CBSE Class 10 Answered

First period contains 2 elements and is called very short period. Second and third periods contain 8 elements each and are called as short periods.

The occurrence of these elements in first, second and third period can be explained by keeping the following points in mind:

• All the elements in the periodic table are arranged in the order of their increasing atomic number.
• Each period in the periodic table starts with the beginning of a new energy level. Example, in case of first period the K-shell starts getting filled while in case of second period L-shell starts getting filled.
• Filling of electron shells takes place according to well defined set of rules.
  • The first period begins with the filling of first energy shell (K-shell). First energy shell can accommodate only 2 electrons in it. Hence, there are only 2 elements in the first period.
  • The second period begins with the filling of second energy shell (L-shell). Second energy shell can accommodate only 8 electrons in it. Hence, there are only 8 elements in the second period.
  • After the addition of 8 electrons to the second shell, the second shell becomes completely filled. Now, the next electron enters the third shell and the elements now go to the third period. The third shell has the capacity to accommodate 18 electrons. However, after the third shell receives 8 electrons, the filling of forth shell begins. This is because the outermost shell does not have capacity to hold more than 8 electrons. Therefore, third period has only 8 elements.