}]]}]}]}] 2f:T38be,

### History & Atomic Models - **Dalton's Atomic Theory (1808):** 1. Matter consists of indivisible atoms. 2. Atoms of same element are identical in mass & properties. 3. Atoms of different elements differ in mass & properties. 4. Atoms combine in simple whole-number ratios to form compounds. 5. Atoms are neither created nor destroyed in chemical reactions. - **Limitations:** Failed to explain subatomic particles, isotopes, isobars. - **Thomson's Plum Pudding Model (1898):** - Atom is a uniform sphere of positive charge with electrons embedded in it. - **Limitations:** Failed to explain Rutherford's $\alpha$-scattering experiment. - **Rutherford's Nuclear Model (1911):** - Based on $\alpha$-scattering experiment (gold foil experiment). - **Observations:** 1. Most $\alpha$-particles passed undeflected $\implies$ most of atom is empty space. 2. Few $\alpha$-particles deflected at small angles $\implies$ there's a positively charged core. 3. Very few $\alpha$-particles deflected at large angles or bounced back $\implies$ positive charge and mass concentrated in a very small region (nucleus). - **Postulates:** 1. Atom has a tiny, dense, positively charged nucleus at its center. 2. Electrons revolve around the nucleus in circular paths. 3. Centripetal force for electron's revolution is provided by electrostatic attraction between nucleus and electron. 4. Atom is electrically neutral. - **Limitations:** 1. **Stability of Atom (Maxwell's theory):** Accelerating electron should continuously radiate energy and spiral into the nucleus, making atom unstable. 2. **Line Spectrum:** Could not explain the line spectrum of elements, only continuous spectrum was predicted. ### Electromagnetic Radiation - **Wave Nature of Light:** - Light propagates as electromagnetic waves. - **Properties:** - **Wavelength ($\lambda$):** Distance between two consecutive crests or troughs. Unit: m, cm, nm, Å. - **Frequency ($\nu$):** Number of waves passing a point per second. Unit: Hz or s⁻¹. - **Velocity (c):** Speed of light in vacuum ($3 \times 10^8$ m/s). - **Relationship:** $c = \nu \lambda$. - **Wave Number ($\bar{\nu}$):** Number of waves per unit length. $\bar{\nu} = 1/\lambda = \nu/c$. Unit: m⁻¹, cm⁻¹. - **Amplitude (A):** Height of the crest or depth of the trough. Determines intensity. - **Particle Nature of Light (Planck's Quantum Theory):** - Energy is emitted or absorbed in discrete packets called quanta (photons for light). - Energy of a quantum: $E = h\nu = hc/\lambda$. - $h$ = Planck's constant ($6.626 \times 10^{-34}$ J s). - **Photoelectric Effect:** Emission of electrons from a metal surface when light of suitable frequency falls on it. - **Threshold Frequency ($\nu_0$):** Minimum frequency required to eject an electron. - **Work Function ($\phi_0$):** Minimum energy required to eject an electron. $\phi_0 = h\nu_0$. - **Kinetic Energy of ejected electron:** $KE = h\nu - h\nu_0$. - Number of electrons ejected depends on intensity of light. - KE of ejected electrons depends on frequency of light. ### Bohr's Model for Hydrogen Atom - **Postulates:** 1. Electrons revolve around the nucleus in fixed circular orbits (stationary states) without radiating energy. 2. Energy of an electron in an orbit is constant. 3. Electrons can jump from one stationary state to another by absorbing or emitting energy. - Absorption: Electron jumps to a higher energy orbit. -