Understanding points
Structure 3.1.7—Discontinuities occur in the trend of increasing first ionization energy across a period. (AHL)
Structure 3.1.8—Transition elements have incomplete d-sublevels that give them characteristic properties. (AHL)
Structure 3.1.9—The formation of variable oxidation states in transition elements can be explained by the fact that their successive ionization energies are close in value. (AHL)
Structure 3.1.10—Transition element complexes are coloured due to the absorption of light when
an electron is promoted between the orbitals in the split d-sublevels. The colour absorbed is
complementary to the colour observed. (AHL)
Trends in 1st IE
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Overall: increase across a period due to increasing effective nuclear charge and decreasing atomic radius
1.
Drop b/w 2 & 13 ˙.˙ p orbital is at higher energy level than s orbital and shielded
a.
e.g. Be & B
2.
Drop b/w 15 & 16 ˙.˙ full orbital has e- - e- repulsion compared to half-filled
a.
e.g. N & O
3.
Very high value for Group 18 ˙.˙ noble gases do not want to lose full octet
Transition element
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An element that has an incomplete d sublevel in one or more of its oxidation states
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Zn is not a transition element as it has a complete d orbital (Zn = [Ar] 3d¹⁰ 4s²) and does not form ions with incomplete d-orbitals
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Sc is not a transition element as its common ion, Sc3+, has no d electrons
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Incomplete d-orbitals allow the element to have variable oxidation states, form complex ions with ligands (colored) and have catalytic and magnetic properties
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Physical properties identical to metals
1.
high electrical and thermal conductivity
2.
high melting point
3.
malleable
4.
ductile – they can be easily drawn into wires
Variable oxidation states
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4s and 3d have similar energy levels
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4s is filled first because empty 4s is lower in energy level than empty 3d orbital. But filled 4s is higher in energy level than filled 3d orbital so 4s is lost first
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All the transition metals show both the +2 and +3 oxidation states
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The max oxidation state of the elements increases in steps of +1 and reaches a maximum at manganese
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Electron configuration
Element | 3d | 4s | Max O.S. | ||||
Sc | ↑ | ↑↓ | +3 | ||||
Ti | ↑ | ↑ | ↑↓ | +4 | |||
V | ↑ | ↑ | ↑ | ↑↓ | +5 | ||
Cr* | ↑ | ↑ | ↑ | ↑ | ↑ | ↑ | +6 |
Mn | ↑ | ↑ | ↑ | ↑ | ↑ | ↑↓ | +7 |
Fe | ↑↓ | ↑ | ↑ | ↑ | ↑ | ↑↓ | +6 |
Co | ↑↓ | ↑↓ | ↑ | ↑ | ↑ | ↑↓ | +5 |
Ni | ↑↓ | ↑↓ | ↑↓ | ↑ | ↑ | ↑↓ | +4 |
Cu* | ↑↓ | ↑↓ | ↑↓ | ↑↓ | ↑↓ | ↑ | +3 |
Zn | ↑↓ | ↑↓ | ↑↓ | ↑↓ | ↑↓ | ↑↓ | +2 |
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Half complete and fully complete d orbital is stable .˙. e- is excited from 4s
1.
Challenge: Cu will end up with 5 pairs in either case
2.
Atomic radii across d block doesn’t change much ˙.˙ effective nuclear charge doesn’t change and remain as +2 (+1 for Cr and Cu) as newly added e- is added to d orbital at lower energy level than 4s and adds to ‘shielding’
3.
Alloy is possible b/c transition metal atoms are of similar size
Catalytic properties: increase the rate of chemical reaction
Heterogeneous | ||
Fe | Haber Process | N2(g) + 3H2(g) ⇌ 2NH3(g) |
Ni | Hydrogenation | C2H4(g) + H2(g) → C2H6(g) |
V2O5 | Contact Process | 2SO2(g) + O2(g) ⇌ 2SO3(g) |
MnO2 | Decomposition of H2O2 | 2H2O2(aq) → 2H2O(l) + O2(g) |
Homogeneous | ||
Fe2+ | haemoglobin | O2–Fe2+ bond formation/breakage |
Magnetic properties
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Every spinning e- has its magnetic field
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Not limited to transition metals, e.g. Na (paramagnetic), Na+ (diamagnetic)
Ferromagnetism | Permanent
Unpaired e-s align parallel to each other regardless of external magnetic field
e.g. Fe, Ni, Co |
Paramagnetism | Weak attraction to magnetic field
Unpaired e-s are attracted to external magnetic field
All transition metals are paramagnetic
Strength of paramagnetic field ∝ no. of unpaired parallel d orbital e-s (Cr most paramagnetic) |
Diamagnetism | Weak repulsion to magnetic field
Paired e-s cancel out each other’s spin
Repelled by external magnetic field
Zinc diamagnetic (property of all materials - very weakly reactive to magnetic field), e.g. Cu (I) (in CuCl), Ti4+, Na+ |
Complex Ions
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Transition metals form complex ions with ligands to gain stability (bond formation = energy release)
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Coordination bond: donation of a lone pair
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Ligand: lone pair donor (nucleophiles / Lewis base) that coordinate transition metal ions
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Coordination number
1.
no. of ligands in a complex ion
2.
shape of complex ion depends on coordination no. (linear/square planar/tetrahedral/octahedral)
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Ligands
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Monodentate ligands: CO > CN- > NH3 > SCN- > H2O > OH- > H2O > X-
◦
X- = I- > Br- > Cl- > F-
•
Polydentate ligands: form 2 or more coordination bonds with the metal ion
1.
Bidentate: ethylenediamine, oxalate
2.
Hexadentate: EDTA
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Ligand strength is increased by:
1.
Lower EN of central atom of ligand: less EN means more likely to donate e-
2.
Presence of -ve charge: more e- rich, more likely to donate e-
•
d orbital splitting and colouring of complex
1.
As ligands approach the central ion, repulsion between lone pair and metal ion d orbital e- pairs splits d orbitals into 2 higher and 3 lower energy orbitals
2.
Absorption of visible region light excites an electron from lower energy to higher energy d orbital: the energy gap between lower and higher d orbitals corresponds to wavelength absorbed
3.
Complementary colour is transmitted and observed: opposite on the color wheel
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•
•
e.g. [Cu(H2O)6]2+ is cyan blue b/c light in red region is absorbed
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e.g. [Cu(NH3)4(H2O)2]2+ is deep blue b/c light in yellow region is absorbed
Factors that affect size of the d orbital energy gap & colour of complex
1.
Identity of the central atom (metal): higher Zeff pulls ligands more closely
2.
Oxidation number: higher ionic charge pulls ligands more closely
3.
Strength of ligand: charge density ∝ ΔE
→ greater repulsion b/w ligand and d orbital e-s causes greater d orbital splitting
→ shorter wavelength absorbed
Transition metal ion complexes are colourless if e- promotion not possible ˙.˙ of
1.
No 3d e-, e.g. Sc3+, Ti4+ → no e- for the transition
2.
Full 3d orbital, e.g. Cu1+ → full 3d orbital is too stable for transition to occur
Complex ionic compounds often exist as crystals, e.g.
Crystallisation
Anhydrous CuSO4 ⇌ CuSO4•5H2O
Further evaporation (heat)
•
Crystallisation = evaporating water from saturated CuSO4 solution
•
Anhydrous = colourless