Electrons Behaviour in an Atom. What are orbits, Orbitals, Shells, Sub-shells, Quantum state & Quantum Numbers ??

Electrons behaviour in an Atom. What are Orbits, Orbitals, Shells, Subshells, Quantum states & Quantum Numbers?

Chemistry is mostly the study of electron interactions between atoms and molecules. Understanding the behavior of the electrons in an atom is an important part for understanding chemical reactions. We all know that Electrons are the negatively charged subatomic particles which revolve round the nucleus of an atom in specific orbits.

What are Orbits ????

Orbits are the paths in which an electron travels around the nucleus of an atom. But according to Heisenberg's Uncertainity Principle it is impossible to predict where an electron is and where it's going next.Thus it is impossible to locate the exact path in which the electron revolve round the nucleus and hence orbits cannot exist.

What are Orbitals ????

An Orbital is basically the region where the probability of finding the electron is maximum. Electrons are constantly spinning in these atomic orbitals and those shells or orbitals, are at specific distances from the nucleus.

Below is the very clear and straight forward explaination about shells, subshells and orbitals.

Electrons revolve around the nucleus in the Electronic shells :- K, L, M, N, O, P & Q

Each Electronic shell consists of some sub-shells :-

1. K contains only one sub-shell : s

2. L contains two subshells : s & p

3. M contains three subshells : s, p & d

4. N, O, P & Q contain 4 subshells : s, p, d & f .

Each sub-shell consists of some orbitals where the electrons are

present. Each orbital can hold up to 2 electrons.

1) s contain 1 orbitals and can hold upto 2 electrons.

2) p contain 3 orbitals and can hold upto 6 electrons.

3) d contain 5 orbitals and can hold upto 10 electrons.

4) f contain 7 orbitals and can hold upto 14 electrons.

Thus the Electrons have a fixed amount of energy depending upon the orbitals in which they are present. Hence they do not fall in to nucleus. Electrons in the first orbit closest to the nucleus has the lowest energy whereas electrons in the subsequent orbits posses higher energy. Electrons in an atom occupy shells in the increasing order of their energies.

Quantum Numbers and Quantum States :- 

Quantum numbers are often used to describe specifically the energies and shapes of orbitals in atoms.

Each electron in an atom is described by four different quantum numbers, they are : n , l , ml and ms .

1) The Principal Quantum Number " n " :- 

n is called the Principle Quantum Number. It is also known as the radial quantum number as it also defines the distance of the electron from the nucleus. The principal quantum number has integral values of n = 1, 2, 3... And as the value of n increases, the farther is the electron from the nucleus, the larger is the size of the orbital,and the larger is the Atom. n cannot be 0 or any negative integer, because there exists no atoms with zero or a negative amount of energy.

The shell "K" has been given the value n = 1, the"L" shell has been given the value n = 2 & so on...

Thus,      n 1  2  3 4

          shell K L M N ...

2) The Orbital Angular Momentum Quantum Number " l " :-

l is the orbital angular momentum quantum number as it determines the shape of an orbital, and therefore the angular distribution.It is also known as Azimuthal Quantum number.Each value of l indicates a specific s, p, d or f subshell.

The Azimuthal Quantum number has integral values of l = 0 to l = n - 1 for each value of n. This means that for n = 1, the first shell, there is only l =1-1 = 0 subshells i.e the shell and subshell are identical.

Thus to designate a particular subshell we write the number of the shell itself followed by the subshell designator as nl. This illustrates the relationship between "n" and "l". Thus 1s is the first shell having one orbital type associated with it.

3) The Magnetic Quantum Number "' ml " :-

ml is the magnetic quantum number as it determines the number of orbitals and their orientation within a subshell. The magnetic quantum number has integral values of ml = - l to + l including 0. It splits the subshells into individual orbitals.

4) The Electron Spin Quantum Number " ms " :- 

ms is the fourth and the final Quantum Number. Unlike the above three Quantum Numbers ms does not depend on another quantum numbers.It designates the direction of the electron spin.It has only two possible values of +1/2 or -1/2 .

Thus by specifying the four Quantum numbers the address of any electron in a given atom can be completely defined.

Electronic Configuration :- The arrangement of electrons of each element in their orbitals or shells is known as it's electronic configuration.

Pauli's Exclusion Principle :- In 1925 an Autrian physicist, Wolfgang Pauli, expressed the importance of electron spin in determining electronic configurations. According to Pauli's Exclusion Principle No two electrons in an atom can exist in the same quantum state. Pauli's Exclusion Principle implies that no two electrons can have the same set of four quantum numbers n, l, ml and ms . At least one quantum number should be different. This principle assigns quantum number to electrons.

For eg. 

Let us consider the case of an Helium atom which has 2electrons .

These electrons occupy n=1 orbital, so clearly for n=1 ,l=0 , ml =0 but ms can have 2 values +1/2 or -1/2 .

Hence 1 electron will occupy n=1, l=0 , ml =0 and ms = +1/2 where as the 

 another electron will occupy n=1, l=0 , ml =0 and ms = -1/2 orbital.

Also according to Pauli Exclusion Principle, the maximum number of electrons in an orbital with principal quantum number n is 2n²  . Thus, For 1st shell, n=1,maximum electrons that can be accomodated are 2  . Similarly the maximum no. of electrons that can be accomodated in 2nd , 3rd and 4th shell are 8 ,18 and 32 . Pauli Exclusion Principle is used in arranging the elements in the periodic table .

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