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Modern Periodic Table

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Modern Periodic Table - Lesson Summary

At the time of development of Mendeleev's Periodic Table the sub-atomic particles were not discovered.

Discovery of sub-atomic particles and their properties took place at the start of 20th century. And Isotopes and noble gas elements were also discovered around the same period.

In 1913, an English physicist Henry Moseley observed regularities in the characteristic X-ray spectra of elements.

On plotting the square root of X-ray frequency emitted by elements against their corresponding atomic numbers, Moseley got a straight line graph.

Moseley established that the atomic number which is the number of protons contained in the nucleus or the number of electrons surrounding the nucleus in a neutral atom is a more fundamental property of an element as compared to its atomic mass, as it defines the electronic structure and properties of an element.

The elements were arranged in order of their increasing atomic numbers. And anomalies of Mendeleev's table were removed.


(i) About the position of isotopes was solved.

(ii) The placement of higher atomic weight elements like tellurium, before lower ones like iodine, was also removed.

The placement of hydrogen was one anomaly of Mendeleev's periodic table that was still not resolved.

On the basic of its atomic number, Moseley placed Hydrogen at the top left corner of the table without attaching it to any particular group.

Several scientists drafted various forms of periodic tables that focused on factors such as chemical reactions, valence and electronic configurations of elements.

Of all these periodic tables, the "long form" of the Periodic Table, which is synonymous to the modern Periodic Table today, is the most convenient and widely-used table.

It arranges the elements in horizontal rows and vertical columns based on atomic numbers, while keeping electronic configuration as the basis for periodicity in the properties of elements.

Similar to Mendeleev's Periodic Table, in the long form of periodic table, the horizontal rows were called periods, and the vertical columns were called groups

It has a total of seven periods based on the highest principal quantum number of the elements in that period.

Each period contains specific number of elements as per the principal quantum number.


   Number of elements















In both the sixth and seventh periods, only 18 elements are shown in the periodic table.

Remaining 14 elements of each of these two periods are shown in separate panels at the bottom of the table and are referred to as Lanthanides and Actinides.

The number of groups total 18, numbered from 1 to 18 as per the IUPAC standards.

As, the atomic number increases across a period, the outer electronic configuration changes gradually from one element to another which results gradual change in properties.

Both trans-uranium and trans-fermium elements are man-made elements synthesized in laboratories using costly and sophisticated equipment.

The long form of periodic table has a total of 117 elements. Of these, elements with atomic numbers up to 112, 114 and 116 have already been discovered whereas elements with atomic numbers 113, 115, 117 and 118 are not yet known

All the elements between Uranium (atomic number 92) and Fermium (atomic number of 100) are called trans-uranium elements.

The elements after Fermium, are called trans-fermium elements


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