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According to modern theories of atomic structure, electrons revolve in a three dimensional region around the nucleus.

Shell number indicated by letters K,L,M,N,O… or 1,2,3,4,5…

Each main energy level has sublevels or subshells. In these subshells, there
are regions where there is a high probability of finding electrons. These are known as orbitals. The maximum number of electrons that can be accommodated in an orbital is 2.

  • There are 4 sub shells.(s,p,d,f).
  • In ‘s’ subshell have 1 orbital can accommodate maximum 2 electron
  • In ‘p’subshell there are 3 orbitals can accommodate maximum 6 electrons
  • In ‘d’subshell,there are 5 orbitals,can accommodate maximum 10 electrons
  • In ‘f’subshell,there are 7 orbitals,can accommodate maximum 14 electrons

As the distance from the nucleus increases, the energy of the electrons in the shells increases and the attractive force between the nucleus and the electrons decreases.

When we write the subshell wise electronic configuration, the number on the left side of the subshell denotes the shell number and the number on the top right side denotes the number of electrons.

Electron filling occurs in sub shell is in accordance with increasing in energy

• In 2P,6 Two represents the shell number, P represents the subshell and six represents the number of electrons in that subshell

The d sub shell can accommodate a maximum of 10 electrons.
The completely filled configuration (d”) or the half filled
configuration (d) of this subshell is more stable than the
others.will have some changes in the filling of electrons to attain
stability. Likewise, for f sub shell, f’ and f14 arrangements
are more stable.

For the subshell electronic configurations of chromium and copper, the configurations with half filled d subshell or completely filled d subshell show greater stability.

The block to which the element belongs will be the same as the subshell to which the last electron is added. In the periodic table, elements in groups 1 & 2 belong to s block, those in groups 13 to 18 belong to the p block and those in groups 3 to 12 belong to the d block. The elements in the f block are placed at the bottom of the periodic table in two separate rows.

Period number is equal to the highest shell number in the sub shell electronic configuration

Group number

  • s-block:The no. of electrons in the outer most s sub shell.
  • p-block:by adding 12 to the total number of electrons in the p sub shells of the outer most shell.
  • d-block:adding the total number of electrons in the outer most s sub shell and preceding inner d sub shell.

Most of the coloured salts are compounds of transition elements; the colour is due to the presence of transition element ion
In the case of transition elements the difference in energy between the outemost s sub shell and the penultimate d sub shell is very small. Hence under suitable condition the electrons in d sub shell also take part in chemical reaction This is the reason why transition element shows variable
oxidation state.
f block contains lanthanoids and actinoids.
Most of the actinoids are radioactive and are artificial elements.


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