Students, do you remember about electron shells from Bohr atomic theory?Okay, we discuss now. The picture shows electron shells of some elements that outer shells are not filled.
There is a possibility of these elements react with other elements, because they are not stable.
For stability, the number of electrons in the outer shell should be 2 electrons if it is closely to the helium atom (duplet rule) and 8 electrons if it is closely to the other inert gases (octet rule).
Look at a sodium atom. There is an electron in the outer shell, so if it reacts with a chlorine atom, it will loose the outer electron and it will form sodium ion, Na+. A chlorine atom, Cl will gain electron and form Cl- ion which has 8 electrons in the outer shell. Now Na+ is also has 8 electron in the outer shell (from the inner shell ). Both Na+ and Cl- now become stable. These ions attract one of another; as a result, these form NaCl(s) as a crystal. Na and Cl are chemically reactive.
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