Periodic Table

There are 118 confirmed elements, from hydrogen (1) to oganesson (118). The last four — nihonium, moscovium, tennessine and oganesson — were officially named in 2016.

Elements are ordered by atomic number (the number of protons). Rows are periods — elements in a period have the same number of electron shells. Columns are groups — elements in a group have similar outer electrons and so similar chemistry.

Radioactive elements with no stable isotopes don't have a standard atomic weight, so the table shows the mass number of their longest-lived isotope in brackets, such as [98] for technetium.

Fill orbitals in order of energy — 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on — with up to 2 electrons in s, 6 in p, 10 in d and 14 in f. Shorthand starts from the previous noble gas, e.g. iron is [Ar] 3d6 4s2. Some elements like chromium and copper are exceptions.

Group 1 is the alkali metals, group 2 the alkaline earth metals, groups 3–12 the transition metals, group 17 the halogens and group 18 the noble gases. The lanthanides and actinides are the two rows usually shown below the main table.

They fit between groups 2 and 3 in periods 6 and 7, but placing them inline would make the table very wide. Showing them below keeps it compact without changing the order.