Atom Energy Shells or Levels

"The orbit around the nucleus within which the electron rotate is called shells or Energy levels."

Each discrete distance orbit from the nucleus corresponds to a certain energy level. The electron which rotates in the lowest orbit has lowest energy level and in the outermost orbit, electrons have higher energy levels. Hence energy levels increase as the distance from the nucleus increases.

There are many shells around the nucleus which are arced as K, L, M; N, and so on.

Atom Energy Levels Shells
Atom Energy Levels Shells


The K Shell is the closest shell to the nucleus. It is stable with 2-electrons, corresponding to the structure of Helium whose K Shell is filled with 2-electrons.


The L Shell is the second closest shell to the nucleus. It is stable with 8-elect- on, Corresponding to the atomic structure of Neon whose L shell is filled with 8-electrons.


The I Shell is third closest shell to the nucleus. It is stable with 18 electrons corresponding to the atomic structure of .Argon (Inert gas) whose M shell is filled by 18 electrons. Its electronic configuration is Shells

Total Electrons28

Other shells which can take maximum electrons is shown by the table.

ShellMaximum ElectronsInert Gas
M8 (upto Calcium) or 18Argon
N8, 18 or 32Krypton
O8 or 18Xenon
P8 or 18Radon

For distribution of electrons in the shells, 2n2 rule is used

where n is the number of shell lives maximum electrons that can be placed in any shell

Valance Electron

"The electrons in an incomplete outermost orbit are called valance electrons."


Valance electrons are less tightly bound to the atom than those closer to the nucleus. This is because the force of attraction between the positively charged nucleus and the negatively charged electron decreases with increasing distance from the nucleus. Electrons with the highest energy levels exist in the outermost shell of an atom and are relatively loosely bound to the atom. This outermost shell is known as the valance shell and electrons in this shell are called valance electrons.

A completed outermost shell has valance of zero. Cu has valance of 1 because one electron is in outer shell after completing its inner shells. Similarly carbon has a valance of 4 and all the inert gasses have zero valance.