Summary

1.    With the discovery of new elements, scientists tried to arrange them in a scientific and organized manner, on the basis of their atomic masses. Dobereiner in 1829 was the first to classify the similar elements in the groups of three elements, known as "triads". Newland in 1863 then classified the elements in the increasing atomic masses and put forward the law of octaves. In 1869 German chemist Lothar Meyer improved the Newland's classification and plotted a graph between the atomic volumes and atomic masses of the elements. He included only 56 elements and discovered that the elements with similar properties occupied the similar positions on the curve.

2.    In 1869 Russian scientist Mendeleev arranged the elements in the increasing order of atomic masses and put forward a periodic law, known as Mendeleev's periodic law. In 1873, he published his periodic table which consisted of eight vertical columns, known as groups and twelve (12) horizontal rows known as periods. Many places were left vacant for undiscovered elements in his periodic table and predicted their properties. He also discovered correct atomic masses of many elements.

3.    In (1913-1914), after the discovery of atomic numbers, the elements were then arranged in their increasing atomic numbers and modern periodic law was introduced.
According to modern periodic law. "The physical and chemical properties of the elements are the periodic functions of their atomic numbers.

4.    According to modern periodic classifications, a long-form of periodic table has been introduced. It contains eight (8) groups and seven (7) periods. The groups are further subdivided into A and B sub-groups.

5.    In the periodic table, the properties of the elements gradually change in groups and periods. Down the groups generally the physical properties of the element slightly change with the increasing atomic numbers, while chemical properties remain same to some extent. Along the periods the physical and chemical properties both change to larger extent with the-increasing atomic numbers. The chemical properties depend upon the valence electrons present. The elements with similar electronic configurations in their valence shells have similar chemical properties.

6.    In the periodic table, elements on the left are metals, while on the right side are non-metals and in the middle, the elements are generally metalloids showing both the characteristics of metals and non-metals.

7.    The minimum amount of energy required to remove the most loosely bound electron from the valence electrons in a gaseous atom to form positive ion. i.e. cation is called first ionization energy or ionization potential and the minimum amount of energy released from a gaseous atom when an electron is added in the valence shell to form negative ion i.e. anion is called Electron affinity. Down the groups the values of ionization potential and electron affinity both decrease with the increasing atomic numbers due to increase in atomic sizes. While along the periods the values of ionization potential and electron affinity both increase with the increasing atomic numbers.

8.    The relative power of an atom to attract the shared pair of electrons towards itself is called electronegativity. The most electronegative atoms is that of fluorine (F) with electronegativity = 4.0. The repetition or recurrence of properties after regular intervals is called periodicity.

 

 
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