Electronic Configuration in Periods and Groups - Practice Questions & MCQ

Period

The period of any element is determined by the last shell in which the last electrons enters. For example, Fe atomic number is 26. The electronic configuration can be written as:

1s²2s²2p⁶3s²3p⁶4s²3d⁶

Now, the last electron enters into the d-subshell but electrons are also present in 4th subshell. Therefore, Fe belongs to fourth period.

Block

The block of any element is determined by the last subshell in which the last electron enters. For example, Na has atomic number 11, thus its electronic configuration can be written as: 

1s²2s²2p⁶3s¹

Now, its last electron enters into the s-subshell, therefore, Na belongs to the s-block.

Group

The group of any element is determined in different ways.

• For s-block

If the last electron of any element enters into the s-subshell, then the group number is equal to the number of electrons in the last s-subshell.

For example, K has atomic number 19, thus its electronic configuration can be written as: 

1s²2s²2p⁶3s²3p⁶4s¹ 

Now it has 1 electron in the s-subshell, therefore K belongs to Group 1.

• For p-block

If the last electron of any element enters into the p-subshell, then the group number is equal to (12 + the number of electrons in the last p-subshell).

For example, Ge has atomic number 32, thus its electronic configuration can be written as: 

1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p²

           Now, it has 2 electrons in the last p-subshell, therefore its group number is:

12 + 2 = 14

           Thus, Ge belongs to Group 14

• For d-block

If the last electron of any element enters into the d-subshell, then the group number is equal to (2 + the number of electrons in (n-1)d-subshell.

For example, Mn has atomic number 25, thus its electronic configuration can be written as: 

1s²2s²2p⁶3s²3p⁶4s²3d⁵

Now it has 5 electrons in the d-subshell, therefore its group number is:

2 + 5 = 7

Thus, Mn belongs to Group 7.

• For f-block

There are only two series of f-block i.e, lanthanide and actinide. If the last electron of any element enters into the f-subshell and if the atomic number is between 57-71, then the element belongs to lanthanide series i.e, 6th period. Further, if the last electron of any element enters into the f-subshell and if the atomic number is between 89-103, then the element belongs to the actinide series i.e, 7th period. All the elements from both these series belong to group 3.