Period 1 element Period 1 element 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
Periodic table the last member of the f-block for period 6". Later spectroscopic work found that the electron configuration of ytterbium was in fact [Xe]4f6s. This meant that ytterbium and lutetium—the latter with [Xe]4f5d6s—both had 14 f-electrons, "resulting in a d- rather than an f- differentiating electron" for lutetium and making it an "equally valid candidate" with [Xe]5d6s lanthanum, for the group 3 periodic table position below yttrium. Lanthanum has the advantage of incumbency since the 5d electron appears for the first time in its structure whereas it appears for the third time in lutetium, having also made a brief second appearance