Electron Configuration

Electron configuration is the distribution of electrons in atomic shells (also called orbits or energy levels) and subshells (also called orbitals and sub energy levels) . Electron configuration is written in standard notation  which shows shell number, subshell of that shell and number of electrons as superscript as given below.

1s²

Coefficient 1 showing n=1 i.e “K”

S showing subshell

Superscript 2 is showing number of electrons

1s2 notation is telling us that "s" subshell of K (n=1) shell is having 2 electrons.  In Electron configuration electron containing subshells with number of electrons present in it, are placed in standard sequence( increasing energy level).

Firstly we take detail review of shells and subshells.

 Shells:

• Shells are major energy pathways around the nucleus in which electrons revolve.

• The maximum number of electrons present in a shell is determined by principal quantum number (n).

• Number of electrons in a shell are calculated by a formula 2n2, here "n" is number of shell can be n=1,2,3,4, and so on.  

• Shells are designated by alphabet K,L,M,N O...etc.   

Table showing the shell number, name and number of electrons present in it.

Shell number showing electrons number in it.

Subshells:

• ^{Shells are further splitted into subshells - there are four Subshells s,p,d and f- Subshells values are determined by Azimuthal quantum number"l". The formula to calculate the number of Subshells is l=n-1. So,}Azimuthal quantum number (l) depends on  principal quantum number (n).

• Azimuthal quantum number (l) Values for subshells s,p,d and f are :

If calculated value for 

I=0 here 0 is value for "s" Subshell and just one subshell is present.

l=1 here 1 is for p and telling upto p both  subshells i.e s and p are present.

l=2 here 2 is for d and telling upto d three Subshells i.e s,p, and d are present. 

l=3 here 3 is for f and telling go upto f forth subshell i.e sp,d,and f are present .

If, n=1 then l=0 shows "K" shell is not further divided having one subshell. This can be calculated by formula as:

n=1 then
 l= n-1

l= 1-1=0. 

0 value is representing  two things

1. First Shell (K) is not further divided into subshell.
2.  Just one subshell is present which is "1s". Given above that l=0 mean subshell is "s". 1coefficient with "s" representing shell number. 

n =2 then 

 l= n- 1

 l= 2-1=1

l=1 showing two informations 

1. Second shell  L(n=2) shell is having 2 subshells
2. Two subshells with values of 0 upto 1 i.e s and p. ( 0 for s, 1 for p). Here these subshell will be 2s and 2p as shell number n=2.First four shells values are given in table below.

By careful look at the table we will conclude that n value is equal to number of subshells present in shell.

• Number of electrons in each subshell is calculated by 2( 2l+1).

Numbers of electrons in subshells.

Rules /Principles use in Electron configuration.

1. Electron configuration shows the arrangement of shells and subshells in increasing energy order.energy order is decided by Auf bau principle, states that electrons will occupy less energy subshells before entering into high energy subshells .To arrange shells and Subshells in increasing  energy order we have n+l rule.To arrange shells and subshells in increasing energy order take some example for understanding:
   

for 1s , n=1 and s=0

hence n+l = 1 + 0 =1 

for 2s, n=2 and s=0

hence n+l = 2 +0 =2

for 3s, n=3, and s =0

hence n+l = 3 +0 =3

for 2p, n=2, p=1

hence n + l = 2 + 1 = 3

In case of 3s and 2p both are having same value i.e 3 in such case orbital with small principle number will be lower in energy.

for 4s , n=4 and s =0

hence n+l=4+0 =4

for 3p, n=3, p=1

hence n+l=3+1 = 4

for 3d, n=3 and l=2

hence, n+l = 3 +2 =5 

Above 4s and 3d are having 4 and 5 values respectively so 4s will be before the 3d. 

1s,2s,2p,3s,3p,4s,3d, 4p,......

The energy order for filling the electrons in orbitals according to "Auf bau" principle can be illustrated by following diagram.   

To arrange shells and Subshells in increasing energy order read from arrow tail to head then order will be 

Is,2s, 2p 3s, 3p, 4s, 3d, 4p 5s, 4d, 5p ,6s, 4f, 5d,6p, 7s....

There are noted exceptions in electron configuration of few elements like chromium and copper. This exception can be explained by the stability of half filled or completely filled orbitals.

 

2. Pauli Exclusion Principle states

• Maximum 2 electrons with opposite spin can accommodate in an orbital. 
• Or this principle can be stated as no two electrons with all same quantum numbers can be in an orbital. If two electrons which are accommodating same orbital is having same first three quantum number the forth quantum number ( spin quantum number) must be different. 

3. Hund's Rule states that in case of filling the degenerate orbitals firstly each degenerate orbital will receive the one electron with same spin and then pairing with opposite spin will occur

• 
  
  

 For example we take electron configuration of carbon, nitrogen and oxygen, flourine and neon.

Look at carbon electron configuration last two electrons separately fill the 2px and 2py with same spin rather going to 2px with opposite spin. 

Similarly in case of nitrogen electron configuration three electrons with same spin occupy the 2px,2pyand 2pz. 

In case of oxygen, flourine and neon electrons now start pairing up with opposite spins. This all is explained by Hund's rule.

Electron configuration of first ten (10) elements are given as

₁H¹ = 1s¹

₂He= 1s²

₃Li= 1s^2, 2s¹

₄Be= 1s^2, 2s²

₅B = 1s^2, 2s², 2p¹

₆C = 1s^2, 2s², 2p²

₇N = 1s^2, 2s², 2p³

₈O = 1s^2, 2s², 2p⁴

₉F = 1s^2, 2s², 2p⁵₁

₁₀ Ne = 1s^2, 2s², 2p⁶