Periodic table of elements

What it's for

A quick reference for students: instantly find the symbol and atomic number of the main chemical elements.

Note

The most common, studied elements are included. Search by name (oxygen), symbol (O) or number.

Why it's called «periodic»

Because lining the elements up by increasing atomic number makes the properties repeat at regular intervals: every so often an element reappears that behaves like one already seen. Mendeleev noticed this in 1869 and arranged the elements in a grid where similar ones end up one under another. The columns (the groups) gather elements with the same number of outer electrons, and it's the outer electrons that decide how an element reacts; the rows (the periods) say how many electron shells the atom has.

Everything studied afterwards follows from this structure: the first column is the alkali metals, all fiercely reactive with water; the last is the noble gases, which react with nothing because their outer shell is full. It isn't a table to memorize, it's a map.

The symbols that have nothing to do with the name

Eleven elements have a symbol that looks wrong, and the reason is that it comes from the Latin name: iron is Fe (ferrum), gold Au (aurum), silver Ag (argentum), copper Cu (cuprum), lead Pb (plumbum), tin Sn (stannum), sodium Na (natrium), potassium K (kalium), mercury Hg (hydrargyrum), antimony Sb (stibium). Tungsten is W, from the German Wolfram. Knowing this turns eleven exceptions to memorize into a single rule.

The capital letter changes the element

An element's symbol always has a capital first letter and a lower-case second one, and it isn't a typographic convention: «Co» is cobalt, «CO» is carbon monoxide, a compound of two different elements. It's the same reason «Ni» is nickel and «NI» means nothing. When a chemical formula «doesn't work», the first place to look is the capitals.

Nearby tools

For the mass of a formula, with each element's contribution, there is Molar mass, and to balance a reaction Balance a chemical equation. The Greek letters of formulas are in Greek alphabet and unit multiples in SI prefixes.