Periodic Law
Ck12 Science
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Printed: November 18, 2013
AUTHOR
Ck12 Science
www.ck12.org
C ONCEPT
Concept 1. Periodic Law
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Periodic Law
• State the periodic law.
• Describe the organization of the periodic table.
How are these items related to one another?
We have all enjoyed playing the game “Clue.” The object of the game is to obtain information about a murder – who
did it, where they did it, and what was used as the murder weapon. As the game progresses, clues are obtained by
each player and they then must assemble these clues into a guess as to the criminal. Individual pieces of information
takes on broader significance when put together with other parts of the puzzle.
The Periodic Law
When Mendeleev put his periodic table together, nobody knew about the existence of the nucleus. It was not until
1911 that Rutherford conducted his gold foil experiment that demonstrated the presence of the nucleus in the atom.
Just two years later, in 1913, English physicist Henry Moseley (1887-1915) examined x-ray spectra of a number
of chemical elements. He would shoot X-rays through crystals of the element and study the wavelengths of the
radiation he detected. Moseley found that there was a relationship between wavelength and atomic number. His
results led to the definition of atomic number as the number of protons contained in the nucleus of each atom. He
then realized that the elements of the periodic table should be arranged in order of increasing atomic number rather
than increasing atomic mass.
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When ordered by atomic number, the discrepancies within Mendeleev’s table disappeared. Tellurium has an atomic
number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic
mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with
being most responsible for the modern periodic law: When elements are arranged in order of increasing atomic
number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we
know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a
new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular
intervals, within the vertical columns called groups.
Summary
• Elements of the periodic table are arranged in order of increasing atomic number.
• The periodic law states “When elements are arranged in order of increasing atomic number, there is a periodic
repetition of their chemical and physical properties.”
Practice
Use the link below to answer the following questions:
http://www.famousscientists.org/henry-moseley/
1. Where did Moseley attend college?
2. Who did he do research with after graduating from college?
3. What is Moseley’s law?
Review
1.
2.
3.
4.
5.
Did Mendeleev know about the nucleus of an atom?
Who discovered the relationship between wavelength of X-rays and atomic number?
What did Moseley conclude from his research?
What is the “periodic law”?
What do the vertical columns (groups) in the periodic table represent?
• group: Elements with similar chemical properties appear at regular intervals, within the vertical columns.
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Concept 1. Periodic Law
• period: A period is a horizontal row of the periodic table.
• periodic law: When elements are arranged in order of increasing atomic number, there is a periodic repetition
of their chemical and physical properties.
References
1. User:Necessary Evil/Wikimedia Commons. Possible Weapons from Game Clue. Public Domain
2. User:Cepheus/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Periodic_table.svg. Public Domain
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