Periodic Table of Chemical Elements
Representation of the periodic table of chemical elements with color coding according to families. It helps predict the chemical properties of an element, understand periodic trends (electronegativity, atomic radius…), facilitate the study of chemical reactions, and guide the search for new materials.
History of the Periodic Table
The Periodic Table of Chemical Elements, one of the most important tools in chemistry, was developed by Dmitri Mendeleev (1834-1907) in 1869. Its original version classified the 63 known elements at the time by increasing atomic mass and highlighted the periodicity of their chemical properties. Mendeleev's great innovation was to leave blank spaces for elements unknown at the time, whose properties he predicted with remarkable accuracy.
10 Elements Unknown to Mendeleev But Essential Today
| Element | Symbol | Atomic No. | Discovery | Modern Uses |
|---|---|---|---|---|
| Helium | He | 2 | 1868 (solar spectroscopy) | Balloons, MRI, welding |
| Gallium | Ga | 31 | 1875 | Semiconductors (electronic chips) |
| Germanium | Ge | 32 | 1886 | Optical fibers, infrared lenses |
| Radium | Ra | 88 | 1898 (Pierre and Marie Curie) | Formerly in medicine (radiotherapy) |
| Neon | Ne | 10 | 1898 | Lighting (neon signs) |
| Plutonium | Pu | 94 | 1940 | Nuclear energy (civil and military) |
| Technetium | Tc | 43 | 1937 (1st artificial element) | Nuclear medicine (scanners) |
| Francium | Fr | 87 | 1939 | Fundamental research (very rare) |
| Uranium | U | 92 | 1789 (but properties unknown) | Nuclear power plants, armaments |
| Platinum | Pt | 78 | Known but not studied | Automobile catalysts, jewelry |
Historical Remarks
Mendeleev had predicted with remarkable accuracy the properties of gallium (which he called "eka-aluminum"), germanium ("eka-silicon"), and scandium ("eka-boron"). These subsequent discoveries validated his periodic table.
Structure and Organization
The modern table organizes the 118 known elements (to date) according to their atomic number (number of protons) and their recurring chemical properties. It consists of:
- 7 periods (horizontal rows)
- 18 groups (vertical columns)
- 4 blocks (s, p, d, f) according to the electronic subshell being filled
Classification of Elements
The elements are classified into three main categories:
- Metals (left and center) - good conductors, malleable
- Metalloids - intermediate properties
- Non-metals (right) - poor conductors, brittle
Families of Elements
Certain groups have particularly similar chemical properties:
- Alkali metals (Group 1) - very reactive
- Alkaline earth metals (Group 2)
- Halogens (Group 17) - easily form salts
- Noble gases (Group 18) - very stable, low reactivity
Synthetic Elements
Elements beyond uranium (atomic number >92) are generally synthetic, produced in laboratories. They are often very unstable and rapidly decay through radioactivity.
The energies required for nuclear fusion and why uranium is a limit 
The Atom in All Its Forms: From Ancient Intuition to Quantum Mechanics 
