Question

Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer 1

Hint: The electrons in a shell are filled according to the formula as $2{{n}^{2}}$, where n represents the energy level to which the electron belongs and an atom can have maximum of eight electrons in its valence shell and first fills the one shell completely and then moves to the next shell. Now, write the rules for filling the shells with the electrons.

Complete step by step answer:
The distribution of electrons into the shells, subshells and the orbitals of an atom is called its electronic configuration.
The various rules for writing the distribution of electron in the shells are as given below:
1.The maximum number of electrons which should be present in the shell i.e. K,L, M,N,O---and so on can be calculated by the formula as ;$2{{n}^{2}}$. Here n represents the energy levels. So, therefore, the number of electrons present in the first energy level i.e. when n=1 or the K shell is;
= $2\times {{(1)}^{2}}$
= 2

 The number of electrons present in the second energy level i.e. when n=2 or the L shell is;
= $2\times {{(2)}^{2}}$
= $2\times 4$
= 8

The number of electrons present in the third energy level i.e. when n=3 or the M shell is;
= $2\times {{(3)}^{2}}$
= $2\times 9$
= 18

The number of electrons present in the fourth energy level i.e. when n=4 or the N shell is;
= $2\times {{(4)}^{2}}$
= $2\times 16$
= 32
and similarly, we can count the number of electrons present in the shell by using this formula of $2{{n}^{2}}$.

2. An atom can have a maximum of eight electrons in its outermost valence shell and the presence of more than eight electrons cannot be accommodated by the valence shell of that very atom.
3.The electrons in an atom do not fill the second or the next shell unless and until the first shell is completely filled with the electrons i.e. the electrons first occupy the subshells and the orbitals of first shell and after completely filling it, they move onto the next shells i.e. L,M and N and so on.

Note: the filling of the orbitals in a shell takes place according to the Aufbau principle and the Hund’s rule of maximum multiplicity. Aufbau’s principle states that the electrons are filed in the increasing order of their energies i.e. from lower to higher energy levels and Hund’s law of maximum multiplicity states that the orbitals are first singly filled and after that pairing of electrons occurs.