There are unequal number of elements in the periods in the Periodic Table,
Period is the row of elements in the Periodic table. Along the period the number of electrons increases by one, and therefore the chemical properties vary from the element to element (as the outer electron shell is responsible for the chemical properties). Periodic table consists of seven periods, and they vary in length. The order of filling of the electronic levels (orbitals) influences the length and structure of the period. Each electronic level (first, second…. seventh) contains different number of sublevels (orbitals). Different sublevels, in turn, contain different number of electrons. For example, the first electronic level contains only one sublevel (s-orbitals), the second level – two (s- and p- orbitals), the third – three (s-, p-, d-) etc. according to the magnetic quantum numbers, the orbitals of different types contain different number of electrons: s- orbitals contain maximum 2 electrons, p-orbitals – 6 electrons, d- orbitals – 10 electrons etc. The order of filling, or the energy diagram of orbitals on different elements is rather clear. Anyway, the different maximum numbers of electrons for different levels and specific order the electrons fill that levels hints different length of the period: 2 elements of the first period, 8 elements for the second and forth.