Why do atoms combine ?

Answer 1

Many atoms have outer shell electrons that are unstable in their orbits. Nature seeks to find a more stable condition and so atoms combine to form stable molecules.

Answer 2

Atoms, other than those of noble gases do not have their 'octet' or they have unfilled-outermost shells.
If these atoms are able to decrease their potential energy by chemical combination, then they tend to combine and form molecules. Remember that every process tends to the state of lowest energy Thus the overall potential energy decreases by chemical combination.

Answer 3

My explanation is based on covalent bonds.

Elements have different valence numbers. This describes how readily the element is to combine with other elements. It actually describes how many electrons it has in its outer energy shell. The valence describes how many more electrons may be added to that shell to complete it. Elements share electrons to form covalent bonds.

Noble gases (formerly known as Inert gases) have a complete set of electrons for their outermost energy shell. Thus, they do not readily combine with other elements.

Answer 4

To fullfill its required numbers of electrons.

Answer 5

First answer and visit the following
http://curriculum.calstatela.edu/courses/builders/lessons/less/les4/Vles4na.html