Periodic Classification — Study Notes

Overview

The periodic table is chemistry's foundational tool, organizing all known elements by atomic number and electron configuration. For Railway Group D exams, questions test your knowledge of the modern periodic table's structure, element groups, and how properties change predictably across periods and down groups. Understanding periodic trends — atomic size, valency, metallic character, reactivity — lets you predict element behavior without memorizing every single element's properties.

This topic typically yields 1–2 direct questions in the General Science section. Expect questions on group names (alkali metals, halogens), period/group numbers for given elements, or identifying which property increases/decreases in a particular direction. Master the 18-group modern table layout, first 20 elements, and the four major periodic trends. This knowledge also supports other chemistry topics like chemical bonding, metals/non-metals, and reactivity series.

Candidates often confuse Mendeleev's original table with the modern one or mix up trend directions. Focus on the modern periodic law (arrangement by atomic number, not atomic mass) and visualize the table's geography — metals on the left, non-metals on the right, metalloids along the staircase.

Key Concepts

• **Modern Periodic Law**: Elements are arranged in order of increasing atomic number (number of protons). Properties repeat periodically when elements are arranged this way. This corrected Mendeleev's atomic mass-based arrangement.

• **Periods and Groups**: The table has 7 horizontal rows (periods) and 18 vertical columns (groups). Period number = number of electron shells in an atom. Group number often relates to valence electrons (Group 1 = 1 valence electron, Group 17 = 7 valence electrons).

• **Metals, Non-metals, Metalloids**: Metals occupy the left and center (about 78% of elements), non-metals the upper right corner, and metalloids form a diagonal staircase boundary (B, Si, Ge, As, Sb, Te, At).

• **Valency**: The combining capacity of an element. For Groups 1–2, valency equals group number. For Groups 13–17, valency = 18 – group number (e.g., Group 17 has valency 1). Group 18 noble gases have valency 0 (stable, unreactive).

• **Atomic Size (Radius)**: Decreases left to right across a period (increasing nuclear charge pulls electrons closer), increases top to bottom down a group (additional electron shells added).

• **Metallic Character**: Increases down a group (easier to lose electrons with more shells) and decreases across a period left to right (non-metallic character increases toward the right).