The periodic table that we study today is the modern periodic table and was invented by Dmitri Mendeleev. However, Mendeleev was not the first one who grouped and arranged the elements in the form of a periodic table. The following are a few attempts to classify elements that took place prior to the recognition of modern periodic table.
In 1789, Antoine Lavoisier was the first to classify elements based on their properties. He grouped the elements into gases, non-metals, metals and earthly elements.
After several decades, in 1829 Johann Döbereiner made an attempt to group elements. Based on the similarity in chemical properties, he grouped elements into triads. The middle element’s atomic weight in the triad stands approximately as the average of the first and third elements’ atomic weights. This can also help define the properties of the middle element. Eg: Lithium, sodium and potassium. Since Döbereiner failed to classify all known elements into triads, the law of triads did not find success.
The attempt to classify elements did not stop here and it was in 1865 that the English Chemist John Alexander Newland arranged the elements in the order of their increasing atomic weights. He discovered a periodic pattern in the arrangement. He showed that the physical and chemical properties of the eighth element is similar to the properties of the first element in that row. This generalization made by Newlands is known as the 'Law of Octaves'.
It was only in 1869 that Mendeleev published his paper in the Journal of Russian Chemical Society where he classified the elements based on their atomic masses and arranged them into horizontal rows called periods and vertical columns called groups. The periodic law states that the properties of elements are a periodic function of their relative atomic masses. Mendeleev was successful in arranging all 63 elements that were known in his time into a tabular form which contained eight columns and seven rows. It also contained some gaps which were later filled after the discovery of new elements.
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Mendeleev was successful in classifying the elements and showing the periodic similarity in elements. However, there were a few anomalies in the arrangement which were later cleared by the English Physicist, Henry Moseley.
Henry Moseley showed that the chemical and physical properties of elements is determined by the atomic number and not by the atomic mass. He restated the periodic law as-
'Physical and chemical properties of an element are a periodic function of its atomic number.'
The Moseley's periodic law is also known as the Modern Periodic Law and it paved the way to the modern periodic table.
The modern periodic table of elements is the one that we use today. It is based on the concept of Mendeleev's periodic table but differs in the fact that the elements are arranged in the increasing order of atomic number and not atomic mass.
In the study of chemistry, modern periodic table holds prime importance. Following are some of the main features of modern periodic table.
Elements are grouped in ascending order of their respective atomic number.
There are seven horizontal rows called periods and eighteen vertical columns called groups.
The elements in a group show similar physical and chemical properties since they contain the same number of outer electrons. But they show a gradual change as we move from top to bottom in a group.
The elements in a period show a gradual change in properties on moving from left to right. Atomic size gradually decreases as we move from left to right.
The modern periodic table consists of more elements when compared to the 63 elements in Mendeleev's periodic table. Presently it has 118 elements.
It would not be wrong to say that the study of chemistry of elements would have been impossible without the existence of modern periodic table. The classification of elements in the modern periodic table helps in the easier understanding of the properties of elements.
Following are the major classification of elements in the modern periodic table-
Alkali and Alkaline Earth metals: The first two groups on the left side of the periodic table consists of highly reactive elements (except hydrogen). The first group elements contain one electron in the valence shell while the second group elements contain two electrons in their valence shell.
Transition metals: These elements occupy the centre of the periodic table and mostly show the properties of metals. The elements starting from group 3 to 12 falls under transition metals. Some of the transition metals are placed separately in two rows at the bottom of the periodic table. These are known as Lanthanides and Actinides.
Metalloids and non-metals: Metalloids generally appear in a diagonal line at the right side of the periodic table. These are basically the elements that separate metals on the left side of the periodic table from the non-metals on the right side of the periodic table. These elements exhibit the properties of metals and non-metals, thus, they are called metalloids.
Noble gases: The extreme right side of the periodic table is occupied by gases. They are placed in the 18th group and have completely filled valence shells. These gases are non-reactive and are called the inert or noble gases.
The study of modern periodic table (groups and periods) is so significant in the world of chemistry that it can be called as the pillar of chemical world.