nil, respectively. Hence, the symbol and the name respectively are Ubn and unbinilium. . ELECTRONIC CONFIGURATIONS OF ELEMENTS AND THE PERIODIC TABLE In the preceding unit we have learnt that an electron in an atom is characterised by a set of four quantum numbers, and the principal quantum number ( n ) defines the main energy level known as shell .
We have also studied about the filling of electrons into different subshells, also referred to as orbitals ( s, p, d, f ) in an atom. The distribution of electrons into orbitals of an atom is called its electronic configuration . An element’s location in the Periodic Table reflects the quantum numbers of the last orbital filled. In this section we will observe a direct connection between the electronic configurations of the elements and the long form of the Periodic Table.
(a) Electronic Configurations in Periods The period indicates the value of n for the outermost or valence shell. In other words, successive period in the Periodic Table is associated with the filling of the next higher principal energy level ( n = , n = , etc.). It can be readily seen that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled. The first period ( n = ) starts with the filling of the lowest level ( s ) and therefore has two elements — hydrogen (l s ) and helium (l s ) when the first shell ( K ) is completed.
The second period ( n = ) starts with lithium and the third electron enters the s orbital. The next element, beryllium has four electrons and has the electronic configuration s s . Starting from the next element boron, the p orbitals are filled with electrons when the L shell is completed at neon ( s p ). Thus there are elements in the second period.
The third period ( n = ) begins at sodium, and the