The Periodic Law
History of the Periodic Table
Mendeleev and Chemical Periodicity
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Elements arranged in order of increasing
atomic mass, similarities in chemical
properties appeared regularly (periodic).
Mendeleev created a table where
elements of similar properties are grouped
together.
Predicted properties of missing elements.
Mendeleev’s 1st Periodic Table
Mendeleev’s Periodic Table
Moseley and the Periodic Law
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Moseley, working with Rutherford,
discovered that elements fit into patterns
better when arranged in inc. order of
nuclear charge or number of protons.
Led to modern definition of atomic
number.
Periodic Law.
The Modern Periodic Table
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Since Mendeleev, chemists have
discovered new elements and created new
ones in the lab.
Periodic Table: An arrangement of the
elements in order of their atomic numbers
so that the elements with similar
properties fall in the same group.
The Noble Gases
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Rayleigh and Ramsay credited with
discovery of argon (Ar) first, then Ramsay
proposed the new group (18).
The final gas, radon (Rn) was discovered
in 1900 by Friedrich Ernst Dorn.
The Lanthanides
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The lanthanides are the 14 elements with
atomic numbers from 58 (cerium) to 71
(lutetium).
The elements are so similar in chemical
and physical properties, separating and
identifying them took the work of many
chemists.
The Actinides
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The actinides are the 14 elements with
atomic numbers from 90 (thorium) to 103
(lawrencium).
They fall into the 7th period.
Periodicity
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Periodicity with respect to atomic number
can be observed in any group of elements
in the periodic table.
In each of the Groups 1 and 18, the
differences between the atomic numbers
of successive elements are 8,8,18, and 32.
Groups 2 and 13-17 follow similar
patterns.