Electronic Configuration in Periods and Groups

The Period in the Modern Periodic Table-

A horizontal row in the periodic table is called a period. The periodic table is divided into seven periods, each of which starts at the far left. When electrons start to fill a new primary energy level, a new period starts. There are eight elements in periods 2 and 3, compared to only two in period 1 (helium and hydrogen). This is the term defined for the modern periodic table. let us see some examples based on the same topic.

The period of any element is determined by the last shell in which the last electrons enter. For example, Fe's 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 the 4th subshell. Therefore, Fe belongs to the fourth period.

The Block in the Modern Periodic Table-

The atomic orbitals in which an element's valence electrons or vacancies are located unite the elements in a block of the periodic table. Charles Janet appears to have coined the phrase initially. A collection of elements found in nearby element groups is called an element block. The term was first used (in French) by Charles Janet. The spectroscopic lines of atomic orbitals that were described as sharp, primary, diffuse, and fundamental gave rise to the block names s, p, d, and f.
This phrase "block" appears in the current periodic table.

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 the s-subshell, then the group number is equal to the number of electrons in the last s-subshell

• 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 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 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 the 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.

Also Read:

• Nomenclature Of Elements With Atomic Number
• Classification Of Elements And Periodicity In Properties

Recommended topic video on ( Electronic configuration in Periods and Groups)

Solved Examples Based On Electronic configuration in periods and groups-

Example 1:Choose the correct option:

1) The period of the element is determined by its highest shell

2) The period of the element is determined by its last orbital

3) The period of the element is determined by its valence shell electrons

4) The period of the element is determined by its valency

Solution: The period of the element is determined by its highest shell.

Hence, the answer is the option (1).

Example 2: An element with atomic number 23 belongs to which period?

1) 3rd period

2) 2nd period

3) 5th period

4) 4th period

Solution: we have to check the last shell. The shell with a higher value of the principal quantum number(n) is the period for that element. The electronic configuration of an element having atomic number 23, is = [Ar] 3d³ 4s². The shell has a higher value of n is 4s. Therefore the period is = 4.

Hence, the answer is the option (4).

Example 3: 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.

Example 4: 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

Example 5: Mn has the 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.

Example 6: The element with atomic number 35 belongs to which group:

1) 13th group

2) 17th group

3) 16th group

4)2nd group

Solution: The electronic configuration of the element with atomic number 35 is:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵

Now, the group of the element is determined by the following formulae:

(i) For s-block: ns¹ and ns²

(ii) For p-block: 12 + number of electrons in the P-subshell of the last shell.

=12 + 5 =17th group

(iii) For d-block: 2 + number of electrons in (n-1)d subshell.

Hence, the answer is the option (2).

Practice more Questions from the link given below:

Conclusion

Understanding how electrons are arranged in atoms reveals why elements behave chemically as they do. The distribution of electrons across shells and subshells—shaped by principles like Aufbau, Pauli, and Hund—determines an element’s tendency to lose, gain, or share electrons, thereby influencing bond formation and stability. As electrons fill successive orbitals, predictable trends emerge: atomic size decreases and ionization energy increases across a period, while the opposite occurs down a group . This foundational concept not only explains chemical behavior but also guides technological advances—such as semiconductor engineering, catalysis, and the development of novel magnetic and pharmaceutical materials .