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2.Structure of Atoms eLearn.Punjab

2.2 ELECTRONIC CONFIGURATION

Before discussing electronic configuration let us first understand the concept of shells and
subshells.
We have learnt about the structure of atom i.e. it consists of a tiny nucleus lying in the center and
electrons revolving around the nucleus. Now we will discuss how the electrons revolve around the
nucleus? The electrons revolve around the nucleus in different energy levels or shells according to
their respective energies (potential energy). The concept of potential energy of an electron shall be
discussed in higher classes.
Energy levels are represented by ‘n’ values 1, 2, 3 and so on. They are designated by the alphabets
K, L, M and so on. A shell closer to the nucleus is of minimum energy. Since K shell is closest to the
nucleus, the energy of shells increases from K shell onwards. Such as:
1st energy level is K shell; it has the lowest energy.
2nd energy level is L shell; it has more energy than K shell.
3rd energy level is M shell; it has more energy than K and L shells.
4th energy level is N shell; it has more energy than K, L and M shells.

In simple words shells are the main energy levels that electrons can occupy.
Shells are represented by circles around the nucleus. They are counted from the center to outward
as shown in figure 2.5. the number of electrons that a shell can accommodate is given by 2n2,
where ‘n’ is the shell number.

Fig 2.5 Showing different energy levels or shells counted from the center
outwards.

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