Comprehensive notes, solved MCQs, Short Questions for Class 9 Chemistry Chapter 8. Covers Periodic Law, Groups, Periods, Periodic Trends (Atomic Radius, Ionization Energy, Electronegativity), and Element Properties.
Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers.
Structure: Elements arranged in order of increasing atomic number from left to right in horizontal rows (periods). Elements with similar properties are placed in same vertical columns (groups).
Groups: 18 vertical columns. Elements in same group have similar chemical properties (same number of valence electrons).
Periods: 7 horizontal rows. Each period (except 1st) starts with alkali metal and ends with noble gas.
Period Details:
Element Families:
s-block: Groups 1-2. Valence electrons enter s-orbital. Highly reactive metals.
p-block: Groups 13-18 (except $He$). Valence electrons enter p-orbital. Contains metals, metalloids, and non-metals.
d-block (Transition Metals): Groups 3-12. Valence electrons enter d-orbital. Variable oxidation states.
f-block (Inner Transition): Lanthanides (Ce to Lu) and Actinides (Th to Lr). Valence electrons enter f-orbital. Called rare earth elements.
Atomic Radius: Half the distance between nuclei of two bonded identical atoms. Unit: Picometer (pm). 1 pm = 10$^{-12}$ m.
Trends: Increases down group (new shells added), Decreases across period (nuclear charge increases).
Ionization Energy: Energy required to remove most loosely bound electron from gaseous atom. Unit: kJ/mol.
Trends: Decreases down group (larger atomic size), Increases across period (stronger nuclear force).
Example: 1st IE of $Na$ = +496 kJ/mol: $Na_{(g)} \to Na^+_{(g)} + e^-$
Electronegativity: Force with which atom attracts shared pair of electrons in a bond. Most electronegative: $F$, $O$, $N$, $Cl$.
Trends: Decreases down group (atomic size increases), Increases across period (atomic size decreases).
Electron Affinity: Energy change when electron is added to outermost shell of gaseous atom. Unit: kJ/mol.
Metallic Character: Tendency to lose electrons and form positive ions.
Trends: Increases down group (easier to lose electrons), Decreases across period (harder to lose electrons).
Group 1 (Alkali Metals): 1 valence electron ($ns^1$). Highly electropositive. React with water: $2Na + 2H_2O \to 2NaOH + H_2$. React with halogens: $2Na + Cl_2 \to 2NaCl$. Oxides are basic. Reactivity increases down group.
Group 2 (Alkaline Earth Metals): 2 valence electrons ($ns^2$). Form $M^{2+}$ ions. Example: $Mg \to Mg^{2+} + 2e^-$. Oxides are basic: $CaO + H_2O \to Ca(OH)_2$.
Group 17 (Halogens): 7 valence electrons ($ns^2np^5$). Electronegative. Form $X^-$ ions. React with metals: $Ca + Cl_2 \to CaCl_2$. Reactivity decreases down group.
Group 16: 6 valence electrons ($ns^2np^4$). Form $M^{2-}$ ions. Oxygen most electronegative in group.