What is the Periodic Law?

The Periodic Law states that when elements are arranged in order of increasing atomic number, their physical and chemical properties recur at regular (periodic) intervals. This is the foundational principle behind the Periodic Table of Elements, one of the most important organising tools in all of chemistry.

The law was first articulated by Russian chemist Dmitri Mendeleev in March 1869, who arranged 63 known elements by atomic weight and observed that properties repeated periodically. Mendeleev's genius was in leaving gaps for undiscovered elements and predicting their properties — predictions confirmed with the discovery of gallium (1875), scandium (1879), and germanium (1886).

The modern form of the Periodic Law is based on atomic number (number of protons) rather than atomic weight, following Henry Moseley's X-ray spectroscopy work in 1913. Today's periodic table contains 118 confirmed elements arranged in 7 periods (horizontal rows) and 18 groups (vertical columns), with elements in the same group sharing similar chemical properties due to identical valence electron configurations.

Key Features

# Feature Details
1 Mendeleev's Law (1869) Properties of elements are a periodic function of their atomic weights
2 Modern Periodic Law Properties of elements are a periodic function of their atomic numbers (Moseley, 1913)
3 Periods 7 horizontal rows; period number indicates the outermost electron shell being filled
4 Groups 18 vertical columns; elements in the same group have similar valence electron configurations
5 Periodic Trends Atomic radius, ionisation energy, electronegativity, and metallic character vary predictably
6 Mendeleev's Predictions Predicted eka-aluminium (gallium), eka-boron (scandium), eka-silicon (germanium)
7 Total Elements 118 confirmed elements; elements 113-118 officially named in 2016
8 Blocks s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12), f-block (lanthanides/actinides)

Important Concepts

  • Periodic trends across a period (left to right): atomic radius decreases, ionisation energy increases, electronegativity increases, and metallic character decreases. These trends result from increasing nuclear charge with the same number of electron shells.
  • Periodic trends down a group (top to bottom): atomic radius increases, ionisation energy decreases, electronegativity decreases, and metallic character increases. These trends result from additional electron shells increasing the distance from the nucleus.
  • Mendeleev vs Modern Table: Mendeleev arranged by atomic weight and encountered anomalies (e.g., tellurium and iodine). Moseley's atomic number arrangement resolved these inconsistencies.
  • Noble gases (Group 18) have completely filled electron shells, making them chemically inert — they were unknown when Mendeleev created his table and were added as a new group when discovered in the 1890s.
  • Lanthanides (elements 57-71) and actinides (elements 89-103) form the f-block, placed separately at the bottom of the table. Lanthanides (rare earth elements) are critical for electronics, magnets, and defence technology.
  • Valency and oxidation states of elements can be predicted from their group number — Group 1 elements have valency 1, Group 2 have valency 2, and so on for main-group elements.
  • Electron affinity — the energy released when an atom gains an electron — generally increases across a period, with halogens (Group 17) having the highest electron affinities.

UPSC Exam Corner

Prelims: Key Facts

  • Mendeleev presented the first periodic table in March 1869, based on atomic weight
  • The Modern Periodic Law (Moseley, 1913) arranges elements by atomic number
  • The modern table has 7 periods and 18 groups with 118 elements
  • Elements in the same group have similar chemical properties due to similar valence electrons
  • Electronegativity increases across a period (left to right) and decreases down a group
  • Atomic radius decreases across a period and increases down a group
  • Elements 113-118 were officially named in 2016 (Nihonium, Flerovium, Moscovium, Livermorium, Tennessine, Oganesson)
  • The table is divided into 4 blocks: s-block, p-block, d-block (transition metals), f-block (inner transition)
  • Ionisation energy increases across a period and decreases down a group
  • Metallic character decreases across a period (left to right) and increases down a group
  • UNESCO declared 2019 as the International Year of the Periodic Table, marking 150 years since Mendeleev

Mains: Probable Themes

  1. Significance of the Periodic Table as a predictive and organising tool in chemistry
  2. Discovery of new elements — role of nuclear physics and international scientific collaboration
  3. Periodic trends and their application in understanding chemical bonding, material science, and drug design
  4. Mendeleev's scientific method — hypothesis, prediction, and verification as a model for scientific inquiry
  5. Rare earth elements (lanthanides) and their strategic importance in modern technology and defence

Sources: Periodic Table — Wikipedia, Britannica — Periodic Table, Chemistry LibreTexts — The Periodic Law