4. FORMATION OF COLOURED IONS

normally the 3d orbitals of transition metal ions have the same energy

• but when ligands bond to the ions, some orbitals are given more energy than others

• splitting the 3d orbital into different energy levels

electrons tend to occupy the lower orbitals

ground state

to jump to the higher orbitals (excited states)

electrons need energy equal to the energy gap △E which is gotten from visible light

the energy absorbed when electrons jump up can be worked out

△E=hv=hc/λ

• △E is the energy absorbed (joules)

• h= planks constant (6.63 × 10^-34)

• v= frequency of light absorbed (hertz)

• c= the speed of light (3 × 10⁸ m s^-1)

• λ= wavelength of light absorbed (m)

the amount of energy needed to make electrons jump depends on:

• central metal ion

• metal ion’s oxidation state

• the ligands

• the coordination number

when visible light hits transition metal ions

frequencies are absorbed as electrons jump up to higher orbitals

the frequency absorbed depends on

the size of the energy gap

the rest of the frequencies is reflected

• reflected frequencies combine to make the complement of the colour of the absorbed frequencies

• this is the colour we see

if there are no 3d electrons or the 3d subshell is full

no electrons will jump so no energy will be absorbed

if there’s no energy absorbed

the compound will look white or colourless because all the light will be reflected

spectroscopy

can be used to determine the concentration of a solution by measuring how much light it absorbs

white light is shone through a filter

this filter is chosen to only let the colour of the light through that is absorbed by the sample

the light then passes through the sample to a colorimeter

this shows how much light was absorbed by the sample,

more concentrated solutions

absorb more light

before finding the unknown concentration of a sample

you must produce a calibration graph