Transcript Chapter 6: Electronic Structure of Atoms

Chapter 6: Electronic
Structure of Atoms
Pages 207-247
Brief Explanation of the Chapter
Explore Quantum Theory
– Nature of Light
– Arrangement of electrons in atoms
– Exploration of reactivity
– History
Wave Nature of Light
Knowledge of electrons come from light
How can we numerically define the speed
of light?
– 3.00 x 108 m/s
How do we explain the term periodic in
reference to waves?
Define Wavelength
Define Frequency
Wavelength of Light
How are the wavelength and frequency of
electromagnetic radiation related?
Why do different types of electromagnetic
radiation have different properties?
How do the wavelength and frequency of
an X-ray compare with those of the red
light from a neon sign?
How do we express frequency?
Practice
How did you determine which of the waves
represented blue, and which represented
red?
Practice
A laser used in eye surgery to fuse
detached retinas produces radiation with a
wavelength of 640.0 nm. Calculate the
frequency of this radiation.
An FM radio station broadcasts
electromagnetic radiation at a frequency of
103.4 MHz. Calculate the wavelength of
this radiation?
Quantized Energy and Photons
Wave model of light does not explain three
very important phenomena.
– The emission of light from hot objects
(blackbody radiation)
– The emission of electrons from metal surfaces
on which light shines (photoelectric effect)
– The emission of light from electronically
excited gas atoms (emission spectra)
Hot Objects and Quantization of
Energy (blackbody radiation)
1900- Max Planck
– Assumed energy either released or absorbed
by atoms in “chunks” of some minimum size.
– termed QUANTUM (fixed amount) to smallest
quantity of energy that can be emitted or
absorbed
– Proposed that energy (E) of a single quantum
equals constant times the frequency of
radiation
E=hv
Planck’s Constant
h= 6.626 x 10-34 joule-second (J-s)
Can be emitted or absorbed in whole number
multiples of hv (hv, 2hv, 3hv)
If 3hv has been released, we say: 3 quanta of
energy have been emitted
Stairs Vs. Ramp
Photoelectric Effect and Photons
Albert Einstein
– Light shining on a clean metal surface causes surface
to emit electrons
– Minimum frequency of light (different for different
metals) is required for emission.
Light with frequency of 4.60 x 1014 s-1 or greater
causes cesium metal to emit electrons.
– Radiant energy striking surfaces behaves like stream
of energy particles (PHOTONS)
– Does light act more like a wave or a particle?
Practice Problems
A laser emits light that a frequency of 4.69
x 10^14/s. What is the energy of one
photon of this radiation?
– If the laser emits a pulse containing 5.0 x
10^17 photons of this radiation, what is the
total energy of that pulse?
– If the laser emits 1.3 x 10^-2 J of energy
during a pulse, how many photons are
emitted?
Bohr and Line Spectra
Neils Bohr (1913)
– Theoretical explanation of line spectra
Line Spectra
Continuous Vs. Line Spectra, how do they
differ?
How do we produce a bright line spectra?
How is that energy emitted?
We know there is a definite change in energy,
this corresponds to:
Definite Frequency
Definite Wavelength
Bohr’s Model
Assumed electrons move in circular orbits
around nucleus
– Goes against physics!!
Charged particle moving in circular orbit should continually
lose energy and spiral into positively charged nucleus
Based model on 3 postulates:
– Only orbits of certain radii (with specific energies)
permitted for electrons in H atom.
– Electron in permitted orbit is in “allowed” energy state
and therefore does not radiate energy
– Energy is emitted or absorbed as a photon only when
moving energy levels
Energy States of Hydrogen Atom
Bohr began calculating possible energy
levels and found they fit into a specific
equation.
December 13th, 2012
Do Now:
1.Einstein’s 1905 paper on the photoelectric effect was
the first important application of Planck’s quantum
hypothesis. Describe his original hypothesis, and
explain how Einstein made use of it in his theory of
the photoelectric effect.
2. A laser emits a wavelength of 987 nm. In what
portion of the spectrum is this found? Its output
energy is absorbed in a detector that measures a total
energy of 0.52 J over 32 seconds. How many photons
per second are being emitted?
Energy States of a Hydrogen Atom
With three postulates and classical equations for
motion and interacting charges, Bohr calculated
energy corresponding to orbits.
E= (-hcRH) (1/n2) = (-2.18 x 10-18 J) (1/n2)
h= plancks constant
c= speed of light
RH= Rydbergs Constant
n= principal quantum number (range from 1infinity)
Energy of Hydrogen cont.
Each orbit = different n value
Energies of electrons of a H atom given by
this equation are negative for all values of
n (most negative being closest to 1)
Lower the energy, more stable the atom
Lowest energy state = ground state
When in higher = excited state
Yikes!
How is the radius effected as n becomes
infinitely larger?
How is the energy effected as n becomes
infinitely larger?
– The state in which the electron is removed
from the nucleus is the reference, or zero
energy, state.
∆E = Ef – Ei = Ephoton = hv
Hydrogen Atom Cont.
Bohr’s model states that specific frequencies of
light satisfy the previous equation. Therefor we
can state:
If nf is smaller than ni, the electron is moving
closer to the nucleus and change in energy is
negative *indicates a release of energy*
This equation can be used to calculate
frequency or wavelength as well.
Limitations to Bohr
Bohr model can not explain line spectra beyond
the Hydrogen atom (except in crude manner)
As we have seen, electrons also have wavelike
properties
Things kept from Bohr Model:
– Electrons exist in certain discrete energy levels
described by quantum numbers
– Energy is involved in moving an electron from one
level to another
Wave Behavior of Matter
Louis de Broglie:
– Studied Dual Nature of Particles (wave and radiant
energy)
– Suggested that the electron is associated with a
particular wavelength.
– Wavelength of electron is dependent upon mass and
velocity ( m and v)
λ= h/ mv
– Quantity mv = momentum
– Termed “matter waves” to describe wave
characteristics of particles
Practice Problem
Calculate the velocity of a neutron whose
de Broglie wavelength is 500 pm. The
mass of a neutron is 1.67492716 x 10-24 g.