Transcript excited state

Mr. Shields
Regents Chemistry
U06 L02
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Development of the Bohr Model
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We saw that Bohr
Was able to equate orbits
With Energy levels
And secondly he could
Then equate energy
Levels with specific
Wavelengths of light.
So how did he arrive at this model?
To see how his maodel came about we need to
step back a little bit in time and look at
some significant Contributions made by other
contemporary scientists
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The Quantum
1900 – Max Plank Studied the light
(electromagnetic Radiation) emitted
by Heated Objects.
He concluded that matter can only
Gain or lose energy in small specific
multiples. He called the minimum
Energy that matter can gain or lose the
“QUANTUM”
Max Plank (1858 – 1947)
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Quantum
Furthermore, the energy of a quantum
Is equal to the frequency of the emitted
Radiation times a constant.
Equantum = h x frequency
h = plank’s constant
So matter can only emit radiation in
multiples of 1(h x freq), or 2(h x freq) etc.
Max Plank (1858 – 1947)
Energies between these values DO NOT exist.
(Sound familiar? Yep … Bohr’s assumption about where
Electrons can reside i.e. quantized orbits)
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Photoelectric effect
Light directed at metal will release electrons from the
Surface. However, only light having a certain minimum
Frequency will do this
eV = unit of Energy
(electron volts)
e-
Recall that Frequency is also related to energy
Equantum = Plank’s Constant (h) x Frequency
This is known as the
PHOTOELECTRIC EFFECT
But if light has no mass how does it knock an electron out of the atom?
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Einstein & the Photoelectric effect
In 1905 Einstein explained how this happened.
He said Light could behave either
as a wave or a particle.
As a Particle light is called a PHOTON.
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Einstein & the Photoelectric effect
And, extending Plank’s idea:
Equantum = h x freq
Einstein equated the photon with the quantum, so:
Ephoton = h x freq
Therefore Ephoton = Equantum
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The Bohr model proposed electrons in specific energy
levels. These were designated n=1, 2, 3 etc.
Rule 1:
The number
Of electrons
Allowed in
Each Orbit is
2n2
2 electrons
8 electrons
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Bohr Electron Configurations
Rule 2: When electrons are added to atoms they are
placed in the lowest energy levels first.
- What value of “n” represents the lowest energy level?
Look at Neon on the Periodic Table.
- How many electrons it have? How do you know?
- Into what orbits can I put these electrons?
So, Neon’s “electron configuration” is designated 2-8
- Look at the electron configs in your
reference table
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Bohr Electron Configuration
Try these –
How many electrons does Mg have?
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In what orbits are these electrons located?
n= 1, 2, and 3
What is the electron configuration?
2-8-2
Now answer these same questions for Br …
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Bohr’s Model stated:
E5 E3 E1
E6 E4 E2
•Electrons could only exist
in specific orbits.
•Each orbit has a definite energy.
The orbit closest to the nucleus
has the lowest energy.
•When electrons absorb energy
they moves to higher orbits. The
Atom is said to be in a
“EXCITED STATE”
When all excited state electrons return to their original
energy levels the atom is said to be in the
“GROUND STATE”
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NIELS BOHR
Electrons can not reside
between these orbits.
To move from one orbit
to the next e- must
absorb or release specific
amounts of energy called
x
“quantums“
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Excited State
Ground State
Remember – a photon is electromagnetic energy that
Is behaving as a particle but has no mass and carries a
quantum of energy.
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NOTE
Since energy is quantized the energy lost in the transition
from 3 to 2 and then 2 to 1 is exactly equal to the energy
of transition from 3 to 1 or even 1 to 3.
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Hydrogen e- Transition Series
Paschen: IR
nx to n=3
Balmer: Visible light
nx to n=2
Lyman: UV
nx to n=1
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Hydrogen’s emission spectra
When excited electrons drop back to n=2 from
n= 6, 5, 4 and 3, photons of an energy equal to that
Transition are emitted producing hydrogen’s
Emission spectrum.
For example the E3 – E2 transition produces the red
Emission line at 656.2 nm
And the E4 – E2 transition produces the blue-green
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Emission line at 486.1 nm
Excited State Electron configuration
We saw previously that the electron configuration
Of Bromine is 2-8-18-7
This is know as the “ground state electron configuration”
Why?
n= 1 2 3
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What energy levels are they in
And how many e- are in each ?
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Excited State Electron configuration
We’ve said that an excited state of an atom is one in
which any number of electrons have moved to higher
Energy levels.
Even orbits not previously occupied by electrons!
Let’s look at some examples:
The ground state e- config of Bromine is 2-8-18-7
One possible excited state electron config might be
2-7-18-8
(from which orbit was the e- excited
and to what orbit did it go ?) 18
Excited State Electron configurations
Bromines electron config is 2-8-18-7
Can one possible excited state be 0-8-18-8-1 ?
Sure! All the electrons in any given level can be
excited to higher energy levels having 0 or more eCan one possible excited state for Br be 2-7-19-7?
NO! I can’t have more electrons than are allowed in
Any given level.
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Bohr model - Shortcomings
The Bohr model correctly explained the discontinuous lines
seen in the Emission spectra and even allowed one
To Calculate which lines you should get!
It also took care of the problems in Rutherford’s model
but…
It couldn’t explain the spectra for ANY OTHER ELEMENT!
To solve this problem required one more step to be made in
the development of the Atomic model –
The development of the Quantum Mechanical Concept
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