Sometimes the electronic configuration is also described by box notation form i.e., putting an arrow for single electron in a box or a pair of arrows for two electrons in a box. The direction of the arrows gives the orientation of its spin.Further the box is labelled on top by writing the symbol of the orbital.
Rules for Filling the Orbitals
The principle states that the electron in an atom are so arranged that they occupy orbitals in the order of their increasing energy. Since the energy of a 'n' orbital in the absence of any magnetic field depends on the 'n' and 'l' quantum number values, the order of filling orbitals with electrons may be obtained from the (n + l) rule of Bohr Bury's rule.According to this principle the orbital with the lowest energy will be filled first. The orbital having lower (n + l) value has lower energy. However for orbitals whose (n + l) values are equal, the orbital having lower value of 'n' has lower energy. It is important to remember that because of this rule, this sequence of energy levels pertains to energy level up to '3p' and thereafter, '4s' orbitals comes first instead of '3d'. Thus, the orbitals should be filled in the order:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s
Fig: 3.19 - A simple way to determine the relative energies of different orbitals
Bohr Bury rule
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 shell2nd energy level is L shell
3rd energy level is M shell4th energy level is N shell and so on.
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.
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 = 2n22 x (1) 2 = 2
- For 2nd energy level n=2
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
- For 4th energy level n=4
Maximum no.of electrons in the 4th energy level = 2n2= 2x(4)2
|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 configurations of some important elements
|Element||Symbol||Atomic number||Electronic configuration (or Electron arrangement) KLMN|
Geometric Representation of Atomic Structure
- 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.
K L M
Special case of potassium and calcium elements
Atomic number of potassium is 19 and its electronic configuration isK L M N
1 8 8 1
Atomic number of calcium is 20 and its electronic configuration isK L M N
2 8 8 2
This abnormal behaviour can be explained as follows:It is found that shells have sub shells. The smaller sub shells are termed s, p, d and f. The maximum number of electrons that can go into these are 2, 5, 10 and 14 respectively. These sub shells can overlap, resulting in energies that may differ from that predicted purely on the basis of n=1, 2, 3 etc. Therefore when electrons start filling, they may go to a new outer shell even before the inner shell is filled to capacity.