814 19 The Electronic States of Atoms. III. Higher-Order Approximations
with the formula MCl, where M stands for the chemical symbol of the metal. Beryllium,
magnesium, calcium, strontium, and barium all form oxides with the formula MO and
chlorides with the formula MCl 2. When Mendeleev listed the elements in rows of the
appropriate length, lithium, sodium, potassium, rubidium, and cesium all occurred in
the first column, beryllium, magnesium, calcium, strontium, and barium occurred in
the second column, and so on.
Some elements were unknown at the time of Mendeleev. These elements included all
of the inert gases, most of the inner transition elements, and others scattered about the
table, such as scandium, gallium, and germanium. Mendeleev had sufficient confidence
in the periodicity principle that he left blank spaces in his table for undiscovered
elements. These blanks were later filled in with elements having much the same
properties as those predicted by Mendeleev.
The modern periodic chart lists the elements in order of atomic number (number of
protons in the nucleus), rather than atomic mass. Inside the front cover of this book is
such a periodic table. You can see four pairs of stable elements in which the order of
atomic number is different from the order of atomic mass (Ar and K, Co and Ni, Te and
I, and Th and Pa). Mendeleev was probably unaware of these cases, because argon and
protactinium had not been discovered, because he placed cobalt and nickel together in
a triple column, and because he did not have accurate atomic masses for tellurium and
iodine.
There are several ways of numbering the columns. One scheme is to number the
columns IA through VIIIA, and IB through VIIIB as indicated. This system has been
commonly used in the United States. Three columns under Fe, Co, and Ni are grouped
together as column VIIIB. This numbering corresponds closely to the numbering
scheme used by Mendeleev, although the A and B columns were grouped together
in his table, which had only eight columns. There is also another numbering scheme
in which the A and B designations of columns III through VIII are interchanged. This
system has been commonly used in Europe, and is recommended by the International
Union of Pure and Applied Chemistry. A new scheme has been devised to overcome
the disagreement over the A and B designations. It simply numbers the columns from
1 to 18, and will probably become the standard scheme.
The elements in the columns labeled A are calledrepresentative elements, and those
in the columns labeled B are calledtransition elementsortransition metals. The column
IIB has been assigned to one or the other of these categories, and has sometimes been
called a separate category. The two sets of 14 elements at the bottom of the chart
are calledinner transition elementsorinner transition metals. Figure 19.6 shows a
periodic table in which the inner transition elements are listed in the body of the table
with all elements through element 109 listed in order of ascending atomic number.
Various alternative forms of the periodic table have been proposed, including a version
in which columns IA and IIA are placed at the right of the table and other columns are
listed as in Figure 19.6.^29
Uranium (U, element 92) is the heaviest element occurring in the earth’s crust in
measurable amounts. The elements past U have been synthesized in nuclear reactors.
The elements after element 103 (lawrencium, Lr) were given temporary names: Element
104 was unnilquadrium, element 105 was unnilpentium, and so on, based on the Latin
version of the atomic numbers. Some of these names have been replaced. The names(^29) R. L. Rich,J. Chem. Educ., 82 , 1761 (2005).