behavior of waves in-phase rays reinforce out-of phase rays annihilate each other rays out of phase...
TRANSCRIPT
Behavior of Waves
In-phase rays reinforce
Out-of phase rays annihilate each other
Rays out of phase by an exact number of wavelengths reinforce each other
Shapes of OrbitalsBohr atom assumed
spherical orbits
Wave mechanics make no such assumptions and result
in quantum theory.
Changes from a fixed orbit to a description based on
probability statistics
Energy Levels
Calculations produce a series of solutions to the equations giving a series of energy levels characterized by a series of number options
known as quantum numbers
Quantum numbers
Ist quantum number n=1,2,3………..n2nd quantum number l=0,1,2…………(n-1)3rd quantum number m= -l…….0…….+l4th quantum number s = ±½
Energy LevelsOriginal calculations were made for the hydrogen atom.
All theories of structure and bonding were based on that work
Electronic Structure
Electronic structure is determined by the atomic number
e.g. atomic number = 15 gives 1s2 2s2 2p6 3s2 3p3
Electronic StructureAll properties are determined, ultimately, by the electronic structure,
as it determines the bonding, which determines all properties
1s orbital
2s orbital
Electronic Structure
p orbitals
Electronic Structure
d orbitals
Removing an Electron
COSTS ENERGY
For electron to be removed to make
a compound energy must be obtained from
elsewhere
Adding an electron
Energy is released, but is lower than the energy needed to remove an electron from the other atom
Ionic Bonding
For ions to form and produce a stable structure extra energy must be released.
Energy comes from electrostatic attractions between the ions created.
Electrostatic interaction can be attractive or repulsive
Electronic interaction is always repulsive in nature.
Ionic bonding in terms of forces
Attractive force between anion and cation
Repulsive force between electron clouds of the two ions
Balance occurs when the forces are equal, and determines the bond length
Ionic bonding in terms of energy
Electrostatic attraction produces a lower energy
Electronic repulsion produces a higher energy
Minimum in energy corresponds to stable state and determines the bond length
Equations for ionic bonding
The equation for the electrostatic interaction is simply Coulomb’s equation from electrostatics
Where A is given by
Energy and force approaches are linked by this well-known equation
The repulsive energy equation comes from quantum mechanics
Covalent bondingHybridization of orbitals to new symmetrical states of equal energy
sp3 found in saturated carbon compounds
Sp3 Bonding
Ammonia Methane
Covalent bonding
Hybridization of orbitals to new symmetrical states of equal energysp2 found in unsaturated carbon compounds with double bonds
Sp2 Bonding in Ethylene
Sp2 hybridization produces three sp2 bonds in a triangle for each C-atom
2 C-atoms bond
C-H bonds follow
Each C-atom has one p-orbital unhybridized
They hybridize to produce two molecular orbitals: one filled (shown) and one unfilled
The unfilled orbital is known as an anti-bonding orbital
Covalent bonding
Hybridization of orbitals to new symmetrical states of equal energysp found in unsaturated carbon compounds with triple bonds
Sp bonding in acetyleneTwo sp hybrids are formed leaving 2p orbitals on each C-atom
The sp hybrids result in a C-C bond and two C-H bonds
The four p-orbitals hybridize to four new molecular orbitals, two of which are filled. The two unfilled MOs are known as anti-bonding orbitals
Bonding in benzene
Sp2 hybrids, as in ethylene
Bonding gives a planar ring
Each C-atom has one p-orbital left
The six p-orbitals hybridize to produce six molecular orbital (one shown)
The first three are filled; the other three are unfilled
What really happens in many atom systems?As atoms approach they affect each other’s electronic states.
There are as many states for each type of orbital as there are atoms.
Each orbital type becomes a band
Closer-in states require closer approach for band to form
What determines the band structure of an element?The bond length is determined the atomic size
The atomic size determines the band structure
Only the outermost energy levels become bands
Determines the electronic behavior.
Will a material be
a conductor, or
an insulator, or
a semiconductor?
Band structures
(a) (a) and (b) are conductors
(c) is an insulator
(d) is a intrinsic semiconductor
Conductors
Electrons acquire thermal energy and jump into unpaired states
When raise temperature, atomic or lattice vibrations interfere with motion of electrons. Conductivity decreases with increasing temperature.
Insulators and Intrinsic Semiconductors
Electron cannot acquire enough energy to jump in insulator
Electron can occasionally find enough energy to jump in semiconductor
Increasing temperature allows more electrons to jump; conductivity increases with increasing temperature
Mechanism of Conductivity in a Semiconductor
Concept of electrons and holes as independent species
Extrinsic Semiconductors
N-type
e.g. P atom has extra electron
Without an impurity atom, silicon is an insulator at normal
temperatures
Extrinsic Semiconductors
(2) P-typee.g. B atom has one fewer electron