5.3 Physics and the Quantum Mechanical Model

Light • By 1900 enough experimental evidence to convince scientists that light consists of waves

• Wavelength ( λ, Greek symbol Lambda) • Measured in m or nm • Frequency ( ν, Greek symbol nu) • # waves per unit of time • Measured in cycles per second • SI unit = hertz (Hz) • Reciprocal second, s -1 (1/s) • Amplitude = height from zero to crest

C = νλ • Frequency and wavelength are inversely proportional to each other • All electromagnetic waves travel in a vacuum at a speed of c = 2.998 x 10 8 m/s

• Calculate the wavelength of the yellow light emitted by sodium lamp if the frequency of the radiation is 5 x 10 14 Hz.

Electromagnetic Spectrum

Electromagnetic Spectrum • Wave model  light is made up of electromagnetic waves • Visible light ranges from red (low energy, 700 nm) to violet (high energy, 380 nm)

Atomic Spectra • Atoms absorb energy, e energy levels.

move to higher • e lose energy by giving off light as they return to lower energy levels • • Light emitted by atoms is a mixture of specific frequencies, each frequency corresponds to a certain color Emission spectrum

• Emission spectrum of an element is like a fingerprint, no two elements have the same • Used to identify elements

• Lowest possible energy of electron is its Ground State • Absorbs energy and moves to EXCITED STATE • Quantum of energy (light) emitted when electron drops back to lower energy level • Emission takes place in an abrupt step called electronic transition

Energy and Frequency • Photon - A quantum of light (packet of energy) • The light emitted has a frequency directly proportional to energy change of electron > Frequency >Energy

• Energy per photon ∆E = hc/ λ ∆E = h ν h = 6.626 x 10 -34 J ∙ s/photon (h = Planck’s constant)

• Lyman series- transition to n = 1 (ultraviolet) • Balmer series- transition to n = 2 (visible) • Paschen series- transition to n = 3 (infrared)

Quantum Mechanics • De Broglie predicted that all moving objects have wavelike behavior • Classical mechanics describes the motions of bodies much larger than atoms • Quantum mechanics describes the motions of subatomic particles and atoms as waves

• Heisenberg uncertainty principle-it is impossible to know exactly both the velocity and the position of a particle at the same time