Section 4.7—Light & Matter

Electrons Absorbing Energy Photon coming into atom collides with electron. Photons are energy. - +

Electrons Absorbing Energy Photon coming into atom collides with electron. Photons are energy. - + The electron is “excited” to a higher energy level with is newly increased energy from absorbing the photon.

Excitation The process of an electron absorbing a photon of light (energy) and being promoted to a higher energy level from its “ground state”

And later… The electron cannot remain in that excited state indefinitely - +

And later… The electron cannot remain in that excited state indefinitely - + Energy is released during relaxation

Relaxation The process of an electron releasing a photon of light (energy) and falling back down to a lower energy level.

Energy of photon and levels jumped The higher the energy of the photon, the greater the electron jump! A photon of UV light has more energy than a photon of Infrared light The UV photon would cause a higher energy jump (jump up more levels) than the IR photon.

Total energy in = Total energy out However much energy was absorbed must be released again, but it can be released in smaller packets A high energy photon might be absorbed, but two lower energy photons might be released as the electron falls in a “step-wise” manner.

Photons must match energy changes The energy of the photon must exactly match the energy change of the electron. If the photon is not an exact match, the photon will pass through unabsorbed. - +

Measuring light absorption Not all colors come out All colors of light go in Sample of hydrogen

Hydrogen Spectrum The black bars are the colors that a hydrogen atom absorbs. The other colors pass through the atom un-absorbed.

Absorption of Molecules Because the structure of a molecule is much more complicated than a single atom, they absorb regions of light rather than single wavelengths. Absorption spectrum of water Picture from http://www.lsbu.ac.uk/water/vibrat.html

Ways of producing light Fluorescence: visible light is absorbed and visible light is emitted at the same time—the relaxation happens very quickly after excitation Phosphorescence: Visible light is absorbed and then a while later is emitted—relaxation occurs after a period of time

Ways of producing light Incandescence: Energy is put in from heat and given off as visible light Chemiluminescence: Energy released during a chemical reaction is absorbed to cause excitation. Relaxation produces visible light Biolouminescence: Chemiluminescence that occurs in a biological organism. Triboluminescence: Physical pressure or torque provides energy for excitation. Relaxation produces visible light.

What did you learn about glowing things?

Glow in the dark Light Electron structure Periodic Table Is based on Gives off Light Electron structure Changes in produce Which can be determined using Periodic Table Is a part of Atomic structure Arranged to show Is based on Periodicity Atomic theory