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CISCEISC (Science) ISC Class 12
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अभ्यासक्रम

Overview of d-and f-Block Elements

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Estimated time: 32 minutes
CISCE: Class 12

Definition: Transition Elements

The elements in which the differentiating electron enters the (n − 1) d-subshell of the penultimate shell are called transition elements.

CISCE: Class 12

Definition: d-Block Elements

The elements belonging to groups 3 to 12 of the periodic table, in which electrons are filled in the d-subshell, are called d-block elements.

CISCE: Class 12

Definition: Inner-Transition Elements

The elements in which the differentiating electron enters the (n − 2) f-subshell are called inner-transition elements.

CISCE: Class 12

Definition: f-Block Elements

The inner-transition elements that involve filling of the f-subshell and are placed separately at the bottom of the periodic table are called f-block elements.

CISCE: Class 12

Definition: Lanthanoids

The series of fourteen inner-transition elements in which the 4f-subshell is progressively filled is called the lanthanoid series.

CISCE: Class 12

Definition: Actinoids

The series of fourteen inner-transition elements in which the 5f-subshell is progressively filled is called the actinoid series.

CISCE: Class 12

Definition: Transition Series

A horizontal row of transition elements in which electrons are progressively filled in a particular (n − 1) d-subshell is called a transition series.

CISCE: Class 12

Key Points: Physical Properties of Transition Elements

Property Meaning Trend / Behaviour Reason / Explanation
Atomic Radii The size of atoms of transition elements is called atomic radii Slight decrease across a series; nearly constant in later elements Increase in nuclear charge is balanced by shielding effect of (n−1)d electrons
Ionic Radii The size of ions formed by transition elements is called ionic radii Decreases with increase in oxidation state Increase in effective nuclear charge
Metallic Character & Lattice Structure The tendency of transition elements to exhibit metallic nature and close-packed crystal structures is called metallic character High hardness, tensile strength, and metallic lustre Presence of unpaired d-electrons strengthens metallic bonding
Density Mass per unit volume of transition elements is called density Generally increases across a transition series Increase in atomic mass with slight decrease in atomic volume
Melting and Boiling Points The temperatures at which transition metals melt and boil are called melting and boiling points Very high compared to s- and p-block elements Strong metallic bonding due to participation of d-electrons
Enthalpy of Atomization The energy required to convert one mole of solid metal into gaseous atoms is called enthalpy of atomization High values, maximum near the middle of series Strong interatomic bonding
Ionisation Enthalpy The energy required to remove an electron from a gaseous atom is called ionisation enthalpy Intermediate between s- and p-block elements Small atomic size and high nuclear charge
Electrode Potentials The tendency of a metal to lose electrons measured in volts is called electrode potential Irregular variation across the series Variable oxidation states and hydration energies
CISCE: Class 12

Key Points: Chemical Properties of Transition Elements

Property Meaning Characteristic Feature Reason / Explanation
Oxidation States The valency shown by transition elements in their compounds is called oxidation state Variable oxidation states Participation of both ns and (n−1)d electrons
Formation of Coloured Ions The property of transition metal ions to exhibit colour in compounds is called formation of coloured ions Most compounds are coloured d–d electronic transitions
Magnetic Properties The behaviour of transition elements in a magnetic field is called magnetic property Mostly paramagnetic Presence of unpaired electrons
Paramagnetism The attraction of substances towards a magnetic field due to unpaired electrons is called paramagnetism Maximum near middle of series Maximum number of unpaired electrons
Catalytic Properties The ability of transition elements to increase the rate of a chemical reaction without being consumed is called catalysis Many metals and compounds act as catalysts Variable oxidation states and surface adsorption
Complex Formation The tendency of transition metals to form coordination compounds is called complex formation Large number of complexes formed High charge density and availability of vacant orbitals
Interstitial Compounds Compounds formed when small atoms occupy interstitial sites in metal lattice are called interstitial compounds Hard, high melting, non-stoichiometric Small size of H, B, C, N atoms
Formation of Alloys Homogeneous mixtures of transition metals with other metals are called alloys Improve strength and hardness Similar atomic size and crystal structure
Formation of Oxides Compounds formed by the reaction of transition metals with oxygen are called oxides Basic, acidic or amphoteric nature Variable oxidation states
Formation of Sulphides Compounds formed by reaction of transition metals with sulphur are called sulphides Generally formed in lower oxidation states Lower oxidation states are more stable
CISCE: Class 12

Key Points: Physical Characteristics of Lanthanoids

Physical Property Description Important Trend / Explanation
Occurrence The natural distribution of lanthanoids in the earth’s crust is called their occurrence Though called rare earths, they are fairly abundant; cerium is the most abundant; promethium is radioactive and does not occur naturally
Electronic Configuration The arrangement of electrons in lanthanoid atoms is called electronic configuration General configuration: [Xe]4f1–145d0–16s2; uncertainty due to very small energy difference between 4f and 5d orbitals
Atomic Radii The size of lanthanoid atoms is called atomic radii Decreases gradually from Ce to Lu
Ionic Radii The size of lanthanoid ions is called ionic radii Regular decrease for trivalent (Ln³⁺) ions
Lanthanoid Contraction The progressive decrease in atomic and ionic radii across the lanthanoid series is called lanthanoid contraction Caused by ineffective shielding of nuclear charge by 4f-electrons
Density The mass per unit volume of lanthanoids is called density Density generally increases with increase in atomic number
Ionisation Potential The energy required to remove an electron from a lanthanoid atom is called ionisation potential Shows slight increase across the series due to lanthanoid contraction
Magnetic Properties The behaviour of lanthanoid ions in a magnetic field is called magnetic property Most Ln³⁺ ions are paramagnetic; La³⁺ and Lu³⁺ are diamagnetic
Colour of Ions The property of lanthanoid ions to show colour is called colour of ions Most Ln³⁺ ions are coloured in solid and aqueous states
Cause of Colour The reason for colour in lanthanoid ions is called cause of colour Due to f–f electronic transitions
CISCE: Class 12

Key Points: Chemical Characteristics of Lanthanoids

Chemical Property Description Important Feature / Explanation
Oxidation States The valencies exhibited by lanthanoids in their compounds are called oxidation states +3 is the most common oxidation state
Stability of +2 Oxidation State The persistence of the divalent state in some lanthanoids is called stability of +2 state Shown by Ce, Nd, Sm, Eu, Tm and Yb; these ions act as strong reducing agents
Stability of +3 Oxidation State The predominance of the trivalent state is called stability of +3 state Most stable in aqueous solution due to high hydration and lattice energies
Stability of +4 Oxidation State The existence of the tetravalent state in some lanthanoids is called stability of +4 state Shown by Ce, Pr, Nd, Tb and Dy; Ce⁴⁺ is a strong oxidising agent
Basicity of Oxides and Hydroxides The basic nature of lanthanoid oxides and hydroxides is called basicity Basic strength decreases from La(OH)₃ to Lu(OH)₃ due to contraction
Complex Formation The tendency of lanthanoids to form coordination compounds is called complex formation Lanthanoids form fewer complexes than d-block elements
Reason for Poor Complex Formation The cause of weak complex formation is called reason for poor complex formation Large ionic size and non-participation of 4f orbitals in bonding
Formation of Alloys The ability of lanthanoids to form alloys with other metals is called alloy formation Form useful alloys such as misch metal and pyrophoric alloys
Reactivity The chemical activity of lanthanoids is called reactivity Highly electropositive; react readily with air, water, hydrogen and non-metals
CISCE: Class 12

Key Points: General Characteristics of Actinoids

Characteristic Description Important Feature / Explanation
Occurrence Ac, Th, Pa and U occur naturally; elements beyond U are artificially prepared Elements beyond uranium are called trans-uranic elements
Electronic configuration General configuration: ([Rn],5f^{1–14}6d^{0–1}7s^2) Uncertainty due to nearly equal energies of 5f and 6d subshells
Oxidation states Most common oxidation state is +3; also show +2, +4, +5, +6, +7 Participation of 5f, 6d and 7s electrons
Ionic radii Ionic radii decrease regularly across the series Due to poor shielding by 5f-electrons (actinoid contraction)
Colour of ions Most actinoid ions are coloured Ions with 5f⁰, 5f⁵, 5f⁷ configurations are colourless
Magnetic properties Many actinoid ions are paramagnetic Due to presence of unpaired 5f-electrons
Physical characteristics Silvery-white metals with high melting and boiling points All actinoids are radioactive; high density
Electropositive character and reactivity Actinoids are highly electropositive and reactive Readily react with H, O, halogens and acids
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