Chapter six - Electromagnetic Radiation

Energy that travels through space as waves, such as visible light, ultraviolet radiation, and radio waves

All have wave-like properties

All travel at the speed of light in vacuum, c = 2.998×10^8 m/s

Wavelength: The distance between successive high points or the low points in a waves

• Unit m

Frequency: number of complete waves that pass a point in space in a given time

• Unit: hertz (Hz), or s^-1

c = wavelength x frequency

wavelength = c / frequency

frequency = c / wavelength

v = frequency

c = speed of light

Example from class: What is the frequency of light that has a 396.15 nm as wavelength?

Photoelectric effect

• The ejection of electrons from the metal does not depend on the total energy of the light, but only on the wavelength of the light

Photons

• light travels in discrete packets of energy called photons

• the energy of a single packet is directly related to the frequency of the light

• Planck’s equation relates photon energy and frequency

• If a photon has low energy, it will be unable to knock an electron out

Atomic Line spectra

• When the light emitted by “excited” gas phase elements is passed through a prism through a prism, a line spectrum is observed that shows only very specific wavelength of light

• Number of lines and the wavelength of the lines are different for each element

• Only certain wavelength of light are emitted suggest that energy from an atom

• En = -2.176 × 10^-18 j (1/n²) —> This relationship is only true for species with a single electron such as hydrogen

• The energy change for any transition: delta E = -2.179 × 10^-18 j (1/n² final - 1/n² initial)

• Sometimes instead of -2.167 × 10^-8, it can be RHC

• absorption: The electron transition from a lower energy level to a higher energy level

• emission: when electrons go from higher energy level to a lower energy level

• The sign for delta E:for absorption is is positive, for emission it is negative. It follows the direction of energy