Page 2 - LN 9

 Node

The region in space around the nucleus in 3D where the probability of
finding electron is minimum is called node.

 Electronic configuration.
The distribution of electrons into orbitals of an atom is called its
electronic configuration.

Example
2
2
2
carbon (C, 1s 2s 2p )
3
2
2
nitrogen (N, 1s 2s 2p )
4
2
2
oxygen (O, 1s 2s 2p )
2
2
5
fluorine (F, 1s 2s 2p )
2
6
2
neon (Ne, 1s 2s 2p )
 The completely filled and completely half filled sub-shells are stable due
to the following reasons:
1.Symmetrical distribution of electrons: It is well known that symmetry
leads to stability. The completely filled or half filled subshells have
symmetrical distribution of electrons in them and are therefore more
stable. Electrons in the same subshell (here 3d) have equal energy but
different spatial distribution. Consequently, their shielding of
oneanother is relatively small and the electrons are more strongly
attracted by the nucleus.

2. Exchange Energy :The stabilizing effect arises whenever two or more
electrons with the same spin are present in the degenerate orbitals of a
subshell. These electrons tend to exchange their positions and the
energy released due to this exchange is called exchange energy. The
number of exchanges that can take place is maximum when the subshell
is either half filled or completely filled . As a result the exchange energy
is maximum and so is the stability.