The periodic table has evolved over time as scientists' understanding of the chemical elements has increased. Dmitri Mendeleev is generally credited with developing the first recognizable periodic table in 1869 by arranging the elements based on their atomic masses and properties. Later, Henry Moseley improved upon Mendeleev's work by arranging the elements according to their atomic numbers, which are based on their nuclear charge, rather than atomic mass. This ordering better reflected the actual chemical properties of the elements. Moseley's work also revealed gaps in the periodic table that were later filled by the discovery of new elements.

1. ARTICLE

2. HISTORY OF THE PERIODIC TABLE
The periodic table is an arrangement of the chemical elements, organized on
the basis of their atomic numbers, electron configurations and recurring chemical
properties. Elements are presented in order of increasing atomic number. The standard
form of the table consists of a grid of elements, with rows called periods and columns
called groups.
The history of the periodic table reflects over a century growth in the
understanding of chemical properties. The most important event in its history occurred
in 1869, when the table was published by Dmitri Mendeleev, who built upon earlier
discoveries by scientists such as Antoine-Laurent de Lavoisier and John Newlands,
but who is nevertheless generally given sole credit for its development.
Ancient times
A number of physical elements (such as gold, silver and copper) have been
known from antiquity, as they are found in their native form and are relatively simple
to mine with primitive tools. However, the notion that there were a limited number of
elements from which everything was composed originated in around 330 BCE, when
the Greek philosopher Aristotle proposed that everything is made up of a mixture of
one or more roots, an idea that had originally been suggested by the Sicilian
philosopher Empedocles. The four roots, which were later renamed as elements by
Plato, were earth, water, air and fire. While Aristotle and Plato introduced the concept
of an element, their ideas did nothing to advance the understanding of the nature of
matter.
Antoine-Laurent de Lavoisier
Lavoisier’s Traite Elementaire de Chimie (Elementary Treatise of Chemistry),
which was written in 1789 and first translated into English by the writer Robert Kerr,
is considered to be the first modern textbook about chemistry. It contained a list of
“simple substances” that Lavoisier believed could not be broken down further, which
included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur, which

3. formed the basis for the modern list of elements. Lavoisier’s list also included ‘light;
and ‘caloric’, which at the time were believed to be material substances. He has
classified there substances into metals and non metals. While many leading chemists
refused to believe Lavoisier’s new revelations, the Elementary Treatise was written
well enough to convince the younger generation. However, Lavoisier’s descriptions of
his elements lack completeness, as he only classified them as metals and non-metals.
Johann Wolfgang Dobereiner
In 1817, Johann Wolfgang Dobereiner began to formulate one of the earliest
attempts to classify the elements. In 1829, he found that he could form some of the
elements into groups of three, with the members of each group having related
properties. He termed these groups triads. Some of the triads that were classified by
Dobereiner are:
1. chlorine, bromine and iodine
2. calcium, strontium and barium
3. sulfur, selenium and tellurium
4. lithium, sodium and potassium
In all of the triads, the atomic weight of the middle elements was almost
exactly the average of the atomic weights of the other two elements.
John Newlands
In 1864, the English chemist John Newlands classified the sixty-two known
elements into seven groups, based on their physical properties.
Newlands noted that many pairs of similar elements existed, which differed by
some multiple of eight in mass number, and was the first to assign them an atomic
number. When his ‘law of octaves’ was printed in Chemistry News, likening this
periodicity of eights to the musical scale, it was ridiculed by some of his
contemporaries. His lecture to the Chemistry Society on 1 March 1866 was not
published, the Society defending their decision by saying that such ‘theoretical’ topics
might be controversial.

4. The importance of Newlands’ analysis was eventually recognized by the
Chemistry Society with a Gold Medal five years after they recognized Mendeleev’s
work. It was not until the following century, with Gilbert N Lewis’ valence bond
theory (1916) and Irving Langmuir’s octet theory of chemical bonding (1919), that the
importance of the periodicity of eight would be accepted. The Royal Chemistry
Society acknowledged Newlands’ contribution to science in 2008, when they put a
Blue Plaque on the house where he was born, which described him as the “discover of
the Periodic Law for the chemical elements.” He contributed the word ‘periodic’ in
chemistry.
Dmitri Mendeleev
The Russian chemist Dmitri Mendeleev was the first scientist to make a
periodic table similar to the one used today. Mendeleev arranged the elements by
atomic mass corresponding to relative molar mass. It is sometimes said that he played
‘chemical solitaire’ on long train journeys, using cards with various facts about the
known elements. On March 6, 1869, a formal presentation was made to the Russian
Chemical Society, entitled The Dependence Between the Properties of the Atomic
Weights of the Elements. In 1869, the table was published in an obscure Russian
journal and then republished in a German journal, Zeitschrift for Chemie. In it,
Mendeleev stated that :
1. The elements, if arranged according to their atomic mass, exhibit an apparent
periodicity of properties.
2. Elements which are similar as regards to their chemical properties have atomic
weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which
increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements, or of groups of elements in the order of their
atomic masses, corresponds to their so-called valencies, as well as, to some
extent, to their distinctive chemical properties, as is apparent among other
series in that of Li, Be, B, C, N, O and F.
4. The elements which are the most widely diffused have small atomic weights

5. 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,
elements analogous to aluminium and silicon-whose atomic weight 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. Thus the atomic weight of tellurium must
lie between 123 and 126, and cannot be 128.
8. Certain characteristic properties of elements can be foretold from their atomic
masses.
Scientific benefits of Mendeleev’s table
 It enabled Mendeleev to predict the discovery of new elements and left spaces
for them, namely eka-silicon (germanium), eka-aluminium (gallium), and eka-
boron (scandium). Thus, there was no disturbance in the periodic table.
 It could be used by Mendeleev to point out that some of the atomic weights
being used at the time were incorrect.
 It provided for variance from atomic weight order.
Shortcomings of Mendeleev’s table
 The table was not able to predict the existence of noble gases. However, when
this entire family of elements was discovered, Sir William Ramsay was able to
add them to the table as Group 0, without the basic concept of the periodic
table being disturbed.
 A single position could not be assigned to hydrogen, which could be placed
either in the alkali metals group, the halogens group or separately above the
table between boron and carbon.

6. Henry Moseley
In 1914, a year before he was killed in action at Gallipoli, the English physicist
Henry Moseley found the relationship between the X-ray wavelength of an element
and its atomic number. He was then able to re-sequence the periodic table by nuclear
charge, rather than by atomic weight. Before this discovery, atomic numbers were
sequential numbers based on an element’s atomic weight. Moseley’s discovery
showed that atomic numbers were in fact based upon experimental measurements.
Using information about their X-ray wavelengths, Moseley placed argon (with
an atomic number Z=18) before potassium (Z=19), despite the fact that argon’s atomic
weight of 39. 9 is greater than the atomic weight of potassium (39.1). The new order
was in agreement with the chemical properties of these elements, since argon is a
noble gas and potassium is an alkali metal. Similarly, Moseley placed cobalt before
nickel and was able to explain that tellurium occurs before iodine, without revising the
experimental atomic weight of tellurium, as had been proposed by Mendeleev.
Moseley’s research showed that there were gaps in the periodic table at atomic
numbers 43 and 61, which are now known to be occupied by technetium and
promethium respectively.