178 DMITRI MENDELEEV
vertically. The following year,
Mendeleev read a paper at the
Russian Chemical Society stating
that: “The elements, if arranged
according to their atomic weight,
exhibit an apparent periodicity
of properties.” He explained that
elements with similar chemical
properties have atomic weights
that are either of nearly the same
value (such as potassium, iridium,
and osmium) or that increase
regularly (such as potassium,
rubidium, and cesium). He further
explained that the arrangement of
the elements into groups in the
order of their atomic weights
corresponds to their valency, which
is the number of bonds the atoms
can form with other atoms.
Predicting new elements
In his paper, Mendeleev made a
bold prediction: “We must expect
the discovery of many yet unknown
elements—for example, two
elements, analogous to aluminum
and silicon, whose atomic weights
would be between 65 and 75.”
Mendeleev’s arrangement
included crucial improvements over
Newlands’ Octaves. Below boron
and aluminum, Newlands had
placed chromium, which made
little sense. Mendeleev reasoned
that there must exist an as-yet
undiscovered element, and
predicted that one would be found
with an atomic weight of about 68.
It would form an oxide (a compound
formed by an element with oxygen)
with a chemical formula of M 2 O 3 ,
where “M” is the symbol for
the new element. This formula
meant that two atoms of the
new element would combine
with three oxygen atoms to
make the oxide. He predicted two
more elements to fill other spaces:
one with an atomic weight of about
45, forming the oxide M 2 O 3 , and the
other with an atomic weight of 72,
forming the oxide MO 2.
Critics were sceptical, but
Mendeleev had made very
specific claims, and one of the
most powerful ways to support
a scientific theory is to makepredictions that are proved true.
In this case, the element gallium
(atomic weight 70, forming the
oxide Ga 2 O 3 ) was discovered in
1875; scandium (weight 45, Sc 2 O 3 )
in 1879; and germanium (weight 73,
GeO 2 ) in 1886. These discoveries
made Mendeleev’s reputation.Mistakes in the table
Mendeleev did make some
mistakes. In his 1869 paper, he
asserted that the atomic weight
of tellurium must be incorrect: it
should lie between 123 and 126,
because the atomic weight of
iodine is 127, and iodine should
clearly follow tellurium in the table,The six noble gases that occur naturally (listed in group 18 of the table) are
helium, neon, argon, krypton, xenon, and radon. They have very low chemical
reactivity because they each have a full valence shell—a shell of electrons
surrounding the atom’s nucleus. Helium has just one shell containing two
electrons, while the other elements have outer shells of eight electrons.
Radioactive radon is unstable.
It is the function of science to
discover the existence of a
general reign of order in nature
and to find the causes
governing this order.
Dmitri MendeleevThe six alkali metals are all soft,
highly reactive metals. The outer layer
of this lump of pure sodium has reacted
with the oxygen in the air to give it a
coating of sodium oxide.He Ne Ar Kr XeNucleus Electron