Chemistry is in the electrons Electronic structure arrangement of electrons in atom Two parameters: Energy Position The popular image of the atom is incorrect: electrons are not miniature planets orbiting a nuclear sun
Learning objectives Describe properties of waves and calculate basic properties like wavelength and frequency Calculate quantities using the photon model of light Describe the basic principles of the Bohr model Distinguish between the classical view and the quantum view of matter Describe Heisenberg Uncertainty principle and debroglie wave-particle duality Calculate wavelengths of particles
Problem: Why don t the electrons collapse into the nucleus? Rutherford s epic experiment revealed the positive nucleus with the electrons occupying the vast void around it
The planetary idea A planet in a stable orbit circulates indefinitely Orbiting charged particles emit energy - spiral into the nucleus emitting energy as it does so
Conventional explanations don t work Yet... atoms exist and electrons are stable outside the nucleus Familiar symbol of planetary atom is incorrect There must be another explanation...
Unlocking the mystery of light Earth s energy provider Chief nourisher of life s feast Inspiration for romance Symbol of good Object of worship Source of wisdom
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 Complementary colors: Green absorbs red Violet absorbs yellow
Let there be light properties of Electromagnetic waves are characterized by: 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 crest waves
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 (c/ms -1 ) = wavelength (λ/m) x frequency (ν/s -1 ) c Wavelength proportional to 1/frequency - as λ ν
The electromagnetic spectrum Harmful radiation 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 Waves in X-ray region have wavelength approximately same as atomic diameter (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
Empirical classification of the spectra of the hydrogen atom All of the lines in the H atom spectrum can be fit to an equation m and n are integers (n > m) R is the Rydberg constant The Balmer Rydberg equation. 1 1 1 R 2 2 m n Inner shell Outer shell
So, everything is cool with waves? NOT Observations of radiation from heated bodies could not be described using classical methods the ultraviolet catastrophe Shortcomings with waves...
Blackbody radiation and the ultraviolet catastrophe Observations of radiation from heated bodies could not be described using classical methods Intensity tends to infinity at shorter wavelength! Observed radiation exhibits a maximum that depends on the temperature of the body As T, λ max
Chunky energy and the Planck equation Blackbody radiation explained if energy is emitted in discrete amounts (quanta) instead of changing continuously Quantization of energy E = hν h is Planck s constant = 6.626 x 10-34 Js
Electrical current Quantization and the photoelectric effect Current flows No current flows Frequency Light incident on a metal surface causes electrons to be emitted. Below threshold frequency nothing happens Above threshold, current increases with intensity
Photons: light as particle and wave In photoelectric effect, light behaves like a particle Energy of electron given by: KE h e Energy is quantized into packets - photons Photon energy depends on frequency: E = hν As frequency increases photon energy increases e
Photon energy and the electromagnetic spectrum As frequency increases, photon energy increases Dangerous radiation has high photon energy UV light, X-rays, gamma rays (ionizing radiation) Harmless radiation has low photon energy IR, radiowaves
Light: particle or wave or both? Newton advanced a corpuscular theory of light, even as he discovered the refraction of light in a prism Huygens developed a wave theory of light Maxwell equations cemented mathematical description of electromagnetic waves Discovery of light interference and its description by wave theory made the latter triumphant in the 19 th century Ultraviolet catastrophe and the photoelectric effect establish the photon So what is light exactly? The enduring mystery...