Electronic Configuration

Imagine living in a lawless and unstructured environment where your belongings are scattered throughout your house. Finding your possession would be an absolute nightmare, would it not ?

Would you not prefer to live in an orderly fashion. Thats the same notion on which scientists clubbed all known elements into a periodic table where they could study the physical and chemical properties and try to find some resemblance. For example, we know generally group 18 elements are inert in nature but noble gases having higher atomic mass like Kr, Xe and Rn form some compounds 

(KrF₂, XeF₆, RnF₂. From this information, can it be guessed whether O ganesson (Og) (Z =118) can also form compounds?

Electrons determine the chemical behavior of an atom. The electronic configuration of atoms is very useful in understanding the periodic table, their chemical and physical behaviour, etc. A set of rules are defined to write the correct electronic configuration of any element but only mathematical interpretation is not science. 

Table of contents

• What is Electronic Configuration?
• Prerequisites of writing electronic configurations
• Electronic Configuration of Elements
• Rules to write condensed electronic configuration or noble gas configuration
•  Few Examples of Condensed Electronic Configurations
• Condensed Electronic Configuration or Noble Gas Configuration
• Rules to Identify Group Number, Period Number & Block Name of Elements
• Exceptional Electronic Configurations
• Practice Problems on Electronic Configuration 
• FAQ's on Electronic Configuration

What is Electronic Configuration?

Electrons are filled in orbitals of an element under a set of  parameters dependent on energy, spin orientation and indistinguishability of electrons.

Prerequisites of Writing Electronic Configurations

• Aufbau's principle
• Pauli’s exclusion principle
• Hund’s rule for maximum multiplicity

Electronic Configuration of Elements

Rules to Write Condensed Electronic Configuration or Noble Gas Configuration

• Electrons are filled in valence or outermost shell according to given order

Example: Write the condensed electronic configuration of flerovium (Fl₁₁₄)

Answer: We know, the last element of period 6 is Rn₈₆.

114-86 = 28 only we have arranged 28 electrons in their valence, penultimate & anti-penultimate shells according to increasing (n+1)rule

This element belongs to period 7 because period 6 is completely filled and the last element of period 7 is

Note: subshells having same (n+1) value, subshell with the lower value of n is preferred

Few Examples of Condensed Electronic Configurations 

Rules to Identify Group Number, Period Number & Block Name of Elements

• Period number: Value of n (principal quantum number) of valence shell or outermost shell decide the number of period of element

• Block name: The last electron enters in which subshell, elements generally belongs to that block

Eg.  

• Group number: Generally, the group number is decided by the number of valence electrons or valence electrons and electrons of the penultimate shell.

1. For s block elements- the group number is equal to the number of valence electrons(n).
2. For p block elements- the group number is equal to the number of valence electrons (n)+ 10.
3. For d block elements- the group number is equal to the number of valence electrons(n) + number of electrons in the penultimate shell (n-1) d orbital.

E.g  

Exceptional Electronic Configurations

These configurations are not an exception of chemistry but the exception of only (n+1) theory. We can’t predict nature with an equation of simple mathematics. These configurations are written on the basis of their practical behavior measured in the lab. To satisfy those lab results we write arrangements of electrons that slightly deviates from (n+1) rule.

Practice Problems on Electronic Configuration

Question 1. Predict the group number, period number & block name respectively of elements (moscovium) having an atomic number 115

(A). 13, 6 & s block
(B). 14, 7 & p block
(C). 15, 7 & p block
(D). 16, 7 & d block

Answer: (C)

We know, the last element of period 6 is Rn₈₆.

115-86 = 29 only we have arranged 29 electrons in their valence, penultimate & antepenultimate shells according to increasing (n+1) rule

This element belongs to period 7 because period 6 is completely filled and the last element of period 7 is Og₁₁₈. 

So, n = 7, (n-1) = 6 and (n-2) =5

Note: subshells having same (n+1) value, subshell with the lower value of n is preferred

Block name = p block (last electron enter’s in p subshell)

Group number = 10 + 5 = 15

Period number = 7

Question 2. Predict the group number, period number & block name respectively of elements having an atomic number 69. 

(A). 3, 6 & s block
(B). 3, 6 & f block
(C). 14, 6 & p block
(D). 13, 6 & d block

Answer: (B)

 Writing electronic configuration and finding its group number and the period number is not a good approach.

In these types of questions always remember the given element is Lanthanide or actinide or not. It will save your time

Lanthanide: Ce₅₈ to Lu₇₁ (all elements belong to group 3, period 6, and f block)

Actinides: Th₉₀ to Lr₁₀₃ (all elements belong to group 3, period 7, and f block)

Question 3. Find the number of electrons in the penultimate shell of chromium. 

(A). 1
(B). 13
(C). 2
(D). 5

Answer: (B)

Question 4. Find the number of electrons in the ‘L’ shell of calcium (Sc₂₁). 

(A). 2
(B). 8
(C). 9
(D). 5

Answer: (C)

FAQ's on Electronic Configuration

Question 1. Why electronic configuration of Palladium is instead of ?

Answer: Arrangements of electrons are based on elements natural behavior 

Case 1: , number of unpaired electrons = 2

Case 2: , number of unpaired electrons = 0

In these cases due to different numbers of unpaired electrons their magnetic behavior can be changed. In the case of fully paired electrons, it behaves as diamagnetic and in the case of unpaired electrons, it behaves as paramagnetic.

Question 2. Does Actinium (Ac₈₆) belong to f block elements?

Answer: Actually, Actinium should be a d-block element, due to its close resemblance with Actinides. So, usually included in all discussions of the lanthanides.

Question 3. Is there any element in the d block which has zero electrons in its valence subshells?

Answer: Yes, pd₄₆ = 

Question 4. How many elements in the 4d series don’t follow (n+1) rule and have exceptional configurations?

Answer: elements having exceptional configurations:

4d series ( Y to Cd)- Nb, Mo, Tc, Ru, Rh, Pd, and Ag (only Y, Zr and Cd follow (n+1)rule)

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