Structure of atoms #2 : Electronic Configuration

The arrangement of electrons in the various shells/orbits/energy levels of an atom of the element is known as electronic configuration.

Bohr-Bury scheme

Electrons revolve around the nucleus in different energy levels or shells and each shell is associated with definite energy. The energy of the K shell is the least while those of L, M, N and O shells increases progressively. We also know that any system that has least energy is the most stable.

1^st energy level is K shell

2^nd energy level is L shell

3^rd energy level is M shell

4^th energy level is N shell and so on.

Electronic configuration of an element

Bohr and Bury Scheme - Important Rules

1. Maximum number of electrons that can be accommodated in a shell is given by 2n² where n=shell number

For 1^st energy level, n = 1

Maximum number of electrons in 1^st energy level = 2n²

2 x (1) 2 = 2

For 2^nd energy level n=2
Maximum number of electrons in the 2^nd energy level = 2n²

2 x 2² = 2 x 4 = 8

For 3^rd energy level n=3

Maximum number of electrons in the 3^rd energy level = 2n²

= 2x(3)²

= 2x9=18

For 4^th energy level n=4

Maximum no.of electrons in the 4^th energy level = 2n²

= 2x(4)²

= 2x16=32

Sl No.	Electron Shell	Maximum capacity
1.	K Shell	2 electrons
2.	L Shell	8 electrons
3.	M Shell	18 electrons
4.	N Shell	32 electrons

2. The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the Octet rule. The presence of 8 electrons in the outermost shell makes the atom very stable.

3. The shells have to be filled from inside to out, i.e. in the sequence K,L,M,.... and a new shell can't be filled before the interior one is filled up to its capacity.

Electronic configurations of some important elements can be derived using 2n^{2 to get the following result:}

Element	Symbol	Atomic number	Electronic configuration (or Electron arrangement) KLMN
Hydrogen	H	1	1
Helium	He	2	2
Lithium	Li	3	2,1
Beryllium	Be	4	2,2
Boron	B	5	2,3
Carbon	C	6	2,4
Nitrogen	N	7	2,5
Oxygen	O	8	2,6
Fluorine	F	9	2,7
Neon	Ne	10	2,8
Sodium	Na	11	2,8,1
Magnesium	Mg	12	2,8,2
Aluminium	Al	13	2,8,3
Silicon	Si	14	2,8,4
Phosphorus	P	15	2,8,5
Sulphur	S	16	2,8,6
Chlorine	Cl	17	2,8,7
Argon	Ar	18	2,8,8
Potassium	K	19	2,8,8,1
Calcium	Ca	20	2,8,8,2

Structural Representation of Atomic Structure

Example:

K L M
2,8,2

Special case of potassium and calcium elements

Atomic number of potassium is 19 and its electronic configuration is

K L M N
2  8 8  1

Atomic number of calcium is 20 and its electronic configuration is

K L M N
2 8 8 2