The Chemical bond
Electronic subshells
Electrons are distributed in shells around the nucleus. These shells are sub divided in subshells, named s, p, d, f... Sub shells present 3D shapes that can be seen here.
The electronic capacities of subshells are as follows:
s -2, p-6, d-10, f-14 .
All numbers are even, as there is space for two sets of electrons, paired according to opposite spins (as required by Pauli exclusion principle).
Sub shells define the spatial distribution of electrons (orbitals). They have well defined shapes, some of which are seen on the left. These shapes are determined by solving the Schroedinger equation (gives quantum mechanical behaviour of electrons).
Elements that have valence electrons in s orbitals constitute the s-block. The same definition applies to the p and d blocks. The d block consists of transition metals, where the distribution of electrons in the 5 different orbitals are responsible for the varied colours and geometries of the complexes made using these elements.
The question you normally get is, for instance, write the electronic configuration of aluminium.
Answer: The shells must be filled in the order of increasing energy: electrons go first to the lowest energy shell, and then start filling the upper levels. A diagram is normally used to find out this order:
Aluminium has Z=13, so that it has 13 electrons. Filling the sub levels following the order given by the arrows above, and respecting the capacity of each sub level, leads to:
1s2 2s2 2p6 3s2 3p1
That shows also that the valence shell, n=3, has 3 electrons.
Other Example:write the electronic configuration of iodine (I).
Iodine has Z=53, so that it has 53 electrons:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
That shows also that the valence shell of iodine, n=5, has 7 electrons.
Cl and Br also have 7 electrons in their valence shells, as discussed before. That is why I, Cl and Br are on the same group (column) of the periodic table!
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