Structure of the Atom

Electronic Configurations of Elements

The arrangement of electrons in the various shells (or energy levels) of an atom of the element is known as electronic configuration of the element.In other words, electronic configuration is the distribution of electrons in various shells (or energy levels) of an atom such as K shell,L shell, M shell, etc.
In order to write down the electronic configuration of an element,we should know two things :
(i) We should know the number of electrons in one atom of the element.
(ii) We should know the maximum number of electrons that can be accommodated in different shells of the atom.
The number of electrons in an atom of the element is given by the atomic number of the element,because the number of electrons in an atom of the element is equal to the atomic number of the element.For example, if the atomic number of an element is 12,then its atom contains 12 electrons.
The maximum number of electrons which can be put in a particular energy level or shell was given by Bohr and Bury. According to Bohr-Bury scheme :
1.The maximum number of electrons which can be accommodated in any energy level of the atom is given by $2n2$ (where n is the number of that energy level). Let us calculate the maximum number of electrons which can be put in the first four energy levels of an atom.
(i) For 1st energy level,n = 1 So,The maximum number of electrons in 1st energy level = $2n2$
= 2 x (1)2
= 2x1
= 2
(ii) For 2nd energy level, n = 2 So, The maximum number of electrons in 2nd energy level = $2n2$
= 2 x (2)2
=2x4
= 8
(iii) For 3rd energy level, n=3
So,The maximum number of electrons in 3rd energy level =$2n2$
= 2 x$(3)2$
= 2x9
= I8
(iv) For 4th energy level, n = 4
So,The maximum number of electrons in 4th energy level =$2n2$
= 2 x (4)2
= 2 x 16
= 32
Thus,the maximum number of electrons that can be accommodated in the first energy level is 2, for second energy level is 8, for third energy level is 18, and for the fourth energy level is 32. Now, the first energy level is called K shell, the second energy level is called L shell, the third energy level is called M shell and the fourth energy level is known as N shell.So, we can also say that the maximum number of electrons which can be accommodated in K shell is 2, for L shell is 8, for M shell is 18 and for N shell is 32.This can be put in the tabular form as follows :2.The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has thecapacity to accommodate more electrons.(If, however,the outermost shell of an atom is the first shell or K shell, then it cannot accommodate more than 2 electrons)
This means that normally, the outermost shell of an atom can take a maximum of 8 electrons only.For example, if M shell is the outermost shell of an atom then it can hold a maximum of 8 electrons only,though its maximum rated capacity is 18 electrons. This is due to the fact that "having 8 electrons in the outermost shell" makes the atoms very stable. If, however, the outermost shell is the first shell or K shell,then it can hold a maximum of 2 electrons only. This is because having 2 electrons in the outermost shell when it is first shell or K shell (there being no other electron shells in the atom) also makes the atom very stable. This happens in the case of helium element which has only one shell (K shell) in its atom.
3.Electrons in an atom do not occupy a new shell unless all the inner shells are completely filled with electrons.
This means that the electron shells in an atom are filled in a step-wise manner. First of all the electrons fill K shell, then L shell, M shell, N shell, and so on. There are, however, some exceptions to this rule (which apply to elements having atomic numbers more than 18). We will study this in higher classes.
Keeping these points in mind, let us learn to write the electronic configurations of the elements.
Sample Problem 1. Write the electronic configuration of an element X whose atomic number is 12.
Solution.Atomic number of the element X is 12 which means that one atom of X has 12 electrons.First of all the electrons will go to K shell which can take a maximum of 2 electrons. Thus, the first 2 electrons will occupy K shell and we write $A=\begin{array}{c}k\\ 2\end{array}$ for it. After filling the K shell, the electrons will go to L shell.Now,L shell can take a maximum of 8 electrons for which we write $A=\begin{array}{c}L\\ 8\end{array}$. In this way 2 + 8 = 10 electrons have been accommodated, and we are left with 2 more electrons. The remaining 2 electrons go to M shell and we write M for it. Writing all the electron shells together, the electronic configuration of the element X becomes :
$A=\begin{array}{c}k\\ 2\end{array}$
$A=\begin{array}{c}L\\ 8\end{array}$
$A=\begin{array}{c}M\\ 2\end{array}$
Please note that it is not necessary to write the shells K, L and M,etc.,while writing the electronic configurations of the elements. For example,the electronic configuration of the element X of atomic number 12 can be simply written as 2, 8, 2. It is, however, better to indicate their shells also. Another point to be noted is that we have given so many details in the solution of this problem just to make you understand the things clearly. There is no need to write so many details in the exam. The arrangement of electrons in the atom of element X has been shown in Figure 23.
Sample Problem 2. Write the distribution of electrons in a carbon atom. (Atomic number of carbon = 6)
Solution. The atomic number of carbon is 6, so a carbon atom has 6 electrons in it. Out of 6 electrons :
(i) the first 2 electrons will occupy K shell. For this we write $A=\begin{array}{c}k\\ 2\end{array}$
(ii) the remaining 6-2 = 4 electrons will go to L shell. For this we write $A=\begin{array}{c}L\\ 4\end{array}$.
So,the distribution of electrons in a carbon atom ( or the electronic configuration of a carbon atom) will be :$A=\begin{array}{c}k\\ 2\end{array}$$A=\begin{array}{c}L\\ 4\end{array}$ or 2, 4
Sample Problem 3. Write the electronic configuration of sodium atom (Atomic number of sodium = 11)
Solution.The atomic number of sodium is 11. This means that a normal sodium atom contains 11 electrons. Out of 11 electrons of sodium atom, the first 2 electrons will occupy K shell, the next 8 electrons will occupy L shell and the remaining 1 electron will go to M shell. So, the electronic configuration of sodium atom will be :
$A=\begin{array}{c}k\\ 2\end{array}$$A=\begin{array}{c}L\\ 8\end{array}$$A=\begin{array}{c}M\\ 1\end{array}$or 2, 8, 1
Sample Problem 4. What would be the electronic configuration of a positively charged sodium ion,Na+ ? What would be its atomic number ?
Solution. The atomic number of sodium is 11. So, a neutral sodium atom (Na) has 11 electrons in it.
(i)A positively charged sodium ion ($Na+$) is formed by the removal of 1 electron from a sodium atom.So, a sodium ion has 11 - 1 = 10 electrons in it. Thus, the electronic configuration of a sodium ion will be
$A=\begin{array}{c}k\\ 2\end{array}$$A=\begin{array}{c}L\\ 8\end{array}$ or 2, 8.
(ii) The atomic number of an element is equal to the number of protons in its atom. Since a sodium atom as well as a sodium ion contain the same number of protons, therefore, the atomic number of sodium ion is the same as that of a sodium atom, which is 11.
Sample Problem 5. Explain why, sodium ion,$Na+$,has completely filled K and L shells.
Solution. A sodium ion, $Na+$,has 10 electrons in it. Now,the maximum capacity of K shell is 2 electrons and that of L shell is 8 electrons. Taken together, the maximum capacity of K and L shells is 2 + 8 = 10 electrons. A sodium ion Na+ has completely filled K and L shells because its 10 electrons can completely fill up K and L shells.
Sample Problem 6. If both K and L shells of an atom are full, what is the total number of electrons in the atom ?
Solution.K shell becomes full with 2 electrons and L shell becomes full with 8 electrons.So,if the Kand L shells of an atom are full, then it will have a total of 2 + 8 = 10 electrons in it.
Sample Problem 7. An element has 2 electrons in the M shell. What is the atomic number of the element ?
Solution. There are 2 electrons in the M shell of the atom of this element. This means that the K and L shells of the atom of this element (which come before M shell) are completely filled with electrons.Now, K shell can accommodate 2 electrons, L shell can accommodate 8 electrons and we have been given that M shell has 2 electrons. So, the electronic configuration of this element becomes 2, 8, 2.Thus, the number of electrons in its atom is 2 + 8 + 2 = 12. Since the number of electrons in a neutral atom is equal to the number of protons,therefore, the atomic number of this element is 12.

Notes

Electron shell	Maximum capacity
K shell L shell M shell N shell	2 electrons 8 electrons 18 electrons 32 electrons