Classification of Elements 1 Flashcards
why do we classify elements
In 1800, only 31 elements were known. By
1865, the number of identified elements had more than
doubled to 63. At present 114 elements are known. Of
them, the recently discovered elements are man-made.
Efforts to synthesise new elements are continuing. With
such a large number of elements it is very difficult to study
individually the chemistry of all these elements and their
innumerable compounds individually. To ease out this
problem, scientists searched for a systematic way to
organise their knowledge by classifying the elements. Not
only that it would rationalize known chemical facts about
elements, but even predict new ones for undertaking further
study.
what were dobereiners traids
The German chemist, Johann Dobereiner in
early 1800’s was the first to consider the idea
of trends among properties of elements. By
1829 he noted a similarity among the physical
and chemical properties of several groups of
three elements (Triads). In each case, he
noticed that the middle element of each of the
Triads had an atomic weight about half way
between the atomic weights of the other two
(Table 3.1). Also the properties of the middle
element were in between those of the other two members. Since Dobereiner’s relationship,
referred to as the Law of Triads, seemed to
work only for a few elements, it was dismissed
as coincidence.
what did aeb chancourtis do
The next reported attempt to
classify elements was made by a French
geologist, A.E.B. de Chancourtois in 1862. He
arranged the then known elements in order of
increasing atomic weights and made a
cylindrical table of elements to display the
periodic recurrence of properties.
law of octaves by newlands
chemist, John Alexander Newlands in 1865
profounded the Law of Octaves. He arranged
the elements in increasing order of their atomic
weights and noted that every eighth element
had properties similar to the first element
(Table 3.2). The relationship was just like every
eighth note that resembles the first in octaves
of music. Newlands’s Law of Octaves seemed
to be true only for elements up to calcium.
who is credited for modern periodic table
The Periodic Law, as we know it today owes
its development to the Russian chemist, Dmitri
Mendeleev (1834-1907) and the German
chemist, Lothar Meyer (1830-1895). Working
independently, both the chemists in 1869 proposed that on arranging elements in the
increasing order of their atomic weights,
similarities appear in physical and chemical
properties at regular intervals.
what did lothar meyer find out wrt the periodic funciton of element
Lothar Meyer
plotted the physical properties such as atomic
volume, melting point and boiling point
against atomic weight and obtained a
periodically repeated pattern. Unlike
Newlands, Lothar Meyer observed a change in
length of that repeating pattern. By 1868,
Lothar Meyer had developed a table of the elements that closely resembles the Modern
Periodic Table. However, his work was not
published until after the work of Dmitri
Mendeleev, the scientist who is generally
credited with the development of the Modern
Periodic Table.
state menedeleev periodic law
the properties of elements are periodic function of their aotmic weights.
give an overview of mendeleev’s periodic table
Mendeleev arranged elements in horizontal
rows and vertical columns of a table in order
of their increasing atomic weights in such a
way that the elements with similar properties
occupied the same vertical column or group.
Mendeleev’s system of classifying elements was
more elaborate than that of Lothar Meyer’s.
what criteria did mendeleev use for classification of elememts
He fully recognized the significance of
periodicity and used broader range of physical
and chemical properties to classify the
elements. In particular, Mendeleev relied on
the similarities in the empirical formulas and
properties of the compounds formed by the
elements.
what did mendeleev do if some elements did not fit into his scheme of classification
He realized that some of the elements
did not fit in with his scheme of classification
if the order of atomic weight was strictly
followed. He ignored the order of atomic weights, thinking that the atomic
measurements might be incorrect, and placed
the elements with similar properties together.
For example, iodine with lower atomic weight
than that of tellurium (Group VI) was placed
in Group VII along with fluorine, chlorine,
bromine because of similarities in properties
how did mendeleev predict new elements
keeping his
primary aim of arranging the elements of
similar properties in the same group, he
proposed that some of the elements were still
undiscovered and, therefore, left several gaps
in the table. For example, both gallium and
germanium were unknown at the time
Mendeleev published his Periodic Table. He left
the gap under aluminium and a gap under
silicon, and called these elements Eka-
Aluminium and Eka-Silicon. Mendeleev
predicted not only the existence of gallium and
germanium, but also described some of their
general physical properties. These elements
were discovered later.
what was given by henry ,osely
In 1913, the
English physicist, Henry Moseley observed
regularities in the characteristic X-ray spectra
of the elements. A plot of ν (whereν is
frequency of X-rays emitted) against atomic
number (Z ) gave a straight line and not the
plot of ν vs atomic mass. He thereby showed
that the atomic number is a more fundamental
property of an element than its atomic mass.
what is the modern periodic law
The physical and chemical properties
of the elements are periodic functions
of their atomic numbers.
how did importance shift to e- configuration
atomic number is
equal to the nuclear charge (i.e., number of
protons) or the number of electrons in a neutral
atom. It is then easy to visualize the significance
of quantum numbers and electronic
configurations in periodicity of elements. In
fact, it is now recognized that the Periodic Law
is essentially the consequence of the periodic
variation in electronic configurations, which
indeed determine the physical and chemical
properties of elements and their compounds.
give an overview of the modern periodic table
A modern version,
the so-called “long form” of the Periodic Table
of the elements (Fig. 3.2), is the most convenient
and widely used. The horizontal rows (which
Mendeleev called series) are called periods and
the vertical columns, groups. Elements having
similar outer electronic configurations in their
atoms are arranged in vertical columns,
referred to as groups or families.
