Classification of Elements and Periodicity - Modern Periodic Law

The Modern Periodic Law states that the physical and chemical properties of the elements are periodic functions of their atomic numbers ($Z$). This shifted the basis of classification from atomic mass ($A$) to atomic number.

Henry Moseley demonstrated that the atomic number is a more fundamental property than atomic mass by plotting $\sqrt{\nu}$ (square root of X-ray frequency) against $Z$, resulting in a straight line.

The Long Form of the Periodic Table is divided into four main blocks based on the orbital being filled: $s$-block, $p$-block, $d$-block, and $f$-block.

The $s$-block elements (Groups $1$ and $2$) and $p$-block elements (Groups $13$ to $18$) are collectively known as Representative Elements or Main Group Elements.

The $d$-block elements (Groups $3$ to $12$) are known as Transition Elements because they represent a transition in properties between the highly electropositive $s$-block and highly electronegative $p$-block.

The $f$-block elements (Lanthanoids and Actinoids) are known as Inner Transition Elements and are placed separately at the bottom of the table.

Periodicity refers to the recurrence of similar properties after regular intervals. This is due to the recurrence of similar outer electronic configurations.