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.
1st energy level is K shell
2nd energy level is L shell
3rd energy level is M shell
4th 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 1st energy level, n = 1
Maximum number of electrons in 1st energy level = 2n2
2 x (1) 2 = 2
For 2nd energy level n = 2
Maximum number of electrons in the 2nd energy level = 2n2
2 x 22 = 2 x 4 = 8
For 3rd energy level n = 3
Maximum number of electrons in the 3rd energy level = 2n2
= 2x(3) 2
= 2 x 9 = 18
For 4th energy level n = 4
Maximum number of electrons in the 4th energy level = 2n2
= 2x(4) 2
= 2x16 = 32
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)
Geometric Representation of Atomic Structure
Example: 1
Steps:
The first 2 electrons will go to the 1st 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.
Maximum number of electrons that can be accommodated in a shell is given by 2n2 where n = shell number
