- The grouping of elements with similar properties together and the separation of elements with dissimilar properties is known as classification of elements. The table, which classifies elements on the basis of their properties, is called the periodic table. Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves. Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.
- Dobereiner's Triads arranged elements in an increasing order of atomic mass, in groups of three. The atomic mass of the middle element was the arithmetic mean of the other two elements of the triad.
- Newland's law of octaves states that on arranging elements in increasing order of their atomic mass, the eighth element resembles the first in physical and chemical properties, just like the eighth node on a musical scale resembles the first note.
- According to Mendeleev's periodic law, the physical and chemical properties of elements are periodic functions of their atomic mass. Mendeleev corrected the atomic masses of a few elements on the basis of their positions in the periodic table. Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.
- Mendeléev’s Periodic Table contains vertical columns called ‘groups’and horizontal rows called ‘periods’. While developing the Periodic Table, there were a few instances where Mendeléev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass. The sequence was inverted so that elements with similar properties could be grouped together. Mendeleev's table could not assign a proper position to hydrogen or to the lanthanides and actinides and isotopes. Isotopes of all elements posed a challenge to Mendeleev’s Periodic Law. Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements — especially when we consider the heavier elements.
- In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law.
- The vertical columns are called groups, while the horizontal rows are called periods. There are 7 periods and 8 groups subdivided into 18 sub groups. The noble gases are on the extreme right of the table and on the table's extreme left, are the alkali metals. Transition elements are placed in the B subgroups in the middle of the table. The inner transition elements - lanthanides and actinides, are placed in two separate series at the bottom of the periodic table. Group number is number of electrons in the valence shell. Elements having the same valence number, are grouped together. The number of shells present in the atom gives period number.
- Atomic size: The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.
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