Periodic Table
Chemistry · आवर्त सारणी · 20 facts
Mendeleev(1869): 63 elements by atomic mass; left gaps for undiscovered
Modern Table by Moseley: arranged by atomic number(Z)
7 periods(rows) and 18 groups(columns)
Group 1: Alkali metals(Li,Na,K); most reactive metals; react with water
Group 2: Alkaline Earth(Be,Mg,Ca); less reactive than Group 1
Group 17: Halogens(F,Cl,Br,I); most reactive non-metals; form salts
Group 18: Noble gases(He,Ne,Ar); stable, full outer shell
Metals: left+centre; Non-metals: right; Metalloids: zigzag line
Lanthanides(4f) and Actinides(5f): inner transition elements at bottom
Lightest: Hydrogen(Z=1); Heaviest natural: Uranium(Z=92)
H unique: both metal+non-metal properties; in Group 1 but non-metal
Au most malleable; Ag best conductor; Hg+Br liquid at room temp
Metalloids: B, Si, Ge, As, Sb, Te
Dobereiner Triads(1829); Newlands Octaves(1866) — early classifications
Mendeleev predicted: Eka-Si=Germanium, Eka-Al=Gallium
Total 118 known elements; beyond 92 are synthetic/man-made
Period number = shells; Group relates to valence electrons
Transition metals(Groups 3-12): variable valency, coloured compounds, catalytic
Fe, Co, Ni: only 3 naturally magnetic elements
Noble gases: He used in balloons, Ne in signs, Ar in welding, Kr in flash