The Development of a New Atomic Model

Objectives Explain the mathematical relationship between the speed, wavelength, and frequency of electromagnetic radiation. Discuss the dual-wave particle nature of light. Discuss the significance of the photoelectric effect and the line-emission spectrum of hydrogen to the development of the atomic model. Describe the Bohr model of the hydrogen atom.

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A New Model Was Formed Why wasn’t the positive nucleus attracted to the negative electron cloud? Investigations into light led to understanding of relationship between light and electrons. Led to revolutionary theory about the nature of light, matter, and atoms

Properties of Light Electromagnetic spectrum Electromagnetic spectrum—all forms of radiation exhibit wavelike behavior as they travel through space. γ-rays (gamma), X-rays, ultraviolet, visible light, infrared light, microwaves, radio waves Travel at 3.0 x 10 8 m/s

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Waves Wavelength Wavelength—λ (lambda)—distance between crests of adjacent waves (nanometers 1 nm=1 x m) Frequency Frequency—ν (nu)—the number of waves that pass a given point in one second (Hz— hertz) Speed of light Speed of light—wavelength is inversely proportional to frequency c = λ ν

Photoelectric Effect Electrons are given off from a metal when light of a certain frequency is shined on it If light is a wave, light of any frequency should make electrons be given off ??why only certain frequencies??

Light as Particles Max Planck—German physicist—1900 Proposed that objects give off energy in small, specific amounts Quantum Quantum—minimum quantity of energy that can be lost or gained by an electron

Albert Einstein dual wave-particle nature Electromagnetic radiation is both a wave and a particle—dual wave-particle nature Light has wavelike properties Light can behave as a stream of particles—each particle with a quantum of energy Photons Photons—particle of electromagnetic radiation having zero rest mass and carrying a quantum of energy Amount of energy depends on frequency

Line-Emission Spectrum Ground state Ground state—lowest energy state of an atom Excited state Excited state—state where an atom has higher potential energy Neon sign— current passes through a gas at low pressure and is excited—when it returns to ground state it gives off energy as electromagnetic radiation

Line-Emission Spectrum When light from an excited state atom is passed through a prism, it is separated into a series of specific wavelengths (not all like a rainbow) When an atom falls back from excited state to ground state—it gives off a photon of radiation Quantum Theory Led to Quantum Theory

Line-Emission Spectrum Specific lines in spectrum mean that the energy differences between energy states was fixed Electrons exist only in specific energy states mon_assets/science/cmc/cim/anima tions/ch5_1.rm Animation of H line spectrum

Bohr Model Neils Bohr—Danish physicist—1913 Linked model of atom with the emission spectrum Electrons can only travel in specific paths (orbits) Electrons in an orbit have a fixed amount of energy Orbits closest to nucleus have smallest energy

Bohr Model Atoms gain energy & electrons move to higher orbits (excited state) As electrons fall back to lower energy orbits they give of photons with a fixed amount of energy Amount of energy photon has is seen as lines in emission spectrum