Ionic and Covalent Compounds
Everything including atoms in existence try to become more stable. They try to reach the minimum state of momentum. Atoms exist or try to reach most stable electronic combination. This stable configuration defines stability for them. Inert gases are observed to be chemically inactive and one of the reasons is their configuration is very stable. Atoms or compounds react with each other in order to form this stable configuration.
According to the octet theory of Valency which was developed independently by Karl and Lewis in 1916, atoms of all elements other than inert gases react because they have an incomplete valance shell. Also while chemically reacting with different atoms, elements tend to combine with each other in order to form a stable configuration of the nearest inert gas. This stable electronic configuration contain either two or eight electrons in their valance shell called duplet or octet. During a chemical reaction or rearrangement, the idea is to reach this stable configuration. During the chemical reaction, an attraction force develops between atoms which binds them together to form molecules. his is called a chemical bond. A chemical bond involves a gain, a loss or sharing of electrons with other atoms. The transfer of electrons forms an ionic bond where as sharing of electrons forms a covalent bond. An ion is an atom that has electronic charge formed either by gain of electrons or loss of electrons. The difference from the original state of the atom gives it a net charge. In case of the atom donating or losing electrons, the atom has electron deficiency with respect to protons giving the atom a positive charge. In case the atom accepts an electron or group of electrons, the atom has more electrons than protons. Hence the atom is negatively charged. An atom is electrically neutral but an ion has charge associated with it. An ion has either a positive charge or negative charge. The gain or loss of electrons result in different electron configurations for an atom or ion. In a solution, an ion gains an independent existence where as an atom may not. The ion and its parent atom has different properties mainly colour, toxicity, atomic structure. An atom and its ion have the same number of protons. Hence there is no difference in their atomic numbers. The inert gas configuration is a stable two or eight electron valance shell. In case of an atom having three or less electrons in a valance shell, it losses electron in order to gain near inert gas configuration. This happens becuase energy considerations do not favor gaining too many electrons. similarly if an element has 5,6 or 7 electrons in its valance shell, then it gains the respective number of electrons. The ion thus formed is a negatively charged ion. Generally non metals have such configurations and the ion formed is generally an anion.
Electrovalent or Ionic Bond
Transfer of one or more electrons from the valance shell of an atom forms an ionic bond. Ionic bond is also known as electrovalent bond. Metallic atoms who have one two or three electrons in their valance shell lose electrons to non metallic atoms who have 5,6 or 7 electrons in a valance shell. The process is accompanied by an energy change which makes the formed compound more stable under the given set of chemical conditions. This electron transfer leave each of the individual atom with an inert gas configuration in its valance shell. The positively charged atom called the cation and the negatively charged atom called the anion attract each other to form an attraction force called as a bond. As the bond or chemical compound so developed is due to electrical force, it is called electrovalent bond. For example the electronic configuration of sodium atom is 2,8,1. It has one electron in its valance shell. In case it loses its electron , its valance shell configuration gets closer to that of neon. In a chlorine atom there are seven electrons. Chlorine's atom has a tendency to the stable configuration of argon. It tries to get an electron in its valance shell. So sodium atom and chlorine atom react with each other to reach their respective inert gas configurations in which sodium loses an electron to chlorine atom to form sodium chloride molecule.
Covalent or Molecular Bond
In a covalent bond the atoms combine in such a way that each atom attains either two electrons or eight electrons in their valance shell. In a covalent bond , the stable valance shell configuration is reached by sharing of electrons. The number of pairs shared determine the type of covalent bond formed namely, single, double or triple covalent bonds. For example chlorine molecule has seven electron in its valance shell. It needs one more electron in its valance shell. It shares one electron with another chlorine atom thus forming a single covalent bond.
Another type of bond exists in nature which is called a coordinate bond. In a coordinate bond one of the participating atoms provides all the electrons needed for stable valance shell configuration. Generally these atoms have a lone pair of electrons and both the electrons are shared or donated during the sharing of another atom. The resultant molecule or compound formed is more stable. However co-ordinate bond not as strong as ionic or covalent bond. Hydronium ion is formed in water using co-ordinate bond.