UNIVERSITY OF ALBERTA
INTRODUCTORY UNIVERSITY CHEMISTRY I (CHEM 101/103)
Quantum Theory & Atomic Structure
Introduction
It is crucial to understand how the electrons are arranged in an atom because:  Electrons are involved in bonding, especially the covalent bond formation  It also helps us to understand the shape of covalent compounds Electromagnetic Radiations

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Light is a form of electromagnetic radiation.
It is important for us to study the properties of light (electromagnetic radiations) because our understanding of electrons in an atom is due to its interaction with matter.

Electromagnetic radiation is a form of energy transmission in which both electric and magnetic fields are propagated in the form of waves.
Some examples: radio waves, microwave, light, X-ray are electromagnetic waves. Two variables for electromagnetic radiation:
Frequency (, Greek nu) is the number of cycles the wave undergoes per second. It has a unit of per second (s-1) or called Hertz (Hz)
Wavelength (λ, Greek lambda) is the distance of one cycle of the wave. It has a unit in meters (m) or very often in nanometers (nm, 10-9 m) or angstroms (Å, 10-10m)

Electromagnetic
wave with longer
wavelength

Electromagnetic
wave with shorter
wavelength
Fig 7.1, p262

1

The speed of a wave (c) is the distance traveled per unit time by that wave. It has a unit of meter per second (m s-1).
All Electromagnetic radiations travel at about 2.998 x 108 m / s Thus, the product of wavelength and frequency gives the speed of the wave. c=xλ
cycle
m
m
=
x
=
s
cycle
s
c
=
λ
Note: A high frequency wave is associated with short wavelength Significance of wavelength - waves with different wavelengths give different color to our perception.

Fig 7.3, p262

2

Important Characteristic of electromagnetic waves
Electromagnetic waves are capable of interfering with each other. Diffraction of light through a small aperture

Fig. 7.5, p265
Constructive interference: When the two waves are in phase with each other Destructive interference: When the two waves are out of phase with each other Interference of waves is the cause of diffraction.

The Quantum Theory (The Particle Nature of Light)
Although light is best described as waves (electromagnetic waves), in the early 20th century, Planck and Einstein proposed light has particle properties. Einstein proposed the photon theory: electromagnetic radiations can be viewed as a stream of particles called photons.

Particles of light = photons
The energy of one photon is given by:
E = h
h = Planck’s constant = 6.626 x 10-34 J s
Since
c
=
λ
hc
E=
λ

3

Work example:
Calculate the energy associated with a photon of blue light which has a wavelength of 450. nm?
6.626 x 10-34 J s x 2.998 x 108 m / s

hc
E=

=

450. x 10-9 m

λ

= 4.41 x 10-19 J photon-1
For one mole of photons, the energy = 4.41 x 10-19 J photon-1x 6.022 x 1023 photon mol-1 = 2.65 x 105 J mol-1 (265 kJ mol-1)
Atomic Spectra
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When energy in the form of electrical discharges or heat is supplied, atoms are ‘excited’
The excited atoms undergo ‘relaxation’ which emits energy in the form of electromagnetic radiation.
The emitted radiation can be examined with a prism, which separates the radiation into its component wavelengths. The patterns of the emitted radiation are in the form of lines and so it is called the line spectrum of the atom.

Fig 7.8, p268

The line spectrum of hydrogen
4

In contrast, sunlight is continuous.

The continuous spectrum of sunlight
The hydrogen atomic spectra
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It is composed of several series of li...

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