Transcript Quantum Cloud Model

Quantum Cloud Model


Why did Bohr propose the orbit or
planetary model?
He based it on experiments with hydrogen
and the line emission spectra that it
produces
Another View
Just the Line Spectrum


Originally only the Balmer Series were
noted as they were within the visible
portion of the electromagnetic
spectrum
Using other detection instruments the
following two series were found:


Lyman Series in the ultraviolet range
Paschen Series in the infrared range
Interpretation…………..




Adding energy to an atom will excite
electrons causing them to jump to a higher
energy level
The original location of the electron is
referred to as the Ground State
The higher level is called the Excited State
When the electron returns to the ground
state it gives off energy equal to the
difference between the two levels (excited
and ground state) as electromagnetic
radiation with its specific wavelength and
frequency




Bohr was able to calculate the radius
from each wavelength detected on the
line emission spectra
Bohr correlated these radii to the
angular momentum of electrons using a
formula that included an “n” term
which was a whole number integer
Assigned the letter K to the first
orbit; L to the second; M to the
third, etc.
Today we call them Energy Levels
Everything Isn’t OK in Bohr Land






The math didn’t work for all elements
Other scientists take up the quest:
De Broglie, French, 1924
Heisenberg, German, 1927
Schrödinger, Austrian, 1926
Led to the Quantum Theory being
applied to electron location





Important contributionsdeBroglie – wavelike properties of
electrons
Heisenberg Uncertainty Principle:
“it is impossible to determine
simultaneously both the position and
velocity of an electron or any other
particle”
Think of a spinning fan, ou know that
there are blades but its position at
any moment isn’t known
 Schrödinger-
formulated the
equations for quantum
mechanics
 Also known as Wave Mechanics
 Employs upper level math
beyond the scope of this
course and introductory
college course
Quantum Mechanics
 “highly
mathematical theories
of atomic structure based on
the belief that energy is
absorbed and radiated in
definite units (quanta)”

Concludes that the energy of an
electron is restricted to certain
definite values
Quantum Numbers



Values used to express the energy of
an electron which describe its
position, shape, magnetic moment, and
spin
Each electron has a unique set of four
quantum numbers
Also tells you the properties of the
atomic orbitals
Principal Quantum
Number





The energy level in which the electron
resides
Symbol is n
Allowed values are 1, 2, 3…∞
The number of electrons that can
reside in an energy level is 2n2
Also tells you how many sublevels or
orbitals found in an energy level- n2
Angular Momentum
Quantum Number







Refers to the shape of the orbital
Symbol is l (like “L” but lower case)
Values it can assume are 0 to n-1
If l = 0 it is a “s orbital”
If l = 1 it is a “p orbital”
If l = 2 it is a “d orbital”
If l = 3 it is a “f orbital”
Magnetic Quantum
Number





Indicates the orientation of an orbital
around the nucleus
Symbolized by m
Allowed values –l...0...+l
For example, p orbitals have an l
value of 1; therefore, -1...0...+1
-1 = px ; 0 = py ; +1 = pz
Spin Quantum Number





Originally not included but was added
to account for the magnetic
properties of the electron
Electrons spin like a top or the Earth
Symbol is s
Only two possible spin states
+1/2 or -1/2
So what do we do with them?



Given the sequence of four numbers
you can tell where an electron is
located
Example: 2,1,-1,+1/2
Tells you the electron is located in
the second energy level in a p orbital;
the orbital is on the x-axis (px), and
has a +1/2 spin
And more…

Can determine the sequence of
electrons in an atom by following
three rules:
 AUFBAU
Principle
 PAULI EXCLUSION Principle
 HUND’S RULE


AUFBAU Principle – an electron
occupies the lowest energy orbital
that can receive it
PAULI EXCLUSION Principle – no two
electrons in the same atom can have
the same set of four quantum
numbers (for electrons to occupy the
same orbital they must have opposite
spin)




HUND’S RULE – orbitals of equal
energy are occupied by one electron
before any orbital is occupied by a
second electron; all electrons in singly
occupied orbitals must have the same
spin state
We will use two notations for electron
sequencing:
Electron Configuration
Orbital Notation