Electronic Configuration of Elements
We already know that electrons orbit the nucleus in an atom, while protons and neutrons are contained within it. How do electrons revolve, is the question? Do they appear to be moving at random? Is there a set path they take? What’s the best way to put them together? Electronic Configuration is the answer to all of these queries. Let’s have a look at how electrons are arranged around a nucleus.
Distribution of Electrons in Different Orbits
The planetary model of an atom was created by Neils Bohr. He was the first to indicate that the properties of the elements have a regular pattern. The electronic structure of an atom is based on the “Bohr atomic model.” He was the first to describe the arrangement of electrons in distinct orbits/shells (electronic configuration). He proposed that electrons are spread in electronic shells that are circular in shape (orbits). These electrons travel a defined distance around the nucleus in orbits.
Before understanding the concept of Electronic Configuration, lets first understand some basic terms used with it as mentioned below:
 Shells: It is based on the principal quantum number (n) which determines the maximum number of electrons that can be placed in a shell. The number of electrons in each shell is given by the formula 2n^{2} where n represents shell number.
 Subshells: It is based on an azimuthal quantum number which is widely denoted by ‘I’. At the same time, the azimuthal quantum number depends on the value of the principal quantum number (n). For example, if the value of shell n=3. The subshells corresponding to it will be s,p, and d with I=0, I=1, and I=2. These are the shells in which the actual distribution of electrons takes place. And the maximum number of electrons accommodated by a subshell is given by the formula 2(2I+1). The possible subshells and their resulting electronic configuration based on quantum number are given below.
Value of principal quantum number  Azimuthal quantum number  Electronic configuration 

n=1 
I=0 
1s 
n=2 
I=0 
2s 

I=1 
2p 
n=3 
I=0 
3s 

I=1 
3p 

I=2 
3d 
n=4 
I=0 
4s 

I=1 
4p 

I=2 
4d 

I=3 
4f 
Electronic Configuration
The representation of electrons distributed in atomic shells of an element is known as electronic configuration. The electrons are mathematically located in these subshells and the notations help in locating the position of these electrons as well as the electronic configuration.
An effective study of these electronic configurations written with specific notation can provide details about the particular element. For example, the specific notation for germanium (Ge) would be,1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{2} .
Notation of Electronic Configuration:
Notation is the representation of the number of electrons present in the subshell. It is written with the shell number, name of the subshell, and a total number of electrons present in the subshell in superscript.
For example, the electronic configuration of oxygen can be written as 1s^{2} 2s^{2} 2p^{4}.
The below table consists of the electronic configuration of the first 20 elements.
Atomic Number 
Name of Elements 
Electronic Configuration 

1  Hydrogen  1s^{1} 
2  Helium  1s^{2} 
3  Lithium  1s^{2 }2s^{1} 
4  Beryllium  1s^{2 }2s^{2} 
5  Boron  1s^{2} 2s^{2} 2p^{1} 
6  Carbon  1s^{2} 2s^{2} 2p^{2} 
7  Nitrogen  1s^{2} 2s^{2} 2p^{3} 
8  Oxygen  1s^{2} 2s^{2} 2p^{4} 
9  Fluorine  1s^{2} 2s^{2} 2p^{5} 
10  Neon  1s^{2} 2s^{2} 2p^{6} 
11  Sodium  1s^{2 }2s^{2} 2p^{6} 3s^{1} 
12  Magnesium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 
13  Aluminum  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{1} 
14  Silicon  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{2} 
15  Phosphorus  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{3} 
16  Sulfur  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4} 
17  Chlorine  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{5} 
18  Argon  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 
19  Potassium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 
20  Calcium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 
Principles for Filling the Shells
Following are the rules or principles that help to fill the shells of an atom:
 Aufbau Principle: The principle states that the electrons will occupy the orbits with lower energy and then the orbits of higher energy levels. The principle gets its name from the Greek word “Aufbeen” which means building up. As per the following principle the electrons are arranged in the order as,
 Pauli Exclusion Principle: The Pauli Exclusion Principle states that the maximum of two electrons can fit into an orbit with opposite spins. It is based on the concept that if two electrons carry equal azimuthal numbers then they will have opposite spins in the orbit.
 Hund’s Rule: The rule states that every orbit is initially occupied by a single electron before a second electron fills the orbit. It’s the rule that determines the order of electrons in the subshell.
How to write the electronic configuration of elements?
So, before drawing an electronic configuration we need to extract some information from the periodic table like the atomic number, no. of electrons, shells, etc. Looking through an example let’s understand the writing method of an electronic configuration. The potassium element has atomic number 19. And has 19 electrons which will be placed in s and p subshell.
The electronic configuration can be written as 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1}. Its 19 electrons can be divided into different shells in a manner as:
 K shell (n=1)= 2,
 L shell (n=2) =8,
 M shell (n=3)=8, and
 N shell (n=4)=1.
Sample Questions
Question 1: Write down the electronic configuration of Aluminum.
Answer:
The electronic configuration of Aluminum (Al) is 1s^{2} 2s^{2 }2p^{6} 3s^{2} 3p^{1} in spdf notation and 2, 8, 3 in orbital diagram.
Question 2: List the significance of electronic configuration.
Answer:
The significance are mentioned below:
 They help to know the reactivity state of the atom.
 It helps to determine its physical as well as chemical properties.
 It predicts an atom’s magnetic behavior, etc.
Question 3: Write down the electronic configuration of ideal gases.
Answer:
Following are the electronic configuration of some ideal gases:
 Helium (He) ⇒ 1s^{2}
 Neon (Ne) ⇒ 1s^{2 }2s^{2} 2p^{6}
 Argon (Ar) ⇒ 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6 }
 Krypton (Kr) ⇒ 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6}
 Xenon (Xe) ⇒ 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 5s^{2} 5p^{6}
 Radon (Rn) ⇒ 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 5s^{2} 5p^{6} 4f^{14} 5d^{10} 6s^{2} 6p^{6}
Question 4: How does copper disobey the Aufbau principle?
Answer:
Since, the electronic configuration of Copper is 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{1}. The violation of the Aufbau principle is due to the energy gap between 3d and 4s orbitals. As the completely filled dorbital offers more stability than the partially filled configuration.
Question 5: How many electrons does each of the subshells hold?
Answer:
The electron filling pattern in subshells is:
 s holds 2 electrons
 p holds 6 electrons
 d holds 10 electrons
 f holds 14 electrons
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