Why Do Atoms Combine?
Molecules of all compounds in nature are made up of more than one atom linked together in a particular ratio. Each molecule is bonded with its elements and changes its properties different to that of its varied constituent elements. For example, a sodium atom combines with a chlorine atom to form a sodium chloride compound which is called as salt. Salt when mixed in water will just dissolve in it silently. When sodium is put in water hydrogen gas is released violently. Chlorine is a toxic gas which was also used as a weapon in World War II. Hence it is understood now that chemical bonding occurs when two or more atoms combine to form a molecule. Water has three atoms combined to form a molecule. Why do these atoms combine? Why can’t they exist alone?
The number of electrons in the outermost shell or orbit of an atom is ready to combine with other atoms to form a bond. Except in transition metals all other elements in periodic table should have electrons in the first orbit as 2 and in all other outer shells as 8 electrons. In the diagram used to describe the outer most shell electrons of an atom, they are represented in the form of dots surrounding the atom symbol. This is called as Lewis dot formula.
The reason for the atoms to form a bond with other atoms is their tendency to have the outer most shell filled up completely and become stable. The above example is given in the case of ionic bonding. There is another type of bonding called as covalent bonding. In this example of formation of NaCl, chlorine atom has 17 electrons surrounding the nucleus in several orbits. The first lower most shell has 2 electrons, the second orbit has 8 electrons, and third one has 7 only. So it has the tendency to accept one electron to become stable. Some atoms have tendency to lose electrons like sodium from their valence shells. Sodium has 11 electrons in its atom. First shell has 2, second one has 8 and third one has only 1 electron. It can either take 7 electrons or give away one electron to form an ionic bond. The former process is difficult and hence gives 1 electron to chlorine to bind to it to form stable sodium chloride or salt. The above facts tell us why atoms combine.