Chemistry is in the electrons Electronic structure – how the electrons are arranged inside the atom Two parameters: –Energy –Position

Learning objectives Describe the properties of waves Describe the photon and the relationship between light frequency and energy

Setting the scene: Why don’t electrons collapse into the nucleus? Like charges attract

The planetary model...doesn’t work Unlike the moon, an orbiting charged particle will spiral inwards to the nucleus Moving charges lose energy by radiation

Conventional explanations do not work Yet... atoms exist and electrons are stable outside the nucleus The iconic symbol of the planetary atom is not correct There must be another explanation

Light and colour White light contains many colours They can be separated by a prism (rainbow)

Absorption determines colour Objects are the colour of the light not absorbed White absorbs nothing Black absorbs everything Blue absorbs red

Making waves Light is an lectromagnetic wave: –Wavelength – distance between two peaks –Frequency - number of wave peaks that pass a fixed point per unit time –Amplitude - height of the peak measured from the center line –Velocity – speed of the wave

Wavelength and colour Different “colours” of electromagnetic radiation are waves with different wavelengths and frequencies. All electromagnetic radiation has the same velocity: the speed of light – 3 x 10 8 m/s Velocity = wavelength x frequency Wavelength proportional to 1/frequency - as wavelength ↑ frequency ↓

The electromagnetic spectrum Continuous range of wavelengths and frequencies: –Radio waves at low-frequency end –Gamma rays at high-frequency end –Visible region is a small slice near the middle Wavelength of X-ray is same as diameter of an atom (10 –10 m) Radiation becomes more dangerous as frequency increases

Atoms emit and absorb radiation at specific wavelengths Absorption is light removed by the atom from incident light Emission is light given out by an energetically excited atom The absorption and emission lines are at the same wavelengths The lines from the H atom form a neat series. Do these spectra have anything to do with the electronic structure?

Each element has a unique spectrum Electronic structures of each element are different Spectra can be used to identify elements – even in very remote locations

Light in coherence: LASERS Light Amplification by Stimulated Emission of Radiation Laser consists of one wavelength only All the waves are “coherent”

Three essentials of a laser Laser medium – solid, liquid, gas Source of “pump” energy to excite medium –light, electrical, chemical Cavity to “build up” power

All shapes and sizes Solid-state lasers –Crystals, semiconductor chips Liquid lasers –Dyes Gas lasers –Helium, Argon, Carbon dioxide

Other forms of radiation: nuclear magnetic resonance (NMR) Magnetic nuclei (like H) in magnetic field absorb radiation Radiation is in the radiowave region Measure absorption peak when radiowave has resonance frequency

Magnetic resonance imaging (MRI) The body is full of H atoms (water) Resonance occurs at different peaks in the body Result is high-resolution map of the inside of the body Non-invasiveSafeExpensive

Problem with light: wave or particle? The photoelectric effect Light incident on a metal surface causes electrons to be emitted. Below threshold frequency nothing happens Above threshold, current increases with intensity No current flows Current flows

Quantization: light as particle and wave In photoelectric effect, light behaves like a particle. Energy is “quantized” into packets Each packet is a photon Photon energy depends on frequency: E = h E = h ν As frequency increases photon energy increases

Photon energy increases from right to left “Dangerous” radiation has high photon energy – UV light, X-rays “Harmless” radiation has low photon energy – IR, radiowaves Blue light has higher energy than red light