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674 Periodic Table (Chemistry)

Once a set of reliable atomic masses was accepted by being in the same column. Newlands even went so far
the scientific community [thanks in large measure to as to leave blank spaces where he thought new elements
the application of Amadeo Avogadro’s Hypothesis by should go, and he made predictions about the properties
Stanislao Cannizzaro (1826–1910)], the time was ripe for of these undiscovered atoms. Unfortunately, Newlands’s
the formulation of the periodic law and the table based idea, self titled the “Law of Octaves,” received mostly crit-
on it. icism from the scientific community. He did not receive
appropriate recognition until 1913, when H. G. Moseley
establishedthephysicalrealitybehindtheordinalnumbers
B. Early Discoveries of Periodicity
of the elements.
The fact that certain elements and compounds exhibit sim-
ilar physical and chemical properties was well known long
C. The Periodic Table of Mendeleev
before the adoption of an accurate set of atomic masses. In-
deed, some elements were first prepared in what appeared Dmitri Ivanovich Mendeleev is one of the most color-
to be families. Humphry Davy’s (1778–1829) experiments ful characters in the history of chemistry. An account
of the early 19th century led to the grouping of potassium of his childhood reads like a plot by Dostoevsky. Of
and sodium into a family that has come to be called al- Russian and Mongolian ancestry, Mendeleev was born
kali metals and now includes lithium, rubidium, cesium, in Siberia in 1834, the youngest of 17 or 18 children.
and francium as well. Not only do these elements share While Dmitri was still a child, his father, director of a
many common chemical and physical properties, but they secondary school, went blind. In order to support her fam-
form many compounds with similar properties. Davy also ily, Maria Kornileva Mendeleeva managed a glass factory.
prepared magnesium, calcium, strontium, and barium and However, after some years, the factory burned down and
subsequently grouped them together to form the alkaline the elder Mendeleev died of tuberculosis. Dmitri had
earths. As analytical methods improved, other elementary just completed his secondary education and his obvious
families were found, among them the halogens, a group of intelligence clearly called for further training. Therefore,
reactive and widely distributed nonmetals including fluo- his mother took her 16-year-old son and one of her daugh-
rine, chlorine, bromine, and iodine. Clearly, the existence ters on a 1000-mile journey by horse-drawn vehicle to
of these families was evidence for order in the great diver- Moscow. Her efforts to obtain a place for Dmitri at the
sity of matter. university there failed, but ultimately he was enrolled in
There were many different approaches to the task of the Central Pedagogic Institute at St. Petersburg.
grouping the elements. In 1817, Johann Wolfgang Following his studies in St. Petersburg and a brief period
D¨obereiner (1780–1849) proposed one of the first systems asaschoolteacher,Mendeleevwasallowedtocontinuehis
to organize the mineral world. His idea was to group ele- education and research in Paris and Heidelberg. Shortly
ments into triads of related substances. Later, he focused after his return to Russia, he was appointed professor of
on triads composed of the elements themselves, for exam- chemistry at the Technological Institute of St. Petersburg.
ple, lithium, sodium, and potassium. He found that within He held that position until 1890, when he resigned in a
the triads he identified, the intermediate atomic mass was policy dispute with the administration. Subsequently, he
close to the arithmetic mean of the atomic masses of the assumed the post of Director of the Bureau of Weights
other two. Moreover, other properties such as density, and Measures. A strong individualist, Mendeleev appears
melting point, and boiling point behaved similarly. to have been bold and outspoken in his educational, so-
Slightly later, another elementary system was proposed cial, and political views. He was similarly courageous in
by Alexandre Emile B´eguyer de Chancourtois (1820– putting forth his scientific ideas, a characteristic which is
1862). His idea was that the properties of elements are the very evident in his approach to the classification of the
properties of numbers. To demonstrate this, he arranged elements. Indeed, it is probably Mendeleev’s boldness in
the elements in a three-dimensional spiral in order of adhering to his classificatory scheme in the face of appar-
increasingatomicmass.Whenthusordered,elementswith ent contradictions and in making predictions based upon
similar properties fell in vertical columns. The success that scheme that has led to his identification with the pe-
of this numerological ordering was soon followed by a riodic table.
system created by J. A. R. Newlands. In 1863 or 1864, Mendeleev also had another attribute essential for the
he assigned “ordinal numbers” to each element in order task at hand—an encyclopedic knowledge of the chem-
of increasing atomic mass. Hence, hydrogen was given ical properties of the elements and thousands of their
an ordinal number of 1, lithium was 2, and so on. Next, compounds. In 1869, he was summarizing much of that
the first seven elements were placed in a row, then the knowledge in a textbook, Principles of Chemistry. As part
next seven were placed in a row directly below the first. of this project, he was searching for a way to organize the
In this way, elements with similar chemistry ended up great diversity of information into a pedagogically and