The Periodic Law
Chapter 5
History of the Periodic Law
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In 1860, chemist had to learn the properties of the 60 elements that had been
discovered.
There was no method for accurately determining an element’s atomic mass or
number of atoms of an element in a particular chemical compound.
Stanislao Cannizzaro presented a convincing method for accurately
measuring the relative masses of atoms that enabled chemists to agree on
standard values for atomic mass and initiated a search for relationships
between atomic mass and other properties of the elements.
Mendeleev and Chemical Periodicity
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Dmitri Mendeleev hoped to organize the elements according to their
properties
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Observed chemical and physical properties of each element and arranged the element
according to the various properties and looked for trends and patterns
He noticed when the elements were arranged in order of increasing atomic
mass, certain similarities in their chemical properties appeared at regular
intervals
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Periodic: a repeating pattern
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Mendeleev created his first periodic table where he grouped elements with
similar properties together.
He placed Iodine (atomic mass 127) after tellurium (atomic mass 128) to
make it where tellurium was in a group of elements that shared similar
properties
The success of his predictions led most chemists to accept his periodic table
and earned him the credit as the discoverer of the periodic law
Two questions remained:
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Why could most of the elements be arranged in the order of increasing atomic mass but a few
could not?
What was the reason for chemical periodicity?
Moseley and the Periodic Law
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In 1911, Henry Moseley was working with Rutherford and examined the
spectra of 38 different metals
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Moseley discovered that the elements in the periodic table fit into patterns
better when they were arranged in increasing order according to nuclear
charge, or the number of protons in the nucleus.
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Moseley’s work led to the definition of atomic number and the recognition that
the atomic number is the basis for the organization of the periodic table
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Moseley’s discovery was consistent with Mendeleev’s ordering of the periodic
table by properties rather than strictly by atomic mass
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Ex: Tellurium with an atomic number of 52 belongs before Iodine (53)
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Mendeleev’s principle of chemical periodicity is correctly stated in what is
known as the periodic law
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Periodic Law: the physical and chemical properties of the elements are periodic functions of
their atomic numbers
- When the elements are arranged in order of increasing atomic number, elements with
similar properties appear at regular intervals
The Modern Periodic Table
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The periodic table has gone under extensive change since Mendeleev’s time.
(new elements have been discovered and synthesized new ones in the lab)
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The Periodic Table is an arrangement of the elements in order of their
atomic numbers so that elements with similar properties fall in the same
column, or group
The Noble Gases
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In 1898, helium was discovered as a component of the sun, based on the
emission spectrum of sunlight
In 1894, John William Strutt (Lord Rayleigh) and Sir William Ramsay
discovered argon, Ar, a gas in the atmosphere that had previously escaped
notice because of its total lack of chemical reactivity.
In 1895, Ramsay showed that helium also exist on Earth
In order to fit argon and helium into the periodic table, Ramsay posed a new
group between Group 17(the fluorine family) and Group 1(the lithium family)
In 1898, Ramsay discovered Krypton and Xenon.
In 1900, Friedrich Ernst Dorn discovered Radon
The Lanthanides
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In the early 1900s the “puzzling” chemistry of the lanthanides was finally
understood
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The Lanthanides are the 14 elements with atomic numbers from 58 (cerium)
to 71 (lutetium). Located in Period 6
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The process of separating and identifying these elements was a tedious task
that required the effort of many chemist because their chemical and physical
properties are so similar
The Actinides
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The Actinides are the 14 elements with atomic numbers from 90 (thorium,Th)
to 103 (lawrencium, Lr).
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Located in period 7
Periodicity
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Periodicity with respect to atomic number can be observed in any group of
elements in the periodic table
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The reason for periodicity is because of the second “mystery” presented by
Mendeleev’s periodic table. It is explained by the arrangement of the
electrons around the nucleus
HOMEWORK
Section Review
pg. 127
(1-4)