Last reviewed 30 Sept 2026 · 7 min read
Why classify elements?
More than 118 elements are known. Classifying them into a table makes it easy to study their properties and to predict those of unknown elements.
History
| Scientist | Contribution |
|---|---|
| Döbereiner (1817) | Law of triads — groups of three elements with similar properties in which the atomic mass of the middle one is about the average of the other two (Li, Na, K; Ca, Sr, Ba; Cl, Br, I) |
| Newlands (1866) | Law of octaves — every eighth element has properties similar to the first (like musical notes); worked only up to calcium |
| Lothar Meyer | plotted atomic volume against atomic mass |
| Dmitri Mendeleev (1869) | Periodic law: the properties of elements are a periodic function of their atomic masses. Arranged elements in order of increasing atomic mass in groups and periods; left gaps for undiscovered elements and predicted their properties (eka-aluminium = gallium, eka-silicon = germanium, eka-boron = scandium) |
| Henry Moseley (1913) | showed that atomic number (not atomic mass) is the fundamental property — the Modern Periodic Law: the properties of elements are a periodic function of their atomic numbers. |
Defects of Mendeleev's table: the position of hydrogen was uncertain; isotopes had no place; some elements were placed out of order of atomic mass (e.g. cobalt before nickel, tellurium before iodine); the noble gases were not included (discovered later).
The modern periodic table
- Periods: the 7 horizontal rows. Elements in a period have the same number of shells, and properties change gradually across it. The number of elements in periods 1 to 7 is 2, 8, 8, 18, 18, 32, 32 (incomplete).
- Groups: the 18 vertical columns. Elements in a group have the same number of valence electrons and similar chemical properties.
- Blocks by the last electron's orbital: s-block (groups 1–2), p-block (groups 13–18), d-block (groups 3–12, the transition elements), f-block (lanthanides and actinides, placed at the bottom).
- Metals lie to the left and centre, non-metals to the top right, and metalloids (semi-metals: boron, silicon, germanium, arsenic, antimony, tellurium) along the zig-zag line between them.
Important families
| Group | Name | Valence electrons | Notes |
|---|---|---|---|
| 1 | Alkali metals (Li, Na, K, Rb, Cs, Fr) | 1 | soft, very reactive, stored in kerosene; react with water to give alkalis and hydrogen; reactivity increases down the group |
| 2 | Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) | 2 | less reactive than alkali metals |
| 3–12 | Transition elements | variable | hard, high melting points, form coloured compounds, variable valency, good catalysts (Fe, Ni, Pt, V₂O₅) |
| 13 | Boron family (B, Al, Ga, In, Tl) | 3 | aluminium is the most abundant metal in the Earth's crust |
| 14 | Carbon family (C, Si, Ge, Sn, Pb) | 4 | |
| 15 | Nitrogen family (pnictogens) (N, P, As, Sb, Bi) | 5 | |
| 16 | Chalcogens (O, S, Se, Te, Po) | 6 | |
| 17 | Halogens (F, Cl, Br, I, At) | 7 | very reactive non-metals ("salt formers"); fluorine is the most reactive; bromine is the only liquid non-metal; reactivity decreases down the group |
| 18 | Noble (inert) gases (He, Ne, Ar, Kr, Xe, Rn) | 8 (2 for He) | complete outer shell; very unreactive; helium is used in balloons, neon in signs, argon in bulbs |
Lanthanides (rare earths, atomic numbers 57–71) and actinides (89–103) form the f-block; transuranic elements (beyond uranium) are man-made.