Periodic Table

The periodic table organizes 118 chemical elements by increasing atomic number, grouping them by similar properties.

Mendeleev's periodic table (1869): arranged elements by atomic mass; left gaps for undiscovered elements (correctly predicted Ga, Ge, Sc).

Modern periodic table (Moseley): arranged by atomic number — solves anomalies of mass-based ordering.

Structure:

• 18 vertical columns = groups (1–18). Elements in a group have similar chemistry because they have the same number of valence (outermost) electrons.
• 7 horizontal rows = periods (1–7). Period number = number of electron shells.

Group highlights:

• Group 1 (alkali metals): Li, Na, K, Rb, Cs, Fr. One valence electron. Very reactive (especially in water — Na fizzes, K ignites). Soft, low density.
• Group 2 (alkaline earth metals): Be, Mg, Ca, Sr, Ba, Ra. Two valence electrons. Less reactive than alkali metals.
• Groups 3–12 (transition metals): Fe, Cu, Zn, Ag, Au, etc. Hard, dense, often coloured compounds. Useful catalysts.
• Group 17 (halogens): F, Cl, Br, I. Seven valence electrons; highly reactive non-metals. Toxic in gaseous form.
• Group 18 (noble gases): He, Ne, Ar, Kr, Xe, Rn. Eight valence electrons (or 2 for He). Inert; rarely react.

Periodic trends:

• Atomic size: decreases left → right across a period (more protons pulling electrons in tighter). Increases top → bottom in a group (more shells).
• Metallic character: decreases left → right; increases top → bottom. Metals on left, non-metals on right.
• Ionization energy: energy to remove an electron. Increases left → right; decreases top → bottom.
• Electronegativity: tendency to attract electrons in a bond. Increases left → right; decreases top → bottom. Fluorine is most electronegative.

Diagonal relationships: certain pairs like Li-Mg, Be-Al, B-Si show similar properties because their charge/size ratios are alike.

Lanthanides and actinides are displayed separately below the main table (atomic numbers 57–71 and 89–103) due to space; chemically belong to f-block.

Element facts often asked in RRB:

Hydrogen (H, Z=1): lightest element; major fuel for stars; used in fertilizer (ammonia synthesis), rocket fuel (liquid H₂).

Helium (He, Z=2): second-lightest, non-flammable noble gas. Used in balloons, MRI machines, deep-sea diving (mixed with O₂ to prevent narcosis).

Carbon (C, Z=6): backbone of all organic life. Allotropes: diamond (hardest natural substance), graphite (electrical conductor), buckminsterfullerene (C₆₀), graphene.

Nitrogen (N, Z=7): 78% of air; inert at room temperature. Liquid N₂ at −196°C for cryogenics.

Oxygen (O, Z=8): 21% of air; essential for respiration. Discovered by Priestley (1774).

Sodium (Na, Z=11): reacts violently with water → NaOH + H₂. Used in street lamps (yellow glow).

Aluminium (Al, Z=13): most abundant metal in Earth's crust. Light, corrosion-resistant; aircraft, foil, cans.

Silicon (Si, Z=14): semiconductor; basis of chips and solar cells. Silicon Valley.

Phosphorus (P, Z=15): P₄ (white phosphorus) ignites in air; used in matches. Component of DNA and bones.

Chlorine (Cl, Z=17): greenish-yellow gas; bleach, water disinfection.

Calcium (Ca, Z=20): bones, teeth; CaCO₃ = limestone, marble, chalk.

Iron (Fe, Z=26): backbone of haemoglobin; structural metal in steel.

Copper (Cu, Z=29): excellent electrical conductor — wires; also alloyed with zinc (brass) and tin (bronze).

Silver (Ag, Z=47): best electrical and thermal conductor; antibacterial.

Gold (Au, Z=79): non-reactive; jewellery; standard reserve currency.

Mercury (Hg, Z=80): only metal liquid at room temperature. Used in thermometers (now phased out due to toxicity).

Lead (Pb, Z=82): dense; X-ray shielding. Toxic — phased out from paint and petrol.

Uranium (U, Z=92): heaviest natural element; nuclear fuel. U-235 (0.7% of natural U) is fissile.

Discovery oddities:

• 11 of the 92 natural elements were known to the ancients (Au, Ag, Cu, Fe, Sn, Pb, Hg, S, C, As, Sb).
• Element 113 (Nihonium) was the first element discovered by an Asian (Japan) team — 2004.