CD - Energy and matter Flashcards
how can we see colour?
by absorbing the right amount of energy, electrons can move from a filled molecular orbital to an empty one - forming an excited state
- this energy can be in the form of UV or visible light
if the energy gap between filled and empty orbitals correspond to frequencies of visible light, then the molecule appears coloured
- its colour will be complementary to the frequency it absorbs
What affects the energy needed to excite electrons?
delocalisation
as delocalisation increases more molecular orbitals form and are closer in energy
less energy is needed to excite electrons and the frequency of light absorbed drops
this is why functional groups that extent the delocalisation in chromophores cause colour change
What affects the energy needed to excite electrons: single covalent bond
single covalent bond forms when two atomic orbitals each hold one electron
since each molecular orbital can hold 2 electrons only one is filled
the energy gap between these orbitals is very large
high frequency UV is needed to excite the electrons
What affects the energy needed to excite electrons: double bond
contains four molecular orbitals
smaller energy gap between the highest filled orbital and the lowest empty orbital than in a single bond
requires lower frequency UV to excite the electrons
What affects the energy needed to excite electrons: delocalised system
in a delocalised system (like benzene) many molecular orbitals are formed
these orbitals are closer in energy than in a double bond
the electrons in them absorb very low frequency UV and visible light to become excited
What is a delocalised system?
not all delocalised systems involve benzene rings
when C-C and C=C alternate electrons become delocalised - this is known as conjugation