Periodic Table | Class IX

Dobereiner's Triads

Groups of three elements with similar properties; middle element's atomic mass ≈ average of the other two (e.g., Ca 40, Sr 88, Ba 137)

Newlands Law of Octaves

Elements arranged by increasing atomic mass; every 8th element repeats properties of the first, like musical octaves; worked for lighter elements but failed for heavier ones

Mendeleev's Periodic Law

Properties of elements are a periodic function of their atomic masses; arranged 63 elements into 8 groups with gaps for undiscovered elements; predicted properties like eka-aluminium (gallium)

Modern Periodic Law

Properties of elements are a periodic function of their atomic numbers; based on Henry Moseley's discovery of nuclear charge; explains isotopes and anomalous pairs

Modern Periodic Table Groups

18 groups (1-18); Groups 1-2/13-17: representative elements; 3-12: transition; 18: noble gases; lanthanides/actinides in Group 3

Periods in Modern Table

7 periods; 1: shortest (2 elements); 2-3: short (8 each); 4-5: long (18 each); 6-7: longest (32 each); number of shells = period number

Group 1: Alkali Metals

Li, Na, K, Rb, Cs, Fr; 1 valence electron; soft, reactive, form MOH + H₂ with water; reactivity increases down group; flame colors (e.g., Na golden yellow)

Group 2: Alkaline Earth Metals

Be, Mg, Ca, Sr, Ba, Ra; 2 valence electrons; harder than Group 1, form M(OH)₂ + H₂ with water; reactivity increases down group; flame colors (e.g., Ca brick red)

Group 17: Halogens

F, Cl, Br, I, At; 7 valence electrons; most reactive non-metals, reactivity decreases down group; form HX acids, diatomic molecules; color intensity increases (F pale yellow to I violet)

Group 18: Noble Gases

He, Ne, Ar, Kr, Xe, Rn; 8 valence electrons (He: 2); unreactive, monoatomic, low melting/boiling points; used in lights (colored discharge) and balloons (He)

Trends Down a Group

Atomic size increases; metallic character increases; valency constant; reactivity ↑ for metals, ↓ for non-metals

Trends Across a Period

Atomic size decreases; metallic character decreases; valency ↑ to 4 then ↓; oxides from basic to acidic