According to atomic structure what determines which element will be the first and which will be the last in a period?

Consider the following examples.

For Sodium:

 

1. First take the Atomic number of the given element. Say Na. Atomic no. 11
2. Then calculate the electronic configuration using the atomic number 2+8+1=11 therefore the electronic configuration will be 2, 8, 1.
3.     Here we find that the no of valence electrons of element is 1 as the valence shell has 1 electron.
4.   So know we easily understand that the group is 1 because the no of valence electrons is 1.
5.    Now to find the period we have to understand that the period is the number of shell the element is having therefore here sodium has 3 shells (K-2, L-8, and M-1). So the period of the given element is 3.

Thus the position of the given element (here sodium) is ….

Group	Group 1	Valence electrons = 1
Period	3rd	Number of shells = 3

 

When the valence shell has 1 electron, it indicates the first element of the period.

For Li: 2,1

For Na: 2,8,1

For K: 2,8,8,1

So sodium is placed in Group 1 because it contains 1 electron in the outermost shell and period 3 since it contains 3 shells.

 

For Chlorine:

 

1. First take the Atomic number of the given element. Say Cl... Atomic no. 17
2. Then calculate the electronic configuration using the atomic number 2+8+7=17 therefore the electronic configuration will be 2, 8, 7.
3.     Here we find that the no of valence electrons of element is 7 as the valence shell has 7 electrons.
4.   So know we easily understand that the group is 7 because the no of valence electrons is 7.
5.    Now to find the period we have to understand that the period is the number of shell the element is having therefore here chlorine has 3 shells (K-2, L-8, and M-7). So the period of the given element is 3.

Thus the position of the given element (here chlorine) is …..

Group	VII A (17th )	Valence electrons = 7
Period	3rd	Number of shells = 3

 

 

When the valence shell has 7 electrons, it indicates the last element of the period.

For F: 2,7

For Cl: 2,8,7

So chlorine is placed in Group 17  because it contains 7 electrons in the outermost shell and period 3 since it contains 3 shells.