Periodicity Of Elements

Concept: The d-block elements are those in which the last electron enters the subshell. These elements are also called transition elements and are located in the middle of the periodic table.
Counting d-block Elements: The d-block consists of four transition series:
3d series: Sc (21) to Zn (30) → 10 elements
4d series: Y (39) to Cd (48) → 10 elements
5d series: Hf (72) to Hg (80) → 10 elements
6d series: Rf (104) to Cn (112) → 10 elements

Total Number:

Concept: A transition element is defined as an element that has:
A partially filled d-orbital in its ground state, or
Forms at least one ion with an incomplete d-subshell.

Electronic Configuration: Scandium (Z = 21):
Contains one electron in the orbital.
d-subshell is partially filled.
Hence, it satisfies the definition of a transition element.

Zinc (Z = 30):
d-orbital is completely filled ( ).
Common ion:
d-subshell remains fully filled even after ion formation.

Conclusion:
Scandium: Partially filled d-orbital → Transition element.
Zinc: Fully filled d-orbital in atom and ion → Not a transition element.

Concept: Transition elements (d-block elements) have partially filled d-orbitals. Their electronic configuration allows flexibility in electron participation during bonding, leading to unique chemical and physical properties.
1. Variable Oxidation States: Reason:
The energy difference between and orbitals is very small.
Both types of electrons can participate in bond formation.
As a result, different numbers of electrons may be lost or shared.
Example:
Iron: and
Manganese: to
Thus, multiple oxidation states are observed.
2. Magnetic Properties: Reason:
Magnetic behavior depends on presence of unpaired electrons.
Partially filled d-orbitals often contain unpaired electrons.
Types of Magnetism:
Paramagnetism: Due to unpaired electrons (most transition metals)
Diamagnetism: When all electrons are paired
Ferromagnetism: Strong magnetic ordering (e.g., Fe, Co, Ni)

Summary:
Variable oxidation states arise from participation of both and electrons.
Magnetic properties arise from unpaired d-electrons.

Concept: Lanthanoids (elements from La to Lu) involve filling of the 4f orbitals. These electrons do not shield nuclear charge effectively, resulting in a steady increase in effective nuclear attraction across the series.
Definition of Lanthanoid Contraction: Lanthanoid contraction refers to the gradual decrease in atomic and ionic sizes of lanthanoid elements from lanthanum (La) to lutetium (Lu) with increasing atomic number.
Reason:
Addition of electrons occurs in the 4f orbitals.
4f electrons have poor shielding effect.
Effective nuclear charge increases across the series.
This pulls electrons closer to the nucleus, reducing size.

Consequences of Lanthanoid Contraction:
Similarity Between 4d and 5d Elements: Elements of second and third transition series (e.g., Zr and Hf) have nearly identical atomic radii and similar properties.
Decrease in Basic Strength of Hydroxides: Basicity of lanthanoid hydroxides decreases from La(OH) to Lu(OH) due to decreasing ionic size.
Difficulty in Separation of Lanthanoids: Similar sizes and chemical properties make their separation challenging.
Effect on Periodic Trends: Influences atomic size and properties of elements following lanthanoids in the periodic table.