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Chemistry · Class 9 · Chapter 3: Periodic Table and Periodicity of Properties

8 min

Key points

Periodic Table — A table in which elements are arranged in order of increasing atomic number
Modern Periodic Law — Physical and chemical properties of elements are periodic functions of their atomic numbers
Period — A horizontal row in the periodic table containing elements with same number of electron shells
Group/Family — A vertical column in periodic table containing elements with same number of valence electrons
Alkali metals (Group IA) are most reactive metals, stored under kerosene to prevent reaction with air
Noble gases (Group VIIIA) are chemically inert due to complete outer electron shells
Atomic radius decreases across a period due to increasing nuclear charge
Ionization energy increases across a period and decreases down a group
Electronegativity increases across a period and decreases down a group
Metallic character decreases across a period and increases down a group

Definitions

Periodic Table: A table in which elements are arranged in order of increasing atomic number
Period: A horizontal row in the periodic table containing elements with same number of electron shells
Group: A vertical column in periodic table containing elements with same number of valence electrons
Ionization Energy: The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom
Electronegativity: The ability of an atom to attract shared pair of electrons in a covalent bond

Worked example

Define atomic radius. Explain the trend of atomic radius in period and group of periodic table. (6 marks)

Atomic radius is half the distance between the nuclei of two identical atoms bonded together. Across a period, atomic radius decreases from left to right because nuclear charge increases while number of electron shells remains same, pulling electrons closer. Down a group, atomic radius increases because number of electron shells increases, making atoms larger despite increased nuclear charge.

Common mistakes

Students confuse periods with groups - remember periods are horizontal rows
Mixing up trends - atomic radius decreases across period but ionization energy increases
Writing wrong electronic configurations - always count electrons equal to atomic number
Forgetting that noble gases are inert, not reactive
Confusing valence electrons with total electrons in an atom

Quick recap

Modern periodic table arranges elements by increasing atomic number following periodic law. Atomic radius decreases across period, increases down group due to nuclear charge vs electron shell effects. Ionization energy and electronegativity increase across period, decrease down group. Electron affinity generally increases across period with some exceptions.

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