Electron Configuration of Atom

Electron configuration of an atom represents that how the electrons are distributed in its atom among the orbits(shells) and subshells. The electron configuration of atom is very important as it helps to predict the chemical, electrical and magnetic behavior of substance. Based on electron configuration of atom, we can predict that 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 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 represents the “principal quantum number”. For different main energy levels the value of ‘n’ and maximum number of electrons are given in table below-

  Sl. No.	Energy level or Orbit (shell)	Principal quantum number ‘n’	Maximum Number of electrons (2n2)
  1	K	1	2×12=2
  2	L	2	2×22=8
  3	M	3	2×32=18
  4	N	4	2×42=32

• The each main shell (energy level) is subdivided into subshells. These subshell are called orbitals. These subshells /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 subshells in any main shell is equal to “principal quantum number” ‘n’.

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

  Sl. No.	Subshell	Quantum number (l)	Electron capacity of subshell 2(2l+1)
  1	s	1	2(2 × 0+1)=2
  2	p	2	2(2 × 1+1)=6
  3	d	3	2(2 × 2+1)=10
  4	f	4	2(2 × 3+1)=14

• 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-

Based on above rules, the electron configuration of atom of some elements is shown in table below-

Element	Chemical Symbol	Atomic No.	Distribution of electrons over orbits (shells), 2n2				Electron configuration of atom
			K(n=1)	L(n=2)	M(n=3)	N(n=4)
Hydrogen	H	1	1	0	0	0	1s1
Helium	He	2	2	0	0	0	1s2
Lithium	Li	3	2	1	0	0	1s2 2s1
Beryllium	Be	4	2	2	0	0	1s2 2s2
Boron	B	5	2	3	0	0	1s2 2s2 2p1
Carbon	C	6	2	4	0	0	1s2 2s2 2p2
Nitrogen	N	7	2	5	0	0	1s2 2s2 2p3
Oxygen	O	8	2	6	0	0	1s2 2s2 2p4
Fluorine	F	9	2	7	0	0	1s2 2s2 2p5
Neon	N	10	2	8	0	0	1s2 2s2 2p6
Sodium	Na	11	2	8	1	0	1s2 2s2 2p6 3s1
Magnesium	Mg	12	2	8	2	0	1s2 2s2 2p6 3s2
Aluminum	Al	13	2	8	3	0	1s2 2s2 2p6 3s2 3p1
Silicon	Si	14	2	8	4	0	1s2 2s2 2p6 3s2 3p2
Phosphorus	P	15	2	8	5	0	1s2 2s2 2p6 3s2 3p3
Sulphur	S	16	2	8	6	0	1s2 2s2 2p6 3s2 3p4
Chlorine	Cl	17	2	8	7	0	1s2 2s2 2p6 3s2 3p5
Argon	Ar	18	2	8	8	0	1s2 2s2 2p6 3s2 3p6
Potassium	K	19	2	8	8	1	1s2 2s2 2p6 3s2 3p6 4s1 Or [Ar]4s1
Calcium	Ca	20	2	8	8	2	1s2 2s2 2p6 3s2 3p6 4s2 Or [Ar]4s2
Scandium	Sc	21	2	8	8	2	1s2 2s2 2p6 3s2 3p6 3d1 4s2 Or [Ar]3d1 4s2

Where, [Ar]= 1s² 2s² 2p⁶ 3s² 3p⁶ represents the electron configuration of Argon which is having completely filled 3-shells. It is used to simplify the electron configuration of elements of higher atomic number.