The classification of elements has evolved to simplify the study of their vast number and properties. Initially, they were grouped as metals, nonmetals, and later, metalloids. As discoveries progressed, more systematic classifications emerged.

Dobereiner’s Triads (1817): German scientist Dobereiner grouped elements with similar chemical properties into sets of three, or triads. The atomic mass of the middle element was approximately the average of the other two (e.g., Li, Na, K). However, not all elements fit into triads.

Newlands’ Law of Octaves (1866): English scientist John Newlands arranged elements in increasing atomic masses. Every eighth element showed similar properties, like notes in a musical octave. For example, sodium resembled lithium. But this law worked only up to calcium, sometimes forced elements into the same slot, grouped dissimilar elements together, and failed to accommodate undiscovered ones.

Mendeleev’s Periodic Table (1869–1872): Russian scientist Dmitri Mendeleev arranged 63 known elements by increasing atomic masses, considering their chemical and physical properties. He formulated the Periodic Law: “Properties of elements are periodic functions of their atomic masses.” His table had groups (vertical) and periods (horizontal).
Merits:
• Corrected some atomic masses (e.g., Be from 14.09 to 9.4).
• Left gaps for undiscovered elements, predicting their properties accurately (e.g., eka-boron = Sc, eka-aluminum = Ga, eka-silicon = Ge).
• Later accommodated noble gases by creating a “zero group.”
Demerits:
• Ambiguity in placement of Co and Ni.
• Failed to explain isotopes.
• Rise in atomic mass was irregular.
• Position of hydrogen was uncertain.

Modern Periodic Table (1913 onwards): Henry Moseley proved atomic number (Z), not atomic mass, is fundamental. This led to the Modern Periodic Law: “Properties of elements are periodic functions of their atomic numbers.”
Features:
• Known as the long form, with elements in increasing atomic number.
• Resolved issues of isotopes and Co/Ni placement, though hydrogen’s position remains debated.
• Consists of 7 periods and 18 groups.
• Lanthanides and actinides shown separately.
• Now complete with 118 elements.
• Divided into s, p, d, and f blocks. D-block elements are transition metals.
• Zig-zag line in p-block separates metals, nonmetals, and metalloids.
• Position of elements depends on electronic configuration. Elements in the same group share valence electrons; those in the same period share electron shells.

Periodic Trends:
• Valency: Increases then decreases across a period; remains constant down a group.
• Atomic Size: Decreases left to right (greater nuclear pull); increases down a group (new shells added).
• Metallic Character: Tendency to lose electrons increases down a group, decreases across a period.
• Nonmetallic Character: Tendency to gain electrons increases across a period, decreases down a group.
• Reactivity: Higher in very electropositive metals or highly electronegative nonmetals.

This modern classification explains the properties and trends of all 118 known elements.