8/14/2019 l14 chapt9-1 web

1/12

Chemistry 5

Chapter-9

Electrons in Atoms

Part-1

25 October 2002

8/14/2019 l14 chapt9-1 web

2/12

Electromagnetic RadiationWhat is it and why are we interested?

8/14/2019 l14 chapt9-1 web

3/12

Electromagnetic Radiation

Learn about about electronic structure

of atoms and molecules using

electromagnetic radiation as a probe.

What are common types ofelectromagnetic radiation?

Key features of electromagnetic radiation:

visible light

radio waves

X-rays

microwaves

can be described by propagation of waves, which are the oscillation of electricand magnetic fields [as wave can transmit energythink about water waves].

The regular waves have a period orwavelength, , defined by the distancebetween two successive peaks.

The number of peaks (crests) that pass a point per unit time is the frequency,. The S.I. unit for frequency is s-1, hertz, Hz [. = velocity] .

8/14/2019 l14 chapt9-1 web

4/12

Frequency & Wavelength

The velocity of electromagnetic radiation in vacuumis constant:

Cell phones operate around ca. 1 GHz frequencies;what is the wavelength of this electromagneticradiation?

This constant is called thespeed of light, c = 3.00 x 108 m s-1

With this constant we can relate wavelength and frequency for any

electromagnetic radiation:

c = .

c = . = c/

= 3.00 x 108 m s-1/1 x 109 s-1

= 0.3 meters

8/14/2019 l14 chapt9-1 web

5/12

Examples: Wavelength & Frequency

8/14/2019 l14 chapt9-1 web

6/12

Electromagnetic Radiation: Interference

Demonstration: Observations evidence for waveproperties?

Interference of Waves

periodic rings of light and dark contrastrings change with separation

constructive interference: wavesadd in phase

destructive interference: waves addout of phase cancel.

8/14/2019 l14 chapt9-1 web

7/12

8/14/2019 l14 chapt9-1 web

8/12

Quantum Theory: Photons

Blackbody radiation:

Planck: Energy is Quantized

Energy-Frequency Relationship

Classical physics predicts thatintensity of heated body shouldincrease continuously withdecreasing wavelength it does not!

Planck made revolutionarysuggestion that energy was discreteor quantized: = nhPlancks constant:

h = 6.63 x 10-34 J s

Plancks work implies that the energy of a unit quantum of

electromagnetic radiation is proportional to frequency:

E = h

8/14/2019 l14 chapt9-1 web

9/12

Photoelectric Effect

Observations/Key Findings:

electron emission only occurs when thefrequency of the incident light exceeds a

particular threshold value, o.

Above threshold, the kinetic energies ofemitted electrons depends on frequency, and

the number of electrons emitted depends onintensity of light.

These observations

consistent with quantizednature of light, andmoreover, led Einstein to

propose that lightparticle-like concept of

photon.

8/14/2019 l14 chapt9-1 web

10/12

Description of Atoms: Bohr

Key Features of Bohr Atom:

Consider H 1 electron + 1 proton

e-

The electron moves in circular orbits aboutthe nucleus with the motion described byclassical physics.

The electron has only a fixed set ofallowed orbits. While in a give orbit, the

energy of an electron is constant. Thediscrete orbits are described by an integer(call principle quantum number, later).

n = 4

n = 3

n = 2

n = 1

An electron can only transition from oneallowed orbit to another this results ineither absorption or emission of energy,h, corresponding to the difference inenergy between the initial and final states.

2

2

nAZEn

8/14/2019 l14 chapt9-1 web

11/12

Hydrogen Atom: Bohr

Energy Levels in H

Absorption/Emission Energy

Ionization

In the hydrogen atom, Z=1

From experimental observations can defineconstant in Bohr model:

En = -RH/n2, RH = 2.179 x 10

-18J

ground state of H has electron in n =1 level

The hydrogen atom can absorb energy topromote electron to higher energy level oremit energy if in excited state.

The difference in energy,

E, or transitionenergy is

E = Ef Ei = -RH(1/nf2 1/ni2) (= h)

To completly remove an electron ionizethe atom promote to n = level as finalstate.

8/14/2019 l14 chapt9-1 web

12/12

Optical Spectra of Hydrogen & Other Atoms

Demonstration Observations?

Hydrogen

Helium

Neon

Limitations of Bohr Model?

From experimental perspective, the Bohr model cannot explainspectra obtained from atoms with more than 1 electron!