Atomic Structure

## Dalton's Atomic Theory

1) Atom is the smallest indivisible particle of a substance .

2) Atom is indestructible .

3) During a chemical reaction , two or more atoms combine in fixed ratios of their atomic masses to form a compound .

**Limitations**

Atom is not the smallest particle of a substance . Atom contains other sub-atomic particles .

Atom is not indestructible . According to Einstein's Mass-Energy Equation , mass can be converted into energy(E=mc2) .Thus , atom can be destroyed and converted into energy . (Nuclear Energy).

## Thomson's Atomic Model

Thomson's Atomic Model stated that Atom consists of a positively charged sphere , in which negatively charged particles are embedded like a plum pudding (Plum Pudding Model) .

Thomson conducted the Cathode Ray Test to discover negatively charged Cathode Rays . These negatively charged particles were later found to be a sub-atomic particle . These were named as Electrons.

After Cathode Ray Test , Crookes conducted Anode Ray Test by reversing the Voltage . This experiment was further investigated by Goldstein and his investigation led to the discovery of Proton .

## Rutherford's Atomic Model

Earnest Rutherford conducted he famous Gold Foil Experiment , in which alpha rays were projected on a gold foil of thickness of 0.00004 m . The results concluded that there exists void space around a positively charged nucleus .

Thus , Rutherford claimed that Electrons revolve around the nucleus in circular orbits .

Later , James Chadwick conducted an experiment which showed the emission of neutral rays . This lead to the discovery of the neutron .

## Bohr's Atomic Model

According to Rutherford's Atomic Model ; an electron would revolve around the nucleus . But in this process , the electron would lose energy and subsequently collide with the nucleus . Thus Neils Bohr stated that :-

1) Electrons revolve around the nucleus in fixed stationary orbits . Each orbit has its own energy level .

2) When an electron loss energy , it comes closer to the nucleus & when it gains energy it moves away from the nucleus . (Proved by Atomic Spectrum)

## Atomic Spectrum

Neils Bohr could provide a stale model of an atom on the basis of Atomic Spectra . The Atomic Spectra of Hydrogen an Hydrogen like atoms is found to be same because both contain the same number of electrons .

## Bohr's Postulates

Rn = n2R0

vn = v0 / n

En = E0 / n2

Ln = nL0

## Planck's Quantum Theory

Planck's Quantum Theory states that Energy exists in discrete bundles or packets called as Quanta . Thus gain or loss of energy is not a continuous process .

## De Broglie Equation

h/p = λ

Every particle having a wavelength is a wave . Every particle having a certain momentum , possesses mass . Thus , De Broglie Wave Equation shows that Electron has a dual nature ; i.e. Matter as well as wave .

## Schrödinger Wave Equation

∇2ψ = + {8π2m/h2} (E- V)ψ = 0

∇2 = d2/dx2 + d2/dy2 + d2/dz2

Si (ψ) is the wave function . ψ2 gives the probability of finding an electron in a 3-D space around the nucleus .

## Heisenberg's Uncertainty Principle

∆x∆p >= h/4π

Where ∆x is the uncertainty in determining position and ∆p is the uncertainty in determining its momentum .

## Quantum Numbers

### **Principle Quantum Number**

Principle Quantum Number indicates the main energy level of an electron . It is denoted by n . n = 1,2,3,4,5,6,7 . An Orbit can hold 2n2 electrons .

### **Azimuthal Quantum Number**

Azimuthal Quantum Number states the sub-shell or sub-energy level in which the electron is present . It indicates the shape of the orbital in which the electron is present . It is denoted by l . l = 0 for s , l = 1 for p , l = 2 for d , l = 3 for p .

### **Magnetic Quantum Number**

Magnetic Quantum Number indicates the energy level of the orbital of a sub-shell in which the electron is present . It is denoted by m and m = 2(2l + 1)

### **Spin Quantum Number**

If two electrons are present in the same orbit , in the same sub shell and in the same orbital , then they can be distinguished by the Spin Quantum Number . The Spin Quantum Number indicates the spin of the electron .

Thus , no two electrons can have the same set of Quantum Numbers .

## Aufbau Rule

Aufbau Rule states that the order of filling of electrons in sub-shells is such that the sub-shells with lower energy level are first filled.

## Hund's Rule

The Hund's Rule states that electrons first occupy independently into orbitals before pairing with other electrons .

## Pauli's Exclusion Principle

Pauli's Principle states that a maximum of two electrons can be occupied in a single orbital ,both having an opposite spin .