The History of The Modern Periodic Table
Like human evolution, the modern periodic table has also undergone a series of periodic developments with the advancement of scientific capabilities and acceptance. In earlier days, elements were classified based on their metallic and non-metallic character. Somewhere around 1829, the German chemist Johann Wolfgang Dobereiner proposed the first scientifically accepted relationship between the properties of elements. He found that in a few sets of three elements, the atomic weight of the middle element was approximately the arithmetic mean of the other two. Also, he noted that the properties of the middle element were intermediate between the other two elements. Such a set of elements was called a triad.​
Triad -1 : Li, Na, K Triad-2 : Cl, Br, I
Triad-3 : S, Se, Te Triad-4 : Ca, Sr, Ba
This relationship was significant only in a limited set of elements. As a consequence, this model and approach were rejected. The rule proposed by Dobereiner was not enough to apply to the bunch of elements that were already known to the scientific community. Later on in 1862, a French Geologist, Alexandre-Emile Beguyer de Chancourtois, introduced a cylindrical table of elements to display the periodic recurrence of properties. He arranged the known elements in their increasing order of atomic weight and made a cylindrical table called the telluric helix. Due to its impracticality and complexity, it was not widely accepted. Scientists started searching for a uniform law for elements.
Mendeleev's periodic table was known as the short form of the periodic table. In his arrangement of elements, he also considered the properties of elements. There were 8 groups and 12 periods in his periodic table, and each group was subdivided into subgroups. Like Dobereiner triads, Mendeleev also kept a few triads in group VIII. He was so futuristic that he kept some gaps in the periodic table for the elements that could be discovered in the future. Eka boron, Eka silicon and Eka aluminium were later discovered as scandium, germanium and gallium. Mendeleev's periodic table was the first elaborated and comprehensive version.
The next advancement in the upgradation of the periodic table was the arrangement proposed by J.A.R. Newlands in 1865. He found that if the known elements are arranged in the increasing order of their atomic weights, then the 8th element showed similar properties to those of the first element, like the octaves in music and this law was known as the law of octaves
Sa re ga ma pa da ni
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca
After the discovery of noble gases, this law became irrelevant to the scientific community. The law of octaves was no longer followed by the elements upon arranging them in the increasing order of their atomic weight. It was only applicable up to calcium without any major difference in the properties. Afterwards, the elements did not follow this trend.
The next important bridge to the modern periodic table was given by Lother Meyer in 1869 through his atomic volume curve. It was a graphical arrangement of atomic volume against atomic weight. He found that elements with similar properties are in similar positions on the curve. For instance, alkali metals on the peaks of the curve, alkaline earth metals on the descending part of the curve and most of the transition elements at the broad minima of the curve. Even though his findings were relevant, it was not practical to study a huge number of elements through a long graphical arrangement.
The next and most important periodic table was Mendeleev's periodic table. In 1869, he arranged 63 elements in the order of their atomic weight and properties. He also proposed a periodic law, which states that the physical and chemical properties of the elements are periodic functions of their atomic weight. This is known as Mendeleev's periodic law.
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Mendeleev's periodic table, 1871 recreated version
However, the periodic law introduced by him was not completely followed in the periodic arrangement of elements. It is found that the higher atomic weight elements were placed before the lower ones. This was done to match their chemical properties, and they were known as inverted or anomalous pairs. This includes Ar-K, Co-Ni, Te-I and Th-Pa. Other major demerits of Mendeleev's periodic table were that the position of hydrogen and lanthanides was not clear. There was no position available for noble gases; isotopes were not included, and there was a lack of similarity between subgroups. Altogether, we came back to the uncertainty in the cause of periodicity. It was concluded from these demerits that the cause of periodicity in the properties of elements is not a periodic function of atomic weight.
The modern, long form of the periodic table was introduced by English physicist Henry Moseley in 1913. Moseley introduced the number of protons in a nucleus as the atomic number by studying X-ray spectra of elements. He formulated an expression connecting the atomic number with the characteristic X-ray frequency emitted by the element. His plot was between the square root of frequency and the atomic number.​
G18(VIIIA)
The modern periodic law was proposed by Henry Moseley after the failures of Mendeleev's periodic law. It stated that the physical and chemical properties of elements are periodic functions of their atomic numbers and electronic configuration. This law was experimentally proved by Henry Moseley and theoretically backed by Niels Bohr. In the modern periodic table, we have 18 groups and 7 periods. In a period, elements are arranged in increasing order of their atomic number. While arranging the elements, the (n + l) value is also taken into consideration. The highest principal quantum number of the element represents the period number.
18 groups in the modern periodic table are divided into main groups and subgroups. Apart from groups and periods, they are also classified into different blocks: s-block, p-block, d-block and f-block. This classification is done based on the filling of electrons in the subshell. The last electron of an element is known as a differentiating electron, and this will define which block the element belongs to depending on its filling according to the (n + l) rule.​