# 27.THE ATOMIC STRUCTURE – Quantum Numbers.

In the last post we saw how quantum numbers were introduced to understand the structure of the atom.These quantum numbers described all the necessary parameters about an electron in a given orbit.Let us begin by understanding what these quantum numbers exactly tell us.

#### A] The Principal Quantum number (n)–

This parameter divulges information about the size of the orbit , its distance from the nucleus and thus orbital energy.

e.g.- If n=1 , then it indicates that we are looking at the orbit which is closest to the nucleus  i.e the ‘K’ shell.

n=2 indicates the second orbit from the nucleus i.e ‘L’ shell.This orbit is farther from the nucleus when compared with the ‘K’Shell. The maximum number of electrons allowed in any principal shell is given by 2n2.

∴ For first      K shell,   n= 1 , allowed no of electrons are 2(1)2= 2.
For second L shell,   n= 2 , allowed no of electrons are 2(2)2= 8.
For third    M shell,  n= 3 , allowed no of electrons are 2(3)2= 18.
For second N shell,  n= 4 , allowed no of electrons are 2(4)232.

#### B] Azimuthal or Orbital quantum number(ℓ)–

According to Bohr-Sommerfield model,not only do electrons travel in certain orbits(quantisation) but the orbits have different shapes.This quantum number denotes the shape or angular distribution of the orbital.It can take values from 0 to n-1 , in steps of unity,where n is the principal quantum number.

When ℓ = 0 , we have an s-orbital , which is spherical.This gives sharp lines in the spectra.
ℓ = 1 , indicates a p – orbital ,which is elliptical/elongated.
ℓ =2 ,indicates a d- orbital ,which has a complex shape.
ℓ = 3, indicates a f – orbital ,which again has a complex shape. The spectroscopic notations for these are  s⇒ sharp lines
p⇒principle lines
d⇒diffuse lines
f⇒fine/fundamental lines.

These are just characterisations of the kind of lines that were observed by the spectroscopists for each orbital and thus the names s,p,d,f .

#### C] Magnetic Quantum number (m) –

Another concept improving the Bohr model was the discovery that the orbitals don’t have to lie in the same plane.They can be  oriented in different directions in space. This orientation of the orbitals in space is described by the magnetic quantum number. So what if the p orbital is elongated and has a dumbbell shape! How does it look in 3- dimensions? How is that shape oriented in space? The magnetic quantum number provides an answer to this question.m = –ℓ..0..+ℓ.

As the s-orbital is spherical, = 0,m=0. This means that the s-orbital is evenly distributed in space i.e its spherical.

But for p-orbital, ℓ=1. So, m= -1,0,+1. Thus, there are three types of orientations in space for the p-orbital(as shown in the figure below).It could either be on the x-axis or y-axis or z-axis.Thus, we have px,py,pz orbitals.

m=-1 ↔ py.
m=0   ↔ pz.
m=+1  ↔px. When  ℓ=1, m=-2,-1,0,1,2, which represent the five d-orbitals dxy,dyz,dxz,dx2-y2,dz2.

The structure of the seven f-orbitals,namely , 4fy3 – 3x2y, 4fxyz , 4f5yz2 – yr2  ,4f5yz2 – yr2,4f5xz2 – xr2,4fzx2 – zy2,4fx3 – 3xy2,is more complex and can be represented diagrammatically  as follows –

The electrons occupy the f-orbitals only after the element Cerium(Atomic weight = 58).These f- orbitals are under the valence shell and rarely play an important role during reactions.Thus, they are not studied in detail at a preliminary level.

#### D]Spin Quantum number (s)–

This quantum number is required to explain the magnetic properties of substances. Just likethe earth, the electron not only revolves around the nucleus but also spins around itself.It can spin in clockwise or anti-clockwise direction.A moving charge always generates a magnetic field(the direction of the field can be found out by right hand rule where 4 fingers of the hand represent the direction of motion and the streched thumb represents the direction of magnetic field thus produced).Two orientations are possible for electrons dependin on whether they are moving clockwise or counter clockwise as follows – In the above figure, one electron is spinning clockwise and other is moving  anti-clockwise,thus  the two electrons have opposite magnetic orientations (see North pole = N and South pole = S of the induced magnetic field).These two spins would cancel each other out.However, in the case of an unpaired electron, the spin is what generates a magnetic field , which gives rise to a number of magnetic properties.

Thus, for each value of magnetic quantum number(m), spin quantum number(s) has two values ,s= +1/2 and s=-1/2.

So, now we have 4 set of quantum numbers which help us visualise the structure of  the atom in a new light.

 Principal Quantum Number Azimuthal/Orbital Quantum number Magnetic Quantum Number Spin Quantum Number Symbols n ℓ m s Spectroscopic notations K,L,M,N.. s,p,d,f – – What does it tell? Size of the orbit,Distance from nucleus & Orbital energy Shape of the orbital,no.of subt-shells present in the principal orbit/shell. Orientations of the sub-shells(X,Y,Z axis). Direction of electron spin i.e Clockwise or Anti-clockwise. Why is it required? To explain the main spectral lines. To explain the splitting and the hyperfine structure of spectral lines To explain the splitting of lines in a magnetic field. To explain the magnetic properties of substances.

How do we use these to write the orbital names ? Check the table below –

 n ℓ m Orbital Name Allowed no of electrons in the shell(no.of orbitals ✖️2) n=1 ℓ=0  (s-orbital) 0 1s (one orbital) 2 n=2 ℓ = 0  (s-orbital). ℓ = 1  (p-orbital). m= -1,0,+1 (py,pz,px) 2s 2py,2pz,2px (Four orbitals) 8 n=3 ℓ = 0 (s-orbital). ℓ = 1 (p-orbital). ℓ = 2 (d-orbital). m=0 m= -1,0,+1. m= -2,-1,0,+1,+2. 3s 3py,3pz,3px 3dxy,3dyz,3dxz, 3dx2-y2,3dz2 (nine orbitals) 18

We shall begin discussing in the next post how these quantum numbers were useful in the further study of the elements.Till then ,

Be a perpetual student of life and keep learning…

Good day !

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