Periodic Table
Chemistry · आवर्त सारणी
📋Quick Overview
The Periodic Table arranges all known elements based on their properties. Mendeleev (1869) arranged 63 elements by increasing atomic mass. The Modern Periodic Table (by Henry Moseley, 1913) arranges 118 elements by increasing atomic number. It has 18 vertical columns (groups) and 7 horizontal rows (periods).
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Modern Periodic Law (Moseley): Properties of elements are a periodic function of their ATOMIC NUMBER (not atomic mass).
📖Evolution of Periodic Table
| Scientist | Year | Contribution | Limitation |
|---|---|---|---|
| Dobereiner | 1829 | Law of Triads — groups of 3 elements where middle one's atomic mass = average of other two | Could find only a few triads |
| Newlands | 1866 | Law of Octaves — every 8th element has similar properties (like musical notes) | Worked only up to Calcium (Ca) |
| Mendeleev | 1869 | Arranged 63 elements by atomic mass. Left gaps for undiscovered elements (Eka-boron, Eka-aluminium, Eka-silicon) | Could not explain isotopes. Some elements out of order (Co before Ni) |
| Moseley | 1913 | Modern Periodic Table — arranged by atomic number (Z). 18 groups, 7 periods, 118 elements | Currently accepted — basis of modern chemistry |
📖Important Groups & Families
| Group | Name | Elements | Key Properties |
|---|---|---|---|
| Group 1 | Alkali Metals | Li, Na, K, Rb, Cs, Fr | Soft, silvery, highly reactive, stored in kerosene, 1 valence electron |
| Group 2 | Alkaline Earth Metals | Be, Mg, Ca, Sr, Ba, Ra | Less reactive than Group 1, 2 valence electrons, Ca in bones |
| Group 17 | Halogens | F, Cl, Br, I, At | Most reactive non-metals, 7 valence electrons, form salts with metals |
| Group 18 | Noble Gases | He, Ne, Ar, Kr, Xe, Rn | Inert/unreactive, complete outer shell (8 electrons, He has 2), used in lighting |
| Lanthanides | Rare Earth Elements | Ce to Lu (Z=58-71) | Placed separately below main table, Period 6 |
| Actinides | Radioactive series | Th to Lr (Z=90-103) | All radioactive, includes Uranium (U) and Plutonium (Pu) |
📖Metals, Non-Metals & Metalloids
| Property | Metals | Non-Metals | Metalloids |
|---|---|---|---|
| Position | Left side of periodic table | Right side | Along the zigzag line (staircase) |
| Nature | Electropositive (lose electrons) | Electronegative (gain electrons) | Both metallic & non-metallic |
| Examples | Fe, Cu, Au, Ag, Al, Na | C, N, O, S, P, Cl | B, Si, Ge, As, Sb, Te |
| Conductivity | Good conductors (heat & electricity) | Poor conductors (except graphite) | Semiconductors |
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Mercury (Hg) is the only metal that is liquid at room temperature. Bromine (Br) is the only non-metal that is liquid at room temperature. Gallium (Ga) melts in your hand (MP = 29.76°C).
📖Mendeleev's Predictions — Proved Right
| Mendeleev's Name | Actual Element | Discovered By | Year |
|---|---|---|---|
| Eka-boron | Scandium (Sc) | Nilson | 1879 |
| Eka-aluminium | Gallium (Ga) | Lecoq de Boisbaudran | 1875 |
| Eka-silicon | Germanium (Ge) | Winkler | 1886 |