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# Chapter 7 - Chapter 7 Atomic Structure and periodicity 7.1...

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Chapter 7 Atomic Structure and periodicity 7.1 Electromagnetic Radiation: One of the ways that energy travels through space is by electromagnetic radiation ( light waves ). Waves have three primary characteristics : 1. Wave length λ ( Lambda ) : It is the distance between two consecutive peaks in a given wave. 2. Frequency ν : It is defined as the number of cycles (waves) per second that pass a given point in space. 1Hz = 1s -1 3. Speed c : All types of radiation travel at the speed of light (c = 2.9979 x 10 8 m/s or = 2.9979 x 10 10 cm/s) λ α 1/ = c/ λ = c

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Classification of electromagnetic radiation : Radiation Wave length in meters Gamma rays 10 -12 X – rays 10 -10 Ultraviolet 10 -8 Visible 10 -6 Infrared 10 -4 Microwaves 10 -2 Radio FM 1 Shortwave 10 2 AM 10 4 Example 7. 1 . Convert wavelength in 650 nm to frequency in Hz (1m = 10 9 nm) Answer : λ ν = c ν = c/ λ = (2.9979 x 10 8 m) / 650 x 10 -9 m = 4.61 x 10 14 s -1 = 4.61 x 10 14 Hz 2
7.2 The Nature of Matter: Max Planck suggested that energy is quantized it means that matter cannot absorb or emit any quantity of energy ; but instead it can only gain or lose a specific quantity of λ ν . Δ E = h ( change in energy of a system) Where, h is called Planck’s constant = 6.626 x 10 -34 Js Then Albert Einstein suggested that electromagnetic radiation is itself quantized and can be viewed as a stream of particles called photons . The energy of each photon is given by: E photon = h = h c/ λ Ptotoelectric Effect : Also Einstein derived the following equation to explain photoelectric effect: E = m c 2 That means light has some characteristic of matter m = E /c 2 = h /c 2 = h c/ λ c 2 = h / λ c 3

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Diffraction pattern of electrons : Experimental work proves that electrons have wave properties (diffraction experiments). De Broglie (1923) proposal that for a particle of mass m moving with a velocity v : m = h / λ v (De Broglie equation) and λ = h / m v (for microscopic particles) 7.3 The Atomic Spectrum of Hydrogen: ___________________ E 2 Δ E = h ν = h c/ λ adsorption emission ( h ) ( h ) ___________________ E 1 The energy of the electron in the hydrogen atom is quantized . This means that only certain energies ( h ) are possible. 4
7.4 The Bohr Model: In 1913 Bohr developed a quantum model for the hydrogen atom.

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Chapter 7 - Chapter 7 Atomic Structure and periodicity 7.1...

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