spectroscopy and proof of molecular structure elements 4 and 5

atomic spectroscopy

• excited state of an atom has a different configuration to ground state

• atoms jump from one state to another by absorption/emission of photons

• whole atom absorbs radiation not just electron

• transition from one state to another dictated by another set of quantum numbers

atomic emission

• very sensitive

• not easily made quantitative

• used for qualitative analysis - eg flame tests

• difference in higher and lower energy levels corresponds to energy of emitted radiation

atomic absorption

• can be quantitative

• atomic emission lamps - produce specific wavelengths that atoms can absorb

• intensity of absorption dependent on:

1. probability of transition occurring

2. populations at different energy levels

3. amount of sample

molecular electronic spectroscopy

• measures transitions of electrons between different molecular orbitals

• populations of electrons in these orbitals determines the molecular electronic state

what is measured in spectroscopy

• wavelength of absorption - gives information on energy gap between ground and excited states, measured as function of wavelength

• intensity of absorption - sometimes referred to as cross section (sigma)

• from absorbance can measure molar extinction coefficient - how strongly a chemical substance absorbs light of a specific wavelength

UV-vis spectra

• quantified by wavelength of an absorption maximum and absorption strength parameter (molar extinction coefficient)

Beer-Lambert law

• when light shone on thin slice of solution, the amount of light absorbed is proportional to the number of light-absorbing molecules

chromophore

• ‘colour bringer’

• substance that causes specific wavelengths of light to be absorbed in a sample