Chemistry for Biology Students (CHEM0010) Notes - Spectroscopy

B1: Light and its Properties
B1: Light and its Properties
 Light
o Eye

Light from external source

Focused by a lens

Image is formed by a set of detector molecules in the retina

Light falls on the molecules – sends an electrical signal to the brain
 Retina – contains light sensing molecules
o Rods – light and dark detection
o Cones – photo-rhodopsin molecules – respond to different wavelengths of
light – detects colour
 Range of colours detected – depends on number and type of photo-
rhodopsin molecules in our eyes
o EM wave
 James Clerk Maxwell – made a set of wave equations = formed the basis of the
electromagnetic theory of light
 Electricity and magnetism – linked together involving time
 Electric fluctuations described by the electric field strength (E) + magnetic
fluctuations described by the magnetic field intensity (B or H) travelled through
space in unison at constant speed (c)
o Constant (c) = 3x108 m/s





 Light waves – also defined by wavelength in metres (λ), frequency in Hz (v)
o c = λv (speed of light = wavelength x frequency)
o Colour
 Refractive index n = c/v
 Light in a vacuum – all different coloured beams travel at the same speed (c)
 Light in a material – slowed down due to interactions with electrons and atomic
nuclei to slower speed (ν)
 Prism – refracts light
 White light – contains all colours
o Light with different colours (different wavelengths) – slowed down by
different amounts
 When a light beam strikes an inclined surface – beams are bent by
different amounts to travel through the prism with different paths
 Newton’s prism experiment
o Mounted a prism with the point downwards
o Had a darkened room with a curtain
o Cut a hole to let a beam of light through
o Directed light through the lens
o Observed the colours of the rainbow
 Split light into different coloured components
 Purple appeared at the top (shortest wavelength)
 Red appeared at the bottom (longest wavelength)
 Absorbance = 2 – log(%T)
 Visible spectrum of light

,B1: Light and its Properties
 Colours of a
rainbow
o




Relationship between wavelength, frequency and speed of light is given by:
 c = λv
 c stays constant
 As λ changes, v changes in the opposite direction
 Wavenumber = 1 / wavelength
o Colours = different wavelengths and frequencies
 Full electromagnetic spectrum
o Contains all wavelengths / frequencies of light that
satisfy c = λv
 Spectroscopy – light can be used in
different regions to interact with different
atomic and molecular properties – to gain
information about them
 Ultraviolet-visible
o Electron transitions between quantized energy levels in atoms and
molecules – to study bonding
 Infrared spectroscopy
o Vibrations between atoms in molecules – to study molecular structure and
functional groups
 Nuclear magnetic resonance (radio waves)
o Study spin flips of atomic nuclei in magnetic fields and gain information
about local structural environments
 Light as a particle and a wave
o 2 models
 Light as a wave – properties of wavelength and frequency
 Light as a particle = photon – properties of mass and momentum
o Wave-particle duality
 Brought 2 models together
 Each photon associated with a pilot wave
 Equation – λ = h/mv
 h = Planck’s constant = 6.626 x 10 -34 J/s
 m = mass of particle
 mv = momentum (mass x velocity)
o Planck’s equation – energy of a single photon
 E = hv = (hc)/λ
 Energy of a photon = Planck’s constant x frequency = (Planck’s constant x speed of light) /
wavelength
 High energy radiation – has high frequency and short wavelength
 Low energy radiation – has low frequency and long wavelength
 Units
o h = Planck’s constant = 6.626 x 10 -34 J/s
o m = mass of particle
o mv = momentum (mass x velocity)

, B1: Light and its Properties
o λ = wavelength in metres (m)
o v = frequency in s
o c = speed of light in metres per second (m/s)
o E = energy of a photon in Joules (eV)
o 1nm = 10-9m

Ideas in Bonding
 Atomic energy levels and transitions between them
o Atoms
 Electrons orbit around the nucleus
 Energies and shapes of orbitals – defined by 4 quantum numbers
o Quantized electron energy (E) – related to the radius of the orbit = principle
quantum number (n)
o Electrons can jump to a
higher energy orbit by
absorbing light in the UV to
visible range
 Energy jump (ΔE) –
related to frequency (λ) or wavelength (v) of light absorbed by
Planck’s equation
 ΔE = hv = (hc)/ λ
 Shapes of Atomic Orbitals (p vs s orbitals)
o Consider shapes of orbitals when combined to form MOs
 Orbitals are quantized according to their angular momentum + labelled s, p, d, etc
 2s orbitals = spherical
 2p orbitals = have + and – lobes that point along the x-, y-, or z- axes = p x, py, pz
o Carbon
 Uses 2px, 2py, 2pz orbitals to make C-C bonds
 Uses 1s orbitals of H for C-H bonds
 Bonding vs Antibonding orbitals and energy levels
o Molecular orbitals – formed by the overlap of atomic orbitals
 Lower energy compared with the starting atomic orbitals = bonding orbital
 Formed by positive overlap between orbitals = constructive
 Higher energy compared with the starting atomic orbitals = antibonding orbital (no electron
density between 2 atoms)
 Formed by negative overlap between orbitals = destructive
 Hybrid orbitals and bonding
o Carbon atoms use combinations of 2s and 2p orbitals = form hybridised orbitals
 Hybrid orbitals = mathematic combinations of wavefunctions for 2s and 2p orbitals
o 3 possibilities for making spn orbitals
 s + px + py + pz = 4 sp3
 New hybridised orbitals have 4 lobed that point towards the corners of a
tetrahedron
 Each hybridised orbital can bond with 1s of H atoms or combine with sp 3 hybrids on
other C atoms
o Forming double and triple bonds
 sp2 orbitals
 s + px + py = sp2
 Double bond
 Sigma bond – sp2 + sp2
 Pi bond – pz + pz