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.

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 2n2electrons.

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.