Molecular Orbital Theory
>> Parts of this equation/concept include:
A - Molecular Orbital Diagrams
>> Magnetism
>> Bond Order
Molecular orbital theory is a different way of looking at bonding. It combines atomic orbitals into molecular orbitals. This chapter only looks at the molecular orbitals of homonuclear (same-atom) diatomic (two-atom) molecules of main group elements. With these molecules only atomic orbitals of the same type combine. That is, s orbitals with s orbitals and the p orbitals with p orbitals of the same orientation.
The atomic orbitals can combine in two ways. If the orbitals overlap on the bonding axis (head on), it will form a bond. If the orbitals meet on either side of the bonding axis (side-to-side), it will form a bond. The s orbitals and the p orbitals oriented along the z-axis will form bonds. The other two p orbitals will form bonds. See Figure 6.16.
A. Molecular Orbital Diagrams
It is traditional to graph the energy of the molecular orbitals in comparison to the atomic orbitals. When the two atomic orbitals are combined, they create two molecular orbitals. One of those molecular orbitals will be lower in energy than the atomic orbitals (bonding orbital) One will be higher in energy than the atomic orbitals (antibonding orbital). For all of the main group, homonuclear diatomic molecules, the molecular orbital diagram is show in Figure 6.19.
The number of valence electrons is determined as usual. The electrons will go into the lowest available orbital. If there are two degenerate orbitals, the electrons will go into each orbital before any electrons pair up.
>> Magnetism
If there are unpaired electrons in the molecular orbital diagram, the molecule is paramagnetic. If all electrons are paired, the molecule is diamagnetic.
>> Bond Order
The molecular orbital diagram can also be used to calculate bond order. (This is similar in idea to single, double, or triple bonds.)
Bond order = (bonding electrons – antibonding electrons)/2
Molecules with a higher bond order have shorter, stronger bonds and are more stable than molecules with a lower bond order.
>> Example 1
What is the molecular orbital diagram for O2? Is it paramagnetic or diamagnetic? What is its bond order?
Solution:
Oxygen has six valence electrons. Therefore O2 has 12 valence electrons. This is sufficient electrons to fill the s (2 e), s*( 2 e), z (2 e), both (4 e) and one in each * (2 e) orbital.
The unpaired electrons in the * orbitals make the molecule paramagnetic. Bond order = (8 – 4)/2 = 2
>> Example 2
What is the molecular orbital diagram for C2+? Is it paramagnetic or diamagnetic? What is its bond order?
Solution:
Carbon has four valence electrons. The C2+ molecule has 4 + 4 – 1 = 7 electrons. This is sufficient to fill the s (2 e), s* (2 e), z (2 e) and one electron in one orbital.
The unpaired electron in the orbital makes the molecule paramagnetic. Bond order = (5 – 2)/2 = 1.5
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gas
Anoun
1 accelerator, accelerator pedal, gas pedal, gas, throttle, gun
a pedal that controls the throttle valve; "he stepped on the gas"
Category Tree:
entity
╚object; physical object
╚artifact; artefact
╚instrumentality; instrumentation
╚implement
╚bar
╚lever
╚pedal; treadle; foot pedal; foot lever
╚accelerator, accelerator pedal, gas pedal, gas, throttle, gun
2 flatulence, flatulency, gas
a state of excessive gas in the alimentary canal
Category Tree:
state
╚physiological state; physiological condition
╚flatulence, flatulency, gas
3 gas
the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency
Category Tree:
phenomenon
╚natural phenomenon
╚chemical phenomenon
╚state of matter; state
╚gas
4 gasoline, gasolene, gas, petrol
a volatile flammable mixture of hydrocarbons (hexane and heptane and octane etc.) derived from petroleum; used mainly as a fuel in internal-combustion engines
Category Tree:
entity
╚substance; matter
╚fuel
╚gasoline, gasolene, gas, petrol
╚unleaded gasoline
╚napalm
╚leaded gasoline
5 gas
a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely
Category Tree:
entity
╚substance; matter
╚fluid
╚gas
╚arsine
╚firedamp
╚afterdamp
╚propane
╚butane
╚oxygen; O; atomic number 8
╚nitrogen; N; atomic number 7
╚hydrogen; H; atomic number 1
╚fluorine; F; atomic number 9
╚chlorine; Cl; atomic number 17
╚noble gas; inert gas; argonon
╚atmosphere
6 natural gas, gas
a fossil fuel in the gaseous state; used for cooking and heating homes
Category Tree:
entity
╚substance; matter
╚fuel
╚fossil fuel
╚natural gas, gas
Bverb
1 boast, tout, swash, shoot a line, brag, gas, blow, bluster, vaunt, gasconade
show off
Category Tree:
act; move
╚interact
╚communicate; intercommunicate
╚inform
╚misinform; mislead
╚overstate; exaggerate; overdraw; hyperbolize; hyerbolise; magnify; amplify
╚boast, tout, swash, shoot a line, brag, gas, blow, bluster, vaunt, gasconade
╚gloat; triumph; crow
╚puff
2 gas
attack with gas; subject to gas fumes; "The despot gassed the rebellious tribes"
Category Tree:
fight; struggle
╚attack; assail
╚gas
╚teargas
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garter belt
garter snake
garter stitch
Gartner's bacillus
Garuda
Garullus garullus
Gary
Gary Cooper
Gary Kasparov
Gary Weinstein
gas
g****g
gas bomb
gas bracket
gas burner
gas chamber
Gascogne
gas company
gasconade
gas constant
Gascony
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Answered By: zaheer_qadri - 3/29/2006 |