Evaluation of Modern Periodic table

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Periodic Table History

First attempts to systematize

In 1789, Antoine Lavoisier published a list of 33 chemical elements. Although Lavoisier grouped the elements in gases, metals, nonmetals, and lands, chemists have spent the next century looking for a more accurate classification system. In 1829, Johann Wolfgang Döbereiner observed that most elements can be grouped into triads (groups of three) depending on their chemical properties. Lithium, sodium and potassium, for example, were grouped together as the soft, reactive metals. Döbereiner also noted that, when arranged by atomic weight, the second member of each triad was about the average of the first and third. This became known as the law of triads. German chemist Leopold Gmelin worked with this system, and by 1843 he had identified ten triads, three groups of four, and a group of five. Jean-Baptiste Dumas book published in 1857 relations between various groups of metals. Although the various chemists have succeeded in identifying the relationships between small groups of elements, they had not yet built a system that encompassed them all. The German chemist August Kekule had observed in 1858 that carbon tends to bond with other elements in a ratio of one to four. Methane, for example, a carbon atom and four hydrogen atoms. This concept became known as valence. In 1864, fellow German chemist Julius Lothar Meyer published a table of 49 known elements arranged by the valence. The table shows that elements with similar properties have often shared the same valence. English chemist John Newlands has produced a series of papers in 1864 and 1865 that described his own classification of elements: He noted that when listed in order of increasing atomic mass, the same physical and chemical properties returned at intervals of eight years, which he likened to the octaves of music. The law of octaves, however, was ridiculed by his contemporaries, and society theChemical refused to publish his work. Nevertheless, Newlands was able to write an atomic table and use it to predict the existence of missing elements, such as germanium. The Chemical Society only recognized the importance of his discoveries nearly five years after they credited Mendeleev.

The  Mendeleyev table

Russian chemistry professor Dmitri Ivanovich Mendeleev and German chemist Julius Lothar Meyer independently released their periodic tables in 1869 and 1870, respectively. They both built their tables in the same way: When listing items in a row or column in order of atomic weight and starting a new row or column, where the characteristics of the elements began to repeat. The success of the Periodic Table came from two decisions he has made: The first was to leave gaps in the table when it seemed that the corresponding element has not yet been discovered.Mendeleev was not the first chemist to do, but it was the first to be recognized as using trends in the periodic table to predict the properties of these missing elements, such as gallium and germanium. The second decision was occasionally ignore the order suggested by the atomic weights and move adjacent elements, such as cobalt and nickel, to better classify them into chemical families. With the development of theories of atomic structure, it became apparent that Mendeleev had listed the elements in order of increasing atomic number or nuclear charge. In 1913, Henry Moseley obtained experimental values ​​of atomic number of X-ray spectra of elements.

The periodic table we use today is based on that developed and published by Dmitri Mendeleev in 1869.
Mendeleev found that he could arrange the 65 elements known at the time in a grid or table so that each element has:
1. A weight higher than that of atomic left. For example, magnesium (24.3 atomic weight) is placed to the right of sodium (23.0 atomic weight):
23.0
24.3 Na
Mg

2. Chemical properties similar to other elements in the same column - in other words of similar chemical reactions. Magnesium, for example, is placed in the column of alkaline earths:
9.01
Be
24.3
Mg
40.1
California
87.6
Sr

Mendeleev realized that the table in front of him was at the heart of chemistry. And more than that, Mendeleev saw the painting was incomplete - there were areas where things should be, but no one had discovered.
Like Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium on paper. He called this new element eka-silicon, after observing a gap in the periodic table between silicon and tin:
28.1
If
?
?
119
Sn

Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) on paper, because it predicts their existence and their belongings before their actual discovery.

Although Mendeleev made a crucial breakthrough, he made little progress. With hindsight, we know the periodic table of Mendeleev was supported by false reasoning. Mendeleev believed, wrongly, that the chemical properties were determined by atomic weight. Of course, this was perfectly reasonable when you consider the science in 1869.
In 1869, the electron itself had not been discovered - what happened 27 years later, in 1896.
In fact, it took 44 years for the correct explanation of regular patterns in the periodic table of Mendeleev to be found.

The explanation came in 1913 by Henry Moseley, who fired electrons to atoms, resulting in the emission of x-rays. Moseley found that each element, he studied X-rays emitted at a single frequency.
When he looked at the frequencies emitted by a series of elements, he found a model that was best explained if the positive charge in the nucleus increased by exactly one unit of an element.

When he looked at the frequencies emitted by a series of elements, he found a model that was best explained if the positive charge in the nucleus increased by exactly one unit of an element.

In other words, Moseley found that the elements are different from each other because their atoms have different numbers of protons. He discovered that the positions of the elements "in the periodic table are better predicted by their atomic numbers than their atomic weights. (Atomic number element is equal to the number of protons, electrons and thus, in one of its atoms.)

Some More Points Added to Mendeleev's table

Moseley's discovery cleared up the problems of cobalt-nickel and argon and potassium.
Considering the problem of potassium in argon, it was known that argon has a higher weight than potassium atom. Following the reasoning of Mendeleev, argon must be placed after the potassium in the periodic table.

Moseley's work showed that the atomic number is 18 argon and potassium is 19. Therefore argon must be placed before potassium in a periodic table on the basis of atomic numbers. Chemists around the world heaved a collective sigh of relief, because in agreement with the observed chemical properties of these elements.

Moseley also emulated the achievement of Mendeleev discover new elements on the paper, looking for four atomic numbers without corresponding elements. He predicted the existence of elements with atomic numbers 43, 61, 72 and 75. These elements were indeed discovered, we now call technetium, promethium, rhenium and hafnium.

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