**Electron:**

Electron was discovered during experiments on the discharge of electricity through rarified gases. The magnitude of electric charge (e) was determined by Millikan. Charge of the electron = 1.602 x 10^{ -19} coulomb

**Bohr Model of the Atom:**

Proposed by Neil Bohr in 1913. Bohr applied the Planck’s quantum theory to the Rutherford nuclear atom with remarkable success.

**First postulate**:

An atom consists of a positively charged nucleus at the centre. The electrons move round the nucleus in certain stationary orbits of definite radii and not all possible radii.

**Second postulate:**

The radius of the orbit is such that the angular momentum of the electron is an integral multiple of h/2p.

**Third postulate**:

Electron may jump from one orbit to the other, in which case the difference in energy between the two states of motion is radiated in the form of a light quantum.

**X-Rays :**

Invisible electromagnetic radiations, Wavelengths range form 0.010A – 100A. These are discovered by Roentgen. When a fast moving electron is suddenly stopped a part of its kinetic energy is converted into X-ray photon the rest of the energy is converted into heat.

**Wave Particle Duality:**

To understand any given experiment, we must use either the wave or the photon theory, but not both. Light sometimes behaves like a wave and some times like a particle

**de Broglie Hypothesis:**

Photons are treated as “packets of light” behaving like a particle. Momentum of a photon: p = E / c = h/l Energy of a photon: E = hc/l

**Heisenberg Uncertainty Principle:**

If position is identified the momentum cannot be measured. If momentum is measured the position is lost. Dx X Dp ³ h / 4π