<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> A period 1 element is one of the chemical elements in the first row (or period) of the 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 elements with similar behaviour 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 mechanical description of atomic structure, this period corresponds to the filling of the 1s orbital . Period 1 elements obey the duet rule in that they need two electrons to complete their valence shell . The maximum number of electrons that these elements can accommodate is two, both in the 1s orbital . Therefore, period 1 can have only two elements . </P> <P> All other periods in the period table contain at least 8 elements, and it is often helpful to consider periodic trends across the period . However, period 1 contains only two elements, so this concept does not apply here . </P>

Why are there only two elements in period 1
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