 |
Introduction |
| |
In the famous atomic theory of John Dalton (1805), it was suggested that the atoms of an element have a characteristic mass. So, attempts were made to classify elements on the basis of their atomic masses. |
|
Dobereiner's Triads |
| |
In the famous atomic theory of John Dalton (1805), it was suggested that the atoms of an element have a characteristic mass. So, attempts were made to classify elements on the basis of their atomic masses. |
|
Newland's Octaves |
| |
John Alexander Reina Newland was a chemist as well as a lover of music. He arranged many of the known elements in the increasing order of their atomic masses. It was noticed that the eighth element was similar in properties to the first element, just like the eighth note in music - Western as well as Indian. |
|
Mendeleev's Periodic Law |
| |
Later, Mendeleev arranged the sixty-three elements known at that time in the increasing order of the atomic masses, in the form of a table called the Periodic Table. The periodic table further classified the elements by arranging the elements with similar properties together and separating the elements with dissimilar properties from one another. |
|
Modern Periodic Table-Long Form Periodic Table of Elements |
| |
Later, Henry Gywn-Jeffreys Moseley showed that the atomic number of an element is numerically equal to the number of electrons round the nucleus. The number of electrons in turn is equal to the number of protons in the nucleus. He suggested that atomic number is a more fundamental property of an element than its atomic mass. When the elements are arranged in the increasing order of their atomic number, most of the defects of Mendeleev's classification get rectified. |
|
The Seven Periods |
| |
1. First period
The first period has two elements - hydrogen and helium.
2. Second and third period
The second and third periods are known as short periods. Each consists of eight elements. The second period starts with lithium (Z = 3) and ends with inert gas neon (Z = 10). The third period starts with sodium (Z = 11) and ends with argon (Z = 18). |
|
Main Features of the Long Form of the Periodic Table |
| |
The properties of an element in the periodic table mainly depend on its outer electronic configuration, except in the case of transition elements. |
|
Representative Periodic Table for 8 Groups upto Calcium (Atomic Number -20) |
| |
Representative elements are those elements whose inner shells are complete and whose outer shell (valency shell) has 1 to 7 electrons. |
|
General Characteristics of Group |
| |
The members of a certain group possess similar properties but the reactivity may differ, e.g., in case of group I A the reactivity increases as we move down the group from lithium to cesium but in case of VII A it decreases from fluorine to iodine. |
|
General Characteristics of Periods |
| |
Valence shell electrons : The valence shell electrons increase form one to seven as we move from left to right in a period. |
|
Advantages of Long form Periodic Table |
| |
1. The classification of elements is based on the atomic number, which is a more fundamental property.
2. The reason for placing isotopes at one place is justified as the classification is on the basis of atomic number. |
|
Defects of the Long form Periodic Table |
| |
The position of hydrogen is still not settled since it shows properties of both alkali metals as well as halogens. |
|
Types of Elements |
| |
On the basis of electronic configuration, the elements of the periodic table are classified into:
(1) Noble gases, (2) Normal elements, (3) Transition elements, (4) Inner-transition elements, (5) Alkali metals and (6) Halogens. |
|
Summary |
| |
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. |
