Structure of the Atom

Arrangement of Electrons in an Atom (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 shell2^nd energy level is L shell3^rd energy level is M shell4^th energy level is N shell and so on

Bohr and Bury Scheme - Important Rules

• 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 = 1Maximum number of electrons in 1^st energy level = 2n²2 x (1) 2 = 2

• For 2^nd energy level n = 2Maximum number of electrons in the 2^nd energy level = 2n²2 x 2² = 2 x 4 = 8

• For 3^rd energy level n = 3Maximum number of electrons in the 3^rd energy level = 2n²= 2x(3) ²= 2 x 9 = 18

• For 4^th energy level n = 4Maximum number 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

• 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.

Electronic Configuration of an Element

The arrangement of electrons in the various shells/orbits/energy levels of an atom of the element is known as electronic configuration. Keeping the Bohr and Bury rules in mind let us write the electronic configuration of elements.

Electronic Configurations of Some Important Elements

(Inert gases are indicated in blue)

Element	Symbol	Atomic number	Electronic configuration (or Electron arrangement) K L M N
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	19	2,8,8
Potassium	K	19	2,8,8,1
Calcium	Ca	20	2,8,8,2

Geometric Representation of Atomic Structure

Example: 1

24
  Mg
12

Steps:

• The first 2 electrons will go to the 1^st shell = K Shell (2n²)
• The next shell L takes a maximum of 8 electrons (2n²)
• In this way 2 + 8 = 10 electrons have been accommodated. The next 2 electrons go to the M Shell.

  K	L	M
  2	8	2

  Example: 2

  16
    O
   8

  

  Atomic parameters of the first twenty elements

  Name and Symbol	Atomic no.(Z)	Mass no.(A)	Sub Atomic Particals	Electronic configuration				Valency	Metal/Non-metal or Noble gases and  Nature of element
  				K	L	M	N
  Hydrogen (H)	1	1	p = 1 e = 1 n = 0	1				±1	Metal as well as Non-Metal (Gas)
  Helium(He)	2	4	p=2 e =2 n =2	2				2	Noblegas (Gas)
  Lithium (Li)	3	7	p = 3 e =3 n =4	2	1			+1	Metal (Solid)
  Beryllium (Be)	4	9	p = 4 e=4 n=5	2	2			+2	Metal (Solid)
  Boron(B)	5	11	p=5 e =5 n = 6	2	3			+3	Metal (Solid)
  Carbon (C)	6	12	p = 6 e =6 n =6	2	4			4	Non-metal (Solid)
  Nitrogen (N)	7	14	p = 7 e =7 n =7	2	5			-3	Non-metal (Gas)
  Oxygen (0)	8	16	p = 8 e =8 n =8	2	6			-2	Non-metal (Gas)
  Fluorine (F)	9	19	p=9 e=9 n=10	2	7			-1	Non-Metal(Gas)
  Neon(Ne)	10	20	p=10 e=10 n=10	2	8			0	Noble Gas(Gas)
  Sodium (Na)	11	23	p = 11 e =11 n = 12	2	8	1		+1	Metal (Solid)
  Magnesium (Mg)	12	24	p = 12 e =12 n = 12	2	8	2		+2	Metal (Solid)
  Aluminium (Al)	13	27	p = 13 e =13 n = 14	2	8	3		+3	Metal (Solid)
  . Silicon (Si)	14	28	p = 14 e =14 n = 14	2	8	4		4	Non- Metal (Solid)
  Phosphorus (P)	15	31	p = 15 e =15 n = 16	2	8	5		-3	Non- Metal (Solid)
  Sulphur (S)	16	32	p = 16 e = 16 n = 16	2	8	6		-2	Non- Metal (Solid)
  Chlorine (Cl)	17	35	p = 17 e = 17 n = 18	2	8	7		-1	Non- Metal (Gas)
  Argon(Ar)	18	40	p=18 e = 18 n = 22	2	8	8		0	Noblegas (Gas)
  Potassium (K)	19	39	p = 19 e = 19 n = 20	2	8	8	1	+1	Metal (Solid)
  Calcium (Ca)	20	40	p = 20 e = 20 n = 20	2	8	8	2	+2	Metal (Solid)

  Atomic structures of the First Twenty Elements