond?
- A covalent bond is a chemical bond resulting from SHARING of electrons between 2 bonding atoms.
• What forms a covalent bond?
- A covalent bond is formed between two nonmetals.
There are five different categories associated with covalent bonds. What are the 5 different categories?
Covalent
Molecular Substance Polar
NonpolarCoordinate Covalent
Network Solids
What are some characteristics of a covalent bond?
1. Very strong2. Low melting and boiling points Below 500 ˚C4. Exist as solids, liquids, or
gases at room temperature5. Do not conduct electricity
unless it is a molecular electrolyte (acid or base)
6. Molecular electrolytes undergo ionization in polar solvents acids and bases.
Covalent Bonds can have multiple bonds, so you should be familiar with the following…
Single Covalent Bond- chemical bond resulting from sharing of an electron pair between two atoms.
Double Covalent Bond- chemical bond resulting from sharing of two electron pairs between two atoms.
Triple Covalent Bond-chemical bond resulting from sharing of three electron pairs between two atoms.
Triple bonds are the strongest and shortest
c
First, we are going to look at Polar Covalent…
What is polar covalent?
-Polar covalent is a description of a bond that has an uneven distribution of charge due to an unequal sharing of bonding electrons.
The boy is not equally sharing with anyone else but rather taking all the food for himself.
Next, we are going to look at Non-Polar Covalent…
What is non-polar covalent?
-Non polar covalent is a covalent bond that has an even distribution of charge due to an equal sharing of bonding electrons.
This couple is non- polar because they are sharing the drink equally between them.
Now, we are going to look at Network Solids…
What is a Network Solid?
-A network solid is a solid that has covalently bonded atoms linked in one big network or one big macromolecule.
Name 3 Characteristics of a Network Solid.
1. Poor conductors of heat and electricity
2. Hard / Strong
3. High melting and boiling points
Diamond and graphite are examples of network covalent compounds.
Just as a summary to what each bond looks like…
The Metallic-Bond Model
The chemical bonding that results from the attraction
between metal atoms and the surrounding sea of electrons is called metallic bonding.
Properties of Metallic Bond
Metallic BondingWhat is a Metallic Bond?
- A metallic bond occurs in metals. A metal consists of positive ions surrounded by a “sea” of mobile electrons.
Name 4 Characteristics of a Metallic Bond.
1. Good conductors of heat and electricity
2. Great strength
3. Malleable and Ductile
4. LusterThis shows what a metallic bond might look like.
Malleability is the ability of a substance to be hammered or beaten into thin sheets.
Ductility is the ability of a substance to be drawn, pulled, or extruded through a small opening to produce a wire.
Metallic Bonding
Chemical bonding is different in metals than it is in ionic, molecular, or covalent-network compounds.
The unique characteristics of metallic bonding gives metals their characteristic properties, listed below.
electrical conductivity
thermal conductivity
malleability
ductility
shiny appearance
Metallic Bonding
Intermolecular Forces
• An attractive force that operates between molecules
• There are 3 kinds of intermolecular forces:– London dispersion force– Dipole-dipole force– Hydrogen-bonding force
London Dispersion Forces• Instantaneous dipole
– A temporary dipole formed when the electrons in an atom or nonpolar molecule happen to be more on one side in an instant in time, causing it to be more negative than normal and the opposite side positive
• Induced dipole– Positive end of the dipole exerts an attractive
force on nearby electrons, causing an adjacent atom to develop into another temporary dipole
London Dispersion Forces
– The attraction between temporary dipoles– Occurs between atoms and molecules– Only intermolecular force in nonpolar
substances– Tend to be stronger the larger the atom or
molecule (the more electrons are in the atom or molecule)
– Relatively weak forces
Dipole-Dipole Forces
• Attraction between polar molecules
• Occurs when the partially positive end of one molecule attracts the partially negative end of another molecule
• Generally stronger than London dispersion forces
Hydrogen Bonding
• Special type of dipole-dipole force
• Only occurs in molecules that contain hydrogen bonded to a small, highly electronegative element (N, O, F)
• Stronger than a regular dipole-dipole force
Type of Force
Type of Interaction Occurrence
London dispersion
force
A temporary dipole in one molecule induces
the formation of a temporary dipole in a
nearby molecule and is attracted to it.
All atoms and molecules
Dipole-Dipole Force
Polar molecules (permanent dipoles) attract one another
Polar molecules
Hydrogen-Bonding Force
Two dipoles, one containing hydrogen to
an electronegative element and the other
containing an electronegative element,
attract one another.
Polar molecules containing unpaired
molecules and a hydrogen bonded to nitrogen, oxygen, or
fluorine
linear 180o
BeCl2
valence e- = 2 + (2 x 7) = 16e-
Cl....
..BeCl....
..
fewer than 8e-
valence pairs on Be bonding e-
linear molecule
two
linear 180o
CO2
valence e- = 4 + (2 x 6) = 16e-
CO....
.. O....
.. CO..
O..
.. ..
valence pairs on C ignore double bondstwo
single and double bonds same
linearmolecular shape
molecular geometry linear
120o
SO2
valence e- = 6+ (2 x 6) = 18e-
valence pairs on Sthree
one lone pair
molecular shape bent
S O....
..O....
:
SO....
.. O....
..:
SO...... O
..
..
:
two bonding pairs
< 120o
109.5o
NH3
valence e- = 5+ (3 x 1) = 8e-
valence pairs on Nfour N HH
H
:
< 109.5o
molecular shape trigonal pyramid
one lone pair
three bonding pairs
tetrahedral 109.5o
CH4
valence e- = 4+ (4 x 1) = 8e-
valence pairs on CfourC HH
H
H109.5o
molecular geometry
molecular shape tetrahedral
tetrahedral
We have studied electron configuration notation and the sharing of electrons in the formation of covalent bonds.
Methane is a simple natural gas. Its molecule has a carbon atom at the center with four hydrogen atoms covalently bonded around it.
What Proof Exists for Hybridization?
Lets look at a molecule of methane, CH4.
What is the expected orbital notation of carbon in its ground state?
(Hint: How many unpaired electrons does this carbon atom have available for bonding?)
Can you see a problem with this?
Carbon ground state configuration
You should conclude that carbon only has TWO electrons available for bonding. That is not not enough!
How does carbon overcome this problem so that it may form four bonds?
Carbon’s Bonding Problem
The first thought that chemists had was that carbon promotes one of its 2s electrons…
…to the empty 2p orbital.
Carbon’s Empty Orbital
However, they quickly recognized a problem with such an arrangement…
Three of the carbon-hydrogen bonds would involve an electron pair in which the carbon electron was a 2p, matched with the lone 1s electron from a hydrogen atom.
A Problem Arises…
This would mean that three of the bonds in a methane molecule would be identical, because they would involve electron pairs of equal energy.
But what about the fourth bond…?
Unequal bond energy
The fourth bond is between a 2s electron from the carbon and the lone 1s hydrogen electron.
Such a bond would have slightly less energy than the other bonds in a methane molecule.
Unequal bond energy
This bond would be slightly different in character than the other three bonds in methane.
This difference would be measurable to a chemist by determining the bond length and bond energy.
But is this what they observe?
Unequal bond energy
The simple answer is, “No”.
Chemists have proposed an explanation – they call it Hybridization.
Hybridization is the combining of two or more orbitals of nearly equal energy within the same atom into orbitals of equal energy.
Measurements show that all four bonds in methane are equal. Thus, we need a new explanation for the bonding in methane.
Enter Hybridization: