ELECTRONIC CONFRURATION OF ATOM.

 Ans-:

1. WHAT IS ELECTRONIC CONFIGURATION OF ATOM ?

The electron configuration of an atom is how the electrons are distributed within the atom’s orbits (shells) and sub shells.

The electron configuration of an atom is important as it helps to predict the chemical, electrical and magnetic behavior of substance.

Based on electron configuration of atom, we can predict that whether two substances will chemically react or not and if they react, we can also predict what kind of reaction is likely to be happen and how strong it will be.

The electron configuration of an atom, describes the arrangement of electrons in space around the nucleus.

The electrons are distributed over different energy level. These energy levels are called the shell or orbits.

Basically the distribution of electrons over various shells (energy levels) is such that, the overall energy of all electrons of an atom remains minimum for stability of the atom.

The electron distribution over various energy level is governed by the following rules –

•     The maximum number of electrons in any main energy level (shell) is given by, ‘2n2’, where, n is an integer and representing the “principal quantum number”. For different main energy levels the value of ‘n’ and maximum number of electrons are given in table below-

 

No.	Energy level or orbit (shell)	Principal quantum number “n”	Maximum Number of electrons (2n2)
1	K	1	2x12=2
2	L	2	2x22=8
3	M	3	2x32=18
4	N	4	2x42=32

 

•     The each main shell (energy level) is subdivided into sub shells. These sub shell are called orbitals. These sub shells /orbitals are designated by s, p, d, f etc. with corresponding orbital quantum number, l = 0, 1, 2, 3, 4,…..(n-1) etc. The number of sub shells in any main shell is equal to “principal quantum number” ‘n’.

•     The maximum electron capacity of sub shells is governed formula of 2(2l + 1). The capacity of sub shells are given in table below-

•     Aufbau Principle
“Aufbau” is a German word whose meaning is “building up”. Therefore, “Aufbau Principle” is also called “building up principle”. According to this principle, the electrons occupy the orbitals in the order of increasing energy. The increasing energy order of various orbitals and order of occupation is as below –
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p
To simplify the understanding of order of increasing energy, “Madelung’s rule” is used, which is given below- 

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 , I :

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: nlSubshell Notation101s20,12s, 2p30,1,23s, 3p, 3d

n	I	Subshell Notation
1	0	1s
2	0,1	2s,2p
3	0,1,2	3s,3p,3d

 

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 

Value of I	0	1	2	3
Subshell Notation	S	P	D	F
No. of Orbitals	1	3	5	7
Values of m	0	-0,0,+1	-2,-1,0,+1,+2	-3,-2,-1,0,+1,+2,+3