Organization of the Periodic Table of the Elements
The Modern Periodic of the Elements
The periodic has undergone extensive change since Mendeleev's time.
Chemists have discovered new elements and in more recent years,
synthesized them in the laboratory. Each of the more than forty new elements,
however, can be placed in a group of other elements with similar properties.
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.
The noble gases Perhaps the most significant addition to the Periodic
Table came with the discovery of the noble gases. Argon (Ar) was discovered
in 1894 by the English physicists John William Strutt (Lord Rayleigh;
1842-1919) and William Ramsay (1852-1916). Although argon makes up
about 1 percent of the atmosphere, it had escaped notice because of its total
lack of chemical reactivity. Helium (He), another noble gas, had been
detected earlier using the spectrum of sunlight. In 1894 Ramsay discovered
that hedium also exists on earth.
In order to fit argon and helium into the Periodic Table, Ramsay found
it necessary to propose a new group. He placed this group between the groups
now known as Group 17 (the flourine family) and Group 1 (the Lithium
family). In 1898 Ramsay discovered two more noble gases, krypton (Kr) and
xenon (Xe). The final gas, Radon (Rn), was discovered in 1900 by the
German scientist Friedrich Ernst Dorn (1848-1916).
The next step in the development of the Periodic Table was completed in
the early 1900s when the puzzling chemistry of the "rare earth" elements was
finally understood. Because the rare earths are so similar in properties, their
separation and identification required the efforts of many chemists during the
next 100 years. Today, the 14 different rare-earth elements are known as the
lanthanides. The lanthanides, or rare-earth metals, are the 14 elements
with atomic numbers from 58 (cerium, Ce) to 71 (lutetium, Lu).
The actinide elements Another landmark in the development of the
Periodic Table was the discovery of the actinide elements, all of which are
radioactive. The actinides are the 14 elements with atomic numbers from
90 (thorium, Th) to 103 (lawrencium, Lr). The lanthanides and actinides
belong in periods 6 and 7, respectively, of the Periodic Table between the
elements of Groups 3 and 4.
Periodicity Periodicity with respect to atomic number can be observed in
any group of elements. Consider, for example, the noble gases of Group 18.
The first noble gas is helium, which has an atomic number of 2. The elements
following helium in atomic number have completely different properties until
the next noble gas, neon (Ne), which has an atomic number of 10, is reached.
The remaining noble gases in order of increasing atomic number are argon
(atomic number 18), krypton (atomic number 36), xenon (atomic number
54), and radon (atomic number 86). The difference in atomic number
between each noble gas is shown in Figure 5-3. Below the noble gases in
Figure 5-3 are the elements of Group 1. These are all silvery metals of low
density that react strongly with water. As you can see, the differences in
atomic number the Group 1 metals follow exactly the same pattern
as for the noble gases. Starting with the first member of Groups 13-17 a similar
pattern is repeated: The atomic number of each successive element is 8, 18, 18,
and 32 higher than that of the element above it.
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