Why Do Atoms form Ions?
Atoms are neutral in general. Every atom has a nucleus which consists of protons and neutrons. The nucleus in every atom is surrounded by electrons. The electrons are arranged around the nucleus in several shells or orbits. Each orbit has a probability of occurrence of specific density of electrons in the orbits called as orbital. The first shell or lower most shell consists of s-orbital which has only 2 electrons. The next shell consists of s-orbital which possesses 2 electrons as well as p-orbital which possesses 6 electrons. The second shell has 8 electrons and so on. If the outer most shell does not have eight electrons, then this atom either give away electrons or accept electrons to make up the outer most shell electrons as total 8.
If the atom has the tendency to give away electrons then the atom attains positivity. The positivity of the atom is represented as positive ion. If the atom has the tendency to accept electrons then it gains lot of negative charge. This negativity results due to acceptance of negative charge and is represented as negative ion.
When any atom form ion, it tries to aquire the electronic configuration of the nearest noble gas in the periodic table. The noble gases follow octet rule. They are stable as they have always 8 electrons in their valence shells. For example, bromine gains one electron to reach the electronic configuration of nearest noble gas Kr. Hence it forms easily the ionic form Br_.
Metal atoms lose electrons to form positive ions and non metal atoms gain electrons to form negative ions. Sodium (Na) loses an electron to form Na+ and attains the electronic configuration of Neon (noble gas). Chlorine gains an electron to form Cl- and attains the electronic configuration of Argon (noble gas). In both the cases Na and Cl try to follow octet rule in the formation of an ion and further into a compound. In order to become stable every atom follows octet rule which is responsible for the formation of ion.