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Russian chemist who arranged the 63 known
elements (A substance composed of a single type of
atom (e.g., iron)) into a periodic table based on
atomic mass, which he published in Principles of
Chemistry in 1869. This organization surpassed
attempts at classification by Beguyer de Chancourtois
and Newlands and was published a year before the
work of Lothar Meyer. Mendeléev left space for new
elements, and predicted three yet-to-be-discovered
elements including eka-silicon and eka-boron. His
table did not include any of the noble gases, however,
which had not yet been discovered. His table placed
elements in their correct position by atomic number,
thus showing variance from atomic weight in a
number of places.
The periodic table displays all chemical elements
systematically in order of increasing atomic number,
i.e., the number of protons in the nucleus. In 1869,
Russian chemist Dmitri Mendeleev first described an
arrangement of the chemical elements now known as
the periodic table. Mendeleev wrote, "The elements, if
arranged according to their atomic weights, show a
distinct periodicity of their properties.... Elements
exhibiting similarities in their chemical behavior have
atomic weights which are approximately equal (as in
the case of Pt, Ir, Os) or they possess atomic weights
which increase in a uniform manner (as in the case of
K, Rb, Cs)."
Isotopes of an element have the same number of
protons but a different number of neutrons. They are
chemically very similar but have different atomic
masses.