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The arrangement of elements in a group of three is known as ____________.

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Hint: In the history of periodic tables, a German scientist found out a group of 3 elements with similar properties. This group of three elements were named after him. You can use this to find the correct answer here.

Complete answer:
We know that the modern periodic table that we study today was compiled together after several attempts by scientists world-wide and new elements are being added in it even today.
To sort the elements in logical order according to their physical properties, a German scientist named Johann Wolfgang Dobereiner found out groups of 3 elements having similar properties and named it Dobereiner’s triads.
His observations stated that strontium had properties intermediate to that of calcium and barium in 1817 and by 1829, he found other elements in a group of three whose physical properties were closely related and named them Dobereiner’s triads. He also stated that the atomic weight and density of the middle element in the triad could be predicted by taking the value of the arithmetic mean of that property of the other two elements.
We can understand from the above discussion that the arrangement of elements in a group of three is known as Dobereiner’s triads and this is the required answer.

Note:
The first ever periodic table was created by a Russian chemist named Dmitri Mendeleev. Mendeleev’s periodic table has 63 elements. They were placed in a tabular form in increasing order of their relative atomic masses. There were certain limitations to Mendeleev’s periodic table which lead to the modern periodic table.
The modern periodic table is a tabular display of chemical elements found in nature. There are 118 elements in the periodic table. They are arranged according to their atomic number, electronic configuration and repetitive chemical properties.