What is a transition element?
A d-block element that can form at least 1 stable ion with a partially filled (incomplete) sub-shell.
What order do transition metals loose electrons and why?
Loose electrons from the 4s shell first because the energy level of 4s is slightly lower than 3d10
What are the properties of transition metals?
- variable oxidation states
- form coloured ions in solution
- form complex ions
Why do transition metals have variable oxidation states?
Electrons stay in 3d and 4s which are very close together. Electrons are gained and lost using a similar amount of energy when they form ions.
What are they colours for the iron ions in solution?
Fe2+ = light green
Fe3+ = yellow
What are the colours for the copper ions in solution?
Cu2+ = blue
Cu+ ions are unstable in solution and will form Cu2+ and Cu readily.
What is a complex ion?
A central transition metals ion is surrounded by ligand bonded by dative covalent bonds.
What is a ligand?
An atom, ion, or molecule that has at least one lone pair of electrons.
They can be monodentate, bidentate, or polydentate.
What is a monodentate ligand?
They only have one lone pair of electrons
E.g. H2O (both pair of electrons are on the same atom so not bidentate)
:NH3
:Cl-
What is a Bidentate ligand?
A ligand that has 2 lone pairs on different atom.
E.g. ethandioate,
ethyldiamine (abriviated to en)
What is a polydentate ligand?
A ligand that has more than 2 lone pairs of electrons on different atoms
E.g. EDTA4- (It has 6 lone pairs of electrons to form bonds with the central metal ion)
What determines the shapes of complex ions?
- coordinate number (no. Bonds not no. Ligands)
- size of ligands ( some ligands are small and fit 6 around the central metal ion, some are larger and only fit 4. Ethandioate and ethyldiamine are larger, normally 3 fit around the central metal ion)
What shape is a complex ion with a coordinate number of 6?
- octahedral shape, with bond angle of 90
What is the bond angle and shape of a complex ion with a coordinate number of 4?
- tetrahedral shape, 109.5 bond angle
Covers most complexes with coordinate no. 4 - square planar, bond angle of 90
E.g. Cis-plantin Pt[(NH3)2(Cl)2]
What is a coordination number?
The no. Lone pairs donated to the metal ion, this is not always equal to the number of ligands.
What is a ligand exchange/ ligand substitution reaction?
A reaction where one ligand in a complex ion is replaced by a different one.
This normally leads to a colour change.
Why are complex metal ions coloured?
As some of the 3d orbitals are closer to the ligands than others they are destabilised, the others are stabilised.
This forms an energy gap which allows different frequencies of light to be absorbed so different colours are visible.
What happens when ligands bond to a central metal ion?
Some electrons in the 3d sub-shell gain energy and move to a higher energy level, others loose energy to stabilise the complex. This creates an energy gap allowing different frequencies of light to be absorbed.
What affects the amount of energy required from light for electrons to move to a different energy level?
- the central metal ion and its oxidation state
- the type of ligand
- the coordinate number.
Why happens to frequencies of light that are not absorbed by the complex ion and how does it affect what we observe?
Frequencies not absorbed are reflected or transmitted. This combination of frequencies causes a complementary colour to the one absorbed to be visible to observe.
Why are some complexes seen as white or colourless?
When complexes have full or empty 3d subshells, no electrons can migrate to other energy levels so no gap can be formed.
What does colorimetry do?
A colorimeter measures the absorbance of light by a coloured sample. The more concentrated a sample the darker it will be (more light is absorbed). This means it can be used to measure the concentration of transition metal ions in solution.
How does a colorimeter work?
- Set colorimeter to 0 using a blank sample
- White light is filtered into a narrow range of colours producing monochromatic light which must be able to be absorbed by the solution
- The monochromatic light passes through the sample in the curette and some light is absorbed
- The light not absorbed travels to the detector which measures the level of absorbance compared to the blank sample.
How can the results of colorimetry be used to find the concentration of an unknown solution?
Plot a calibration cube using known concentrations where the absorbance is measured. Then test the unknown solution and plot the absorbance on the curve to find the concentration.
