Periodic Table
Interactive periodic table of elements. Click any element for detailed information. Free online chemistry reference.
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Understanding the Periodic Table
The periodic table is one of science's most important tools. First published by Dmitri Mendeleev in 1869, it organizes all known chemical elements by increasing atomic number. Elements in the same column (group) share similar chemical behaviors because they have the same number of valence electrons.
Alkali Metals (Group 1)
Highly reactive soft metals that explode in water. Includes lithium, sodium, and potassium. They have one valence electron.
Alkaline Earth Metals (Group 2)
Reactive metals but less so than Group 1. Includes magnesium and calcium. They have two valence electrons and are essential for biology.
Transition Metals (Groups 3–12)
Hard, dense metals with high melting points. Includes iron, copper, gold, and silver. They form colored compounds and are excellent conductors.
Nonmetals
Poor conductors that gain electrons easily. Includes carbon, nitrogen, and oxygen. They form the basis of organic chemistry and life itself.
Halogens (Group 17)
Highly reactive nonmetals that form salts. Includes fluorine, chlorine, and iodine. They have seven valence electrons and readily accept one more.
Noble Gases (Group 18)
Inert, colorless gases that rarely react. Includes helium, neon, and argon. They have full valence shells, making them extremely stable.
Key Periodic Trends
- Atomic Radius: Increases down a group, decreases across a period from left to right.
- Ionization Energy: Decreases down a group, increases across a period.
- Electronegativity: Increases across a period and up a group; fluorine is the most electronegative element.
- Metallic Character: Strongest on the bottom left; nonmetallic character strongest on the top right.