Download MENDELEEV AND THE ATOMIC TABLE Dmitri Ivanovich

MENDELEEV AND THE ATOMIC TABLE
Dmitri Ivanovich Mendeleev, 1834 – 1907, was a Russian chemist and inventor. He is
credited as being the creator of the first version of the periodic table of elements. Using
the table, he predicted properties of elements yet to be discovered.
In the previous session we talked about our growing
understanding of the atom. Our growing realisation that the world
is made up of more and different elements than Aristotle’s four
essences: Earth, Water, Air and Fire.
Already in 1863 there were 56 known elements, with a new one
being discovered at a rate of approximately one per year. After
becoming a teacher, Mendeleev wrote the definitive textbook of
his time: Principles of Chemistry (two volumes, 1868–1870), in Dmitri Mendeleev in 1897
which he attempted to classify the
elements according to their
chemical properties. In compiling
the list he noticed patterns that led
him to postulate his famous periodic
table. On 6 March 1869,
Mendeleev made a formal
presentation to the Russian
Chemical Society, entitled ?The
Dependence between the Properties
of the Atomic Weights of the
Elements”. It listed elements
according to both atomic weight and
valence (from Latin valentia =
"strength, capacity," the chemical meaning refers to the "combining power of an element”). The presentation
stated that
1. The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties.
2. Elements which are similar in regards to their chemical properties have atomic weights which are either of
nearly the same value or increase regularly.
3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to
their so-called valencies, as well as, to some extent, to their distinctive chemical properties.
4. The elements which are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the
molecule determines the character of a compound body.
6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to
aluminium and silicon, whose atomic weights would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous
elements.
8. Certain characteristic properties of elements can be foretold from their atomic weights.
Mendeleev published his periodic table of all the then known elements, and predicted
several new elements to complete the table. For his predicted eight new elements, he
used the prefixes of eka, dvi, and tri (Sanskrit one,
two, three) in their naming. He was puzzled about
where to put the known lanthanides, and also
predicted the existence of another row to the table
which were the actinides which were some of the
heaviest in atomic mass (see below). Gaallium
and Germanium were found in 1875 and 1886
respectively and fitted perfectly into the two
spaces provided by Mendeleev. I have gone into
this in some details, because to me it shows the
Sculpture in honor of Mendeleev and the
uncanny human capacity to understand Nature
periodic table, Bratislava, Slovakia
Sculpture in Saint Petersburg
The modern periodic table is a tabular
display of the chemical elements, organized on
the basis of their atomic numbers and chemical
properties. Elements are presented in
increasing atomic number. The main body of
the table is a 18 × 7 grid, and elements with the
same number of valence electrons are kept
together in groups, such as the halogens and
the noble gases. Due to this, there are gaps that
form four distinct rectangular areas or blocks.
Using periodic trends, the table can help
predict the properties of various elements and
the relations between properties. As a result, it
provides a useful framework for analysing
chemical behaviour, and is widely used in
chemistry and other sciences.
Standard form of the periodic table. The colors represent different categories
of elements: Red = Alkali Metals; Fawn = Earth Metals; Green - Non =
Metals; Light Blue = Noble Gases; Pink = Transition Metals; Grey = Post
Transition Metals; Yellow = Halogens; Brown = Metalloids; Light Grey =
Unknown Properties. The Lanthanides and Actinides fit the spot above.
All versions of the periodic table only include chemical elements, not mixtures, compounds, or subatomic
particles, and isotopes of a given element are represented in the same cell. Isotopes are atoms that have an
excess or deficiency of Neutrons in the atomic nucleus, giving them in-between atomic weights. Last time we
did speak of Neutrons as the ?packing material” between the protons of an atom. The elements are listed in
order of increasing atomic number (the number of protons in the nucleus of an atom). The seven Periods rows
define the number of Electrons in each Shell - 2 - 8 - 8 - 18 - 18 - 32 - 32. A new row (period) is started when
the next electron shell has its first electron. Groups (Columns) are determined by the electron configuration of
the atom; elements with the same number of electrons in the valence shell fall into the same columns (e.g.
oxygen and selenium are in the same column because they both have 4 electrons in the outermost shell. In
general, elements with similar chemical properties fall into the same group in the periodic table, Thus it is
relatively easy to predict the chemical properties of an element if one knows its position in the table and the
properties of the elements around it. Modern quantum mechanical theories of atomic structure explain group
trends by proposing that elements within the same group generally have the same electron configurations in
their valence shell, which is the most important factor in accounting for their similar properties. It has become
the most important method of classifying elements.
By definition, each chemical element has a unique atomic number representing the number of protons in its
nucleus, but most elements have differing numbers of neutrons among different atoms; as mentioned above,
these are referred to as Isotopes. For example, while all atoms of carbon have six protons and usually six
neutrons, about 1% have seven neutrons, and a very small amount have eight neutrons; so carbon has three
different naturally occurring isotopes, Carbon 12 - 13 - 14. Many isotopes are unstable with varying degrees of
half-life. We have talked earlier about the process of Carbon Dating, where the ratio of Carbon 12 to 13 and 14
in a test sample can give a fairly accurate date when the sample was deposited. Isotopes are never separated in
the periodic table; they are always grouped together under a single element.
The significance of atomic numbers to the organization of the periodic table was not appreciated until the
existence and properties of protons and neutrons became understood. Mendeleev's periodic tables instead used
atomic weights, which at that time worked to a fair precision.
The lanthanide elements are the group of elements with atomic number increasing from 57 (lanthanum) to 71
(lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar to
lanthanum. In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two
additional rows below the main body of the table. This convention is entirely a matter of aesthetics and
formatting practicality; The table would unwieldily wide otherwise. The name "rare earths" is sometimes used
to describe all the lanthanides. This name arises from the minerals from which they were first isolated, which
were uncommon oxide-type minerals. The name comes from the Greek lanthanein "to lie hidden". I was later
identified as the first in an entire series of chemically similar elements and gave name to the whole series. The
actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium
through lawrencium. The series derives its name from the group 3 element actinium. In comparison with the
lanthanides the actinides show much more variable valence. All actinides are radioactive and release energy
upon radioactive decay; uranium and thorium are the most abundant actinides on Earth.
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