From the above discussions, you are aware that shells of different energies exist in an atom. The electrons occupy these shells according to the increasing order of their energy. You also know that the first shell can have two electrons whereas the second shell can accommodate eight electrons. Keeping these points in mind, let us now study the filling of electrons in various shells of atoms of different elements.
Hydrogen atom has only one electron. Thus electronic configuration of hydrogen can be represented as 1.
The next element helium (He) has two electrons in its atom. Since the first shell can accommodate two electrons; hence, this second electron can also be placed in first shell. The electronic configuration of helium can be represented as 2.
The third element, Lithium (Li) has three electrons. Now the two electrons occupy the first shell whereas the third electron goes to the next shell of higher energy level, i.e. second shell. Thus, the electronic configuration of Li is 2, 1.
Similarly, the electronic configurations of beryllium (Be) and boron (B) having four and five electrons respectively can be written as follows:

Be	-	4 electrons	Electronic configuration	-	2,2
B	-	5 electrons	Electronic configuration	-	2,3

The next element carbon (C) has 6 electrons. Now the sixth electron also goes to the second shell which can accommodate eight electrons. Hence, the electronic configuration of carbon can be represented as 2, 4. Similarly, the next element nitrogen having 7 electrons has the electronic configuration 2, 5.
The electronic configuration of other elements can be given on the same lines. The electronic configuration of first twenty elements is given in Table 3.2 and depicted in Fig. 3.7.