The matter is made up of molecules that consist of atoms. According to Bohr’s theory, “the atom consists of a positively charged nucleus and a number of negatively charged electrons which revolve round the nucleus in various orbits”. When an electron is raised from a lower state to a higher state, it is said to be excited. While exciting, if the electron is completely removed from the nucleus, the atom is said to be ionized. So, the process of raising the atom from a normal state to this ionized state is called ionization.

According to Bohr’s model, an electron is moved to a particular orbital, whereas according to quantum mechanics, an electron is placed somewhere in the free space of an atom, called an orbital. This theory of quantum mechanics proved to be correct. Therefore, a three-dimensional boundary where an electron is likely to be found is called an atomic orbital.

## Quantum Numbers

Each orbital, where an electron moves, varies in its energy and size. The energy levels of orbitals can be represented using a discrete set of integrals and half-integrals known as quantum numbers. Four quantum numbers are used to define the wave function.

### Principal Quantum number

The first quantum number to describe an electron is the principal quantum number. Its symbol is n. The size or sequence of the number specifies the energy level. As the value of n increases, the average distance from the electron to the nucleus also increases, as does the energy of the electron. The main energy level can be understood as a shell.

### Angular Momentum Quantum number

The symbol for this quantum number is I. This I indicates the size of the orbital. It ranges from 0 to n-1.

I = 0, 1, 2 …n-1

for the first shell

Example : for n-1, I = 0 is the only possible value of I as n = 1.

So, when I = 0, it is called as **S** orbital. The shape of S is spherical. The following figure represents the shape of S.

If n = 2, then I = 0, 1 as these are the two possible values for n = 2.

We know that it is S orbital for I = 0, but if I = 1, it is **P** orbital.

The P orbital where the electrons are more likely to find is in a **dumbbell** shape. The following figure represents the shape of the dumbbell shape.

### Magnetic Quantum number

This quantum number is denoted by ** m_{I}** which represents the orientation of an orbital around the nucleus. The value of

**m**depends on I.

_{I}**m _{I} = ∫ ( −I to +I )**

For I = 0, m_{l} = 0 this represents S orbital.

For I = 1, m_{I} = -1, 0, +1 these are the three possible values and it represents the P orbital.

The following figure represents the shape of three P orbitals.

### Spin Quantum number

It is denoted by **m _{s}** and here the electron rotates on the axis. As shown here, the speed of rotation of the electron can be either clockwise or counterclockwise.

The possible values of this spin quantum number will be,

**m _{s}= +1/2 up**

For a movement called spin up, the result is the positive half.

**m _{s}= −1/2 down**

For a movement called spin down, the result is the negative half.

These are the four quantum numbers.

### Pauli Exclusion Principle

According to the Pauli exclusion principle, no two electrons in an atom can have the same set of four identical quantum numbers. This means that if any two electrons have the same value of n, s, m_{I} then they will definitely have different I values. Therefore, no two electrons will have the same energy.

**Electronic shells**

If n = 1 is a shell, then I = 0 is a sub-shell.

The shells of electrons corresponding to n = 1, 2, 3….. are represented by K, L, M, and N respectively. The subshells or orbitals corresponding to I = 0, 1, 2, 3, etc. are respectively denoted by s, p, d, f, etc.

Let’s take a look at the electronic configurations of carbon, silicon, and germanium **GroupIV–A.**

It is observed that in each case the outermost p sub-shell has only two electrons. But the possible number of electrons is six. Thus, each outermost shell has four valence electrons. Thus, each electron in an atom has a specific energy. The atomic arrangement inside the molecules in any type of matter is almost the same. But the distance between atoms varies from material to material.