Atoms - tiny wonders to explore

Atomic Spectroscopy

• Visible spectrum:
Red Orange Yellow Green Blue Violet _ 700 650 590 540 480 350 4.29 4.62 3.23 5.56 6.25 8.57 1.77 1.91 2.10 2.30 2.58 3.54

A continuous visible spectrum consists of light with wavelength ranging from 700 to 350 nm. The color varies continuously from red to violet, but we generally identify 6 colors as red, orange, yellow, green, blue, violet. Typical wavelength, l, frequency n, and energy of photon E are given in the table here

A continuous spectrum is similar to this one

• Line spectra of elements

When electric charge passes through tubes of H, Hg, or Ne vapor, these tubes emit visible light. When light from these tube is analyzed by a prism, their spectra are as these shown here. These are typical line spectra, because each spectrum shows only a few lines, rather than a continuous spectrum (as those emitted by a hot solid).

• Line spectra of hydrogen

Energy Level diagram
of H atoms
Continuum n = infinity
========== n = 6,7
---------------- n = 5
---------------- n = 4

---------------- n = 3

---------------- n = 2

---------------- n = 1
The visible spectra of H consists of red (656.3 nm) green, (486.1 nm), blue, (343.0 nm), indigo (410.1 nm), and violet (396.9 nm) lines.

Balmer and Rydberg discovered that these lines can be represented by the formula:

1
---
l
= R   1       1
(---   -   ---)
22       n2
(R = 10973731.534 m-1)
R is the Rydberg constant. Later, a Lyman series was found in the UV region, and these lines can be represented by
1
---
l
= R   1       1
(---   -   ---)
12       n2
(Lyman series).
From these and other results, we know that the electrons in H atoms have the energy levels as shown in the diagram shown here.

• X-rays and metallic elements

Whilhelm Conrad Röntgen (1845-1923), while experimenting with cathode rays, discovered X-rays.

X-rays are emitted when fast-moving electrons are suddenly stopped by a target.

A continuous spectrum is emitted when energies of electrons are low.

Characteristic X-rays are generated when energies of electrons reach 20 keV or higher. The frequency of characteristic X-rays depends on the target.

• X-ray Diffractions

Max von Laue (1879-1950) suggested to his students to use crystals to investigate the wave properties of X-rays.

Using X-ray diffractions, W.H. Bragg (1862-1942) and W.L. Bragg (1890-1971) interpreted the salt crystal structure from the X-ray diffraction studies.

• Moseley's law

Frequencies of Charactristic
Lines of Some Elements
Atomic
No.
ElementFrequency
/(1e18) Hz
23 V 1.199
24 Cr 1.310
25 Mn 1.427
26 Fe 1.550
27 Co 1.677
28 Ni 1.810
29 Cu 1.947
30 Zn 2.090
42 Mo 4.021
47 Ag 5.363
79 Au 16.650
Henry Gwyn-Jeffreys Moseley (1887-1915) used various metals as the target in his X-ray tubes, and measured the wavelengths of the most intense (characteristic) lines. He noticed that frequencies of characteristic lines increase with the increase of the atomic weights of the target metals. He plotted the square root of the frequencies against the order of the elements in the periodic table, and got a straight line. This is known as Moseley's law.

The impact of Moseley's law: Characteristic lines not only confirm a substance as element, Moseley's law places the element in the proper location in the periodic table.

Internet Resource

Exploring the table of isotopes
The pictorial periodic table

E-mail: cchieh@uwaterloo.ca