Electron Configuration

Electronic configuration is defined as the distribution of electrons into the orbitals of an atom. Every neutral atom consists of a fixed number of electrons which is equal to the number of protons and is called the atomic number. Apart from electrons and protons, an atom also has neutrons which may or may not be equal to the number of the protons. In an atom, the protons and the neutrons lie in the nucleus and have almost negligible role in governing any chemical reaction. However, the electrons are the ones which lie outside the nucleus of the atom and their precise distribution inside an atom play a very important role in governing the chemical reactions that the atom involves in. Electronic configuration of an atom defines the precise distribution of electrons in an atom and this distribution helps in understanding the reasons for the chemical reactions that the atom or its corresponding molecules get involved in.

Subshells

Before we know how to write the electronic configuration, it is very important to know the concept of shell, subshell, and atomic orbital. 

When the Schrodinger wave equation is solved for a system, the solutions obtained from it give us the possible energy levels that the electrons can occupy and the corresponding wave function(s) of the electrons associated with each energy level. The solution to the Schrodinger wave equation for a system gives us the quantized energy states which an electron can occupy and is characterized by a set of three quantum numbers:

1. Principal quantum number, n: It can be visualized to be the quantum number assigned to the shells or orbits in which the electrons lie and this is similar to the orbit/shell that was discussed by Bohr in his atomic model.

Every shell has a fixed number of atomic orbitals and as the value of n increases, the number of allowed atomic orbitals also increases accordingly. Every shell is designated a value which is basically the principal quantum number. So, for the 1st shell n=1, for the 2nd shell n=2, for the 3rd shell n=3 and so on.

n = 1 2 3 4…

Shell= K L M N…

2. Azimuthal quantum number, l: It is the quantum number which is assigned to the subshells. However, they are basically the orbital angular momentum or subsidiary quantum number. Every shall has a fixed number of subshells/sublevels. The number of subshells is equal to the value of the principal quantum number i.e. n. So, for n=1 i.e. 1st shell, there can be only one subshell, and the corresponding value for l=0. For n=2 i.e., 2nd shell, there can be 2 subshells, and their value corresponds to l=0 and 1. For n=3, i.e. 3rd subshell, there can be 3 subshells and their values correspond to l=0, 1 and 2; and so on.

So, we can say that every subshell is assigned an Azimuthal quantum number, and for every subshell we have a corresponding symbol in order to designate the subshell.

Value of l = 0 1 2 3 4…

Symbol/notation for subshell = s p d f g…

So, the notation for different subshells go this way: 

So the 1st shell has just one subshell i.e. 1s. The 2nd shell has 2 subshells 2s and 2p. The 3rd shell has 3 subshells i.e. 3s, 3p, and 3d and so on.

3. Magnetic orbital quantum number, m: It is basically the quantum number assigned to different atomic orbitals present in a subshell. Every atomic orbital has a particular spatial orientation with respect to the standard set of coordinate axes and this differentiates atomic orbitals within a subshell and every atomic orbital in a subshell is designated with a magnetic quantum number. For a sub-shell defined by value ‘l’, there can be 2l+1 values of ‘m’ i.e. the total no. of orbitals in that subshell can be 2l+1 and their corresponding values of m goes this way: -l to +l.

So, 1st shell, n=1 has 1 subshell i.e. 1s and it can have just one atomic orbital (2*0+1=1 so only 1 orbital) and the corresponding magnetic quantum number, ‘m’ for the single orbital is 0 itself. 2nd shell, n=2 and has 2 subshells i.e. 2s and 2p. 2s subshell can have only one atomic orbital (2*0+1=1 so only 1 orbital) and the corresponding value for ‘m’ is 0. However, 2p subshell can have 3 atomic orbitals which are designated as 2px, 2py and 2pz (2*1+1=3 so 3 orbitals) and the corresponding values of ‘m’ are  -1,0,+1.

4. Spin quantum number, s: The electrons in an atom have a particle property; it spins on its own axis at a particular speed. The spin quantum number, denoted by s, indicates the orientation of the electron’s angular momentum. It indicates the quantum state, energy, and orbital shape and orientation of the electron. There are only 2 possible values of a spin quantum number are +½ or -½ ( meaning 'spin up' and 'spin down').

On the whole: 

Writing the Electronic Configuration

How to write electronic configuration: 3 sets of rules govern the writing of electronic configuration. They govern the electronic configuration of all elements. They are:

1. Aufbau principle

2. Pauli’s exclusion principle

3. Hund’s Rule

However, one can write the electronic configuration just by understanding the Aufbau principle. It states that, in the ground state, the electrons occupy the atomic orbitals in their order of increasing energies, which is given by n+l rule. Higher the value of n+l for the orbital, higher is the energy. If two orbitals have the same value for n+l, the orbital with lower value of n will have the lower energy and so the electrons will occupy that first.

So, the order of filling of the electrons goes this way:

1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,4f,5d,6p,7s…

Each atomic orbital can just accommodate only 2 electrons that are in opposite spin only.

So the distribution of electrons goes this way: 

So, the 1st shell can have 2 electrons in just 1s. The 2nd shell can have 8 electrons, 2 in 2s and 6 in 2p. The 3rd subshell can have 18 electrons, 2 in 3s, 6 in 3p and 10 in 3d.

And the order of filling of the electrons in these orbitals is done according to the rules stated above.

Electron Configuration

Electron configuration can be defined as the distribution  of electrons across the orbitals of an atom. Each neutral atom has a fixed number of electrons which equals the number of protons present and is called the atomic number. Along with the protons and electrons, the atom consists of neutrons as well which may or may not be in the same quantity as the number of the protons. The protons and neutrons lie inside the nucleus in an atom and have a negligible role in regulating any chemical reactions. On the other hand, the electrons lie outside the nucleus of the atom and have precise distribution inside an atom. Thus, they play a crucial role in determining the important role in carrying out chemical reactions which involve the atom. Electronic configuration of an atom can thus be defined as the precise distribution of electrons which are negatively charged in the atom and this distribution helps us understand the reasons behind the chemical reactions of an atom and also its corresponding molecules. For electronic configuration we must know about shells and subshells. Shell has the maximum number of electrons that it can accommodate and is based on the principal quantum number which is written as n. It can be found by the formula 2n² 

Here, the n is the shell number. The values of n, shells and total number of electrons are accommodated differently. Subshells are defined as the shells where the electrons are distributed and this is based on azimuthal quantum number which is denoted as I. The quantum number depends upon the principle quantum number which is denoted as n.

The configuration of atoms has a standard notation in which all the atomic subshells which have elections are written in the subscript. They are played in a sequence.  For example, sodium's electronic configuration is 1s²2s²2⁶3s¹.