Transition Metals

Question 1

What is transition metal

Answer 1

• transition element is a d block element that can form one or more stable ions with partially filled d orbitals

Question 2

Atomic radius and ionisation energy are relatively invariant

Answer 2

• nuclear charge increases from Ti to Cu as the number of protons increases
• additional electrons are being added to inner 3d orbitals which causes an increase in shielding effect
• this cancels out the effect due to increase in nuclear charge
• electrostatic forces of attraction of valence electrons from nucleus remains fairly constant

Question 3

Transition metals have higher melting point compared to s block elements

Answer 3

• have giant metallic lattice structure held together by strong metallic bonds
• in transition metals, both the 3d and 4s electrons are available for metallic bonds since the energy level difference between 3d and 4s orbital is small
• in s block elements, only one or two 4s electrons are available for metallic bonding
• the greater number of delocalised electrons available for metallic bonding in transition elements result in stronger electrostatic attraction between the positively charged ions and ‘sea’ of delocalised electrons
• more energy is required to overcome stronger metallic bonds in transition elements as compared to s block element except Mn with stable d⁵ arrangement

Question 4

Transition metals have higher densities compared to s block elements

Answer 4

• transition elements have relatively smaller atomic radii and higher atomic mass compared to s block elements
• have more closely packed structures due to their stronger metallic bonding as compared to s block elements
• transition elements are denser than s block elements

Question 5

Why transition metals have variable oxidation states in their compounds

Answer 5

• electrons in 3d and 4s subshells are similar in energy
• different number of these electrons are avaliable for use in bond formation
• ions formed by using different number of electrons for bonding are of similar stability
• max oxidation state = 4s electrons + unpaired 3d electrons

Question 6

Catalytic property of transition elements

Answer 6

— heterogenous catalyst
— homogeneous catalyst
- ability to exist in variable oxidation states
- catalyst should have an electrode potential between those of the reactants

Question 7

Formation of stable complexes and ligand

Answer 7

• complex consists of a central atom or ion surrounded by anions or molecules called ligands
• ligands is a neutral molecule or an anion that has at least one lone pair of electrons to be used in forming a dative bond to the central metal atom or ion
• all ligands are Lewis bases as they are lone pair donors

Question 8

Co-ordination number

Answer 8

• total number of dative bonds that are attached to the central atom or ion

Question 9

Why do transition metal ion form complexes

Answer 9

• transition metal ions have small ionic radius and hence high charge densities,
• attracts ligands that contain lone pair of electrons
• first row transition metal ions have vacant 3d orbitals of low energy level that can accommodate lone pair of electrons from ligands

Question 10

Isomerism in complexes

Answer 10

— structural isomerism
- ionisation isomerism
- hydration
— stereochemical isomerism
- cis-trans isomerism(square planar, octahedral complexes)
- enantiomerism(octahedral complexes containing at least 2 bidentate ligands)
— conformational isomerism
- different geometries
— polymerisation isomerism

Question 11

Acidity of transition metal ions in solution

Answer 11

• weak acids
  -transition metal ions have high charge density
• O-H bonds in H₂O ligands are polarised and break
• aqua complex ions undergo partial hydrolysis to give H₃O⁺

Question 12

Why Sc³⁺, Ti⁴⁺, Cu⁺, Zn⁺ are not coloured

Answer 12

• Sc³⁺, Ti⁴⁺ do not have electrons in the d orbitals for d-d transition to occur
• Cu⁺, Zn⁺ do not have a partially filled d orbitals in the higher energy level to allow for excitation from lower energy d orbitals

Question 13

Why are s block elements not coloured

Answer 13

• for s block element to undergo the electron transition, the electron must absorb in the region of very high energy
• 2p-3s transitions are out of the energy range of visible light
• energy from visible light is not absorbed
• solution of Na⁺ is colourless

Question 14

Why an aqueous solution of Cr³⁺ is colourless

Answer 14

• in presence of water ligands, the partially filled 3d orbitals of Cr³⁺ are split into two different levels with a small energy gap
• when energy is absorbed from the visible light region, an electron is promoted from the d orbital of low energy level to a d orbital of high energy level
• colour of Cr³⁺ observed is complimentary of the colour absorbed

Question 15

Why do d-orbitals split into two sets of energy levels in the presence of ligands

Answer 15

• electrostatic repulsion occurs between the lone pair of electrons on the donor atom of approaching ligand and the electrons in d-orbital of transition metal
• strength of repulsion experienced by the 5 d-orbitals is not equal because of the way they arranged differently in space

Question 16

D-orbital splitting in octahedral complex

Answer 16

• six ligands approach the central metal cation along the x, y and z axes
• since lobes of 3d_z² and 3d_x²-y² orbitals lie along these axis,
• greater electrostatic repulsion occurs between the lone pairs of ligands and electrons in these orbitals and have higher energy level
• smaller electrostatic repulsion occurs between the long pairs of the ligands and electrons in the 3d_xy, 3d_zx and 3d_yz orbitals and they have lower energy level

Question 17

Factors affecting colours of complex ions

Answer 17

• extent of d orbital splitting in a complex which is measured by magnitude of energy gap determines the colours absorbed and affects the colours observed
• identity of transition metal
• electronic configuration of transition metal ion
• type of ligands