NAME:PURSHOTTAM . A . THAKUR

STD:X DIV:B

ROLL NO:42

TOPIC: PERIODIC TABLE

Dobereiner's triads

In the history of the periodic table , Dobereiner's triads were an early attempt at organizing the chemical elements by certain physical properties. In 1829, the German chemist Johann Wolfgang Dobereiner noted that there were triads of elements that showed similar chemical properties. He also noted that other properties of the elements followed trends in which the value of the middle element of the triad was in between those of the end members. For example, the atomic weight and density of the central element of the triad was at or nearly the mean of the atomic weights of the other two members. The modern periodic table has these triads of elements in adjacent spaces (either consecutive periods (rows) in a group (column) or consecutive groups in a period)

Example of Döbereiner's triads

John Alexander Reina Newlands

• Newlands was the first person to devise a periodic table of elements arranged in order of their relative atomic weights. Continuing Johann Wolfgang Döbereiner’s work with triads and Jean- Baptiste Dumas' families of similar elements, he published in 1865 his 'Law of octaves', which stated that "any given element will exhibit analogous behaviour to the eighth element following it in the table." Newlands’ arranged all of the known elements into seven groups, which he likened to the octaves of music. In Newlands' table, the elements were ordered by the atomic weights that were known at the time and were numbered sequentially to show their order . Periods were shown going down the table, with groups going across the opposite from the modern form of the periodic table.

• The incompleteness of the table alluded to the possible existence of additional, undiscovered elements, such as the element germanium, which was predicted by Newlands.

Newland’s Example

Mendeleev's Periodic Table

• Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.

• Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered. He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below Aluminum in his table. When this element, called gallium, was discovered in 1875 its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.

Modern Periodic Table

In 1913, Henry Moseley, an English physicist discovered that atomic number is the most fundamental property of an element and not its atomic mass. Atomic number(z) of an element is the number of protons n the nucleus of the atom . It also represents number of electrons present in the outer most shells. This discovery changed the whole perspective about elements and their properties. Accordingly, Mendeleev’s periodic law was modified into ‘Modern Periodic Law’.

The modern periodic law status “The chemical and physical properties of are a periodic function of their atomic numbers”.

The Periodic table was based on the Modern periodic law is called the Modern Periodic Table. Many versions of this table are in use but the one which is most commonly used is long form of modern periodic table.