Editing Period 1 element (section)

{{Short description|Element in first row of periodic table}}
{{Redirect|Period 1|similarly named topics|First period (disambiguation)}}
{{Periodic table (micro)| title=Period 1 in the [[periodic table]] |  mark=H,He}}
{{Sidebar periodic table|expanded=structure }}
A '''period&nbsp;1 element''' is one of the [[chemical element]]s in the first row (or [[Periodic table period|period]]) of the [[periodic table|periodic table of the chemical elements]]. The periodic table is laid out in rows to illustrate periodic (recurring) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that analog elements fall into the same vertical columns. The first period contains fewer elements than any other row in the table, with only two: [[hydrogen]] and [[helium]]. This situation can be explained by modern theories of [[atomic structure]]. In a [[quantum mechanics|quantum mechanical]] description of atomic structure, this period corresponds to the filling of the [[atomic orbital|1s orbital]]. Period&nbsp;1 elements obey the [[octet rule#Exceptions|duet rule]] in that they need two [[electron]]s to complete their [[valence shell]].

Hydrogen and helium are [[Big Bang nucleosynthesis|the oldest]] and the most abundant elements in the [[universe]].