Chapter 6: Electronic Chapter 6: Electronic Structure of AtomsStructure of Atoms

Pages 207-247Pages 207-247

Brief Explanation of the ChapterBrief Explanation of the Chapter

Explore Quantum TheoryExplore Quantum Theory– Nature of LightNature of Light– Arrangement of electrons in atomsArrangement of electrons in atoms– Exploration of reactivityExploration of reactivity– HistoryHistory

Wave Nature of LightWave Nature of Light

Knowledge of electrons come from lightKnowledge of electrons come from light

How can we numerically define the speed How can we numerically define the speed of light?of light?– 3.00 x 103.00 x 1088 m/s m/s

How do we explain the term periodic in How do we explain the term periodic in reference to waves?reference to waves?

Define WavelengthDefine Wavelength

Define FrequencyDefine Frequency

Wavelength of LightWavelength of Light

How are the wavelength and frequency of How are the wavelength and frequency of electromagnetic radiation related?electromagnetic radiation related?

Why do different types of electromagnetic Why do different types of electromagnetic radiation have different properties?radiation have different properties?

How do the wavelength and frequency of How do the wavelength and frequency of an X-ray compare with those of the red an X-ray compare with those of the red light from a neon sign?light from a neon sign?

How do we express frequency?How do we express frequency?

PracticePractice

How did you determine which of the waves How did you determine which of the waves represented blue, and which represented represented blue, and which represented red?red?

PracticePractice

A laser used in eye surgery to fuse A laser used in eye surgery to fuse detached retinas produces radiation with a detached retinas produces radiation with a wavelength of 640.0 nm. Calculate the wavelength of 640.0 nm. Calculate the frequency of this radiation.frequency of this radiation.

An FM radio station broadcasts An FM radio station broadcasts electromagnetic radiation at a frequency of electromagnetic radiation at a frequency of 103.4 MHz. Calculate the wavelength of 103.4 MHz. Calculate the wavelength of this radiation?this radiation?

Quantized Energy and PhotonsQuantized Energy and Photons

Wave model of light does not explain three Wave model of light does not explain three very important phenomena.very important phenomena.– The emission of light from hot objects The emission of light from hot objects

(blackbody radiation) (blackbody radiation) – The emission of electrons from metal surfaces The emission of electrons from metal surfaces

on which light shines (photoelectric effect)on which light shines (photoelectric effect)– The emission of light from electronically The emission of light from electronically

excited gas atoms (emission spectra)excited gas atoms (emission spectra)

Hot Objects and Quantization of Hot Objects and Quantization of Energy (blackbody radiation)Energy (blackbody radiation)

1900- Max Planck1900- Max Planck– Assumed energy either released or absorbed Assumed energy either released or absorbed

by atoms in “chunks” of some minimum size.by atoms in “chunks” of some minimum size.– termed termed QUANTUMQUANTUM (fixed amount) to smallest (fixed amount) to smallest

quantity of energy that can be emitted or quantity of energy that can be emitted or absorbedabsorbed

– Proposed that energy (E) of a single quantum Proposed that energy (E) of a single quantum equals constant times the frequency of equals constant times the frequency of radiationradiation

E=hvE=hv

Planck’s ConstantPlanck’s Constant

h= h= 6.626 x 106.626 x 10-34-34 joule-second (J-s) joule-second (J-s)

Can be emitted or absorbed in whole number Can be emitted or absorbed in whole number multiples of multiples of hvhv ( (hv, 2hv, 3hv)hv, 2hv, 3hv)

If If 3hv3hv has been released, we say: 3 quanta of has been released, we say: 3 quanta of energy have been emittedenergy have been emitted

Stairs Vs. RampStairs Vs. Ramp

Photoelectric Effect and PhotonsPhotoelectric Effect and Photons

Albert EinsteinAlbert Einstein– Light shining on a clean metal surface causes surface Light shining on a clean metal surface causes surface

to emit electronsto emit electrons– Minimum frequency of light (different for different Minimum frequency of light (different for different

metals) is required for emission.metals) is required for emission.

Light with frequency of 4.60 x 10Light with frequency of 4.60 x 101414 s-1 or greater s-1 or greater causes cesium metal to emit electrons.causes cesium metal to emit electrons.

– Radiant energy striking surfaces behaves like stream Radiant energy striking surfaces behaves like stream of energy particles (PHOTONS)of energy particles (PHOTONS)

– Does light act more like a wave or a particle?Does light act more like a wave or a particle?

Practice ProblemsPractice Problems

A laser emits light that a frequency of 4.69 A laser emits light that a frequency of 4.69 x 10^14/s. What is the energy of one x 10^14/s. What is the energy of one photon of this radiation?photon of this radiation?– If the laser emits a pulse containing 5.0 x If the laser emits a pulse containing 5.0 x

10^17 photons of this radiation, what is the 10^17 photons of this radiation, what is the total energy of that pulse?total energy of that pulse?

– If the laser emits 1.3 x 10^-2 J of energy If the laser emits 1.3 x 10^-2 J of energy during a pulse, how many photons are during a pulse, how many photons are emitted?emitted?

Bohr and Line SpectraBohr and Line Spectra

Neils Bohr (1913)Neils Bohr (1913)– Theoretical explanation of line spectraTheoretical explanation of line spectra

Line SpectraLine Spectra

Continuous Vs. Line Spectra, how do they Continuous Vs. Line Spectra, how do they differ?differ?

How do we produce a bright line spectra?How do we produce a bright line spectra?

How is that energy emitted?How is that energy emitted?

We know there is a definite change in energy, We know there is a definite change in energy, this corresponds to:this corresponds to:

Definite FrequencyDefinite FrequencyDefinite WavelengthDefinite Wavelength

Bohr’s ModelBohr’s Model

Assumed electrons move in circular orbits Assumed electrons move in circular orbits around nucleusaround nucleus– Goes against physics!! Goes against physics!!

Charged particle moving in circular orbit should continually Charged particle moving in circular orbit should continually lose energy and spiral into positively charged nucleuslose energy and spiral into positively charged nucleus

Based model on 3 postulates:Based model on 3 postulates:– Only orbits of certain radii (with specific energies) Only orbits of certain radii (with specific energies)

permitted for electrons in H atom.permitted for electrons in H atom.– Electron in permitted orbit is in “allowed” energy state Electron in permitted orbit is in “allowed” energy state

and therefore does not radiate energyand therefore does not radiate energy– Energy is emitted or absorbed as a photon only when Energy is emitted or absorbed as a photon only when

moving energy levelsmoving energy levels

Energy States of Hydrogen AtomEnergy States of Hydrogen Atom

Bohr began calculating possible energy Bohr began calculating possible energy levels and found they fit into a specific levels and found they fit into a specific equation.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 constantc= speed of lightRH= Rydbergs Constantn= principal quantum number (range from 1-

infinity)

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.