1)Why elements combine? How and why? 2)why do some atoms combine but their outershell is not saturated? or it may have more electrons than it can hold?
3) Resemble Noble gas : some noble gases react forming bonds of compounds?

Almost all the elements combine to form compounds, although the reactivity may vary from element to element. These combinations take place because almost all the elements are unstable. This instability is a function of the number of electrons in the outermost shell.

Octet rule - Matter always wants to be in the most stable form. For any atom, stability is achieved by following the octet rule, which is to say all atoms (with a few exceptions) want 8 electrons in their outermost electron shell (just like noble gases). The electrons present in the outermost shell of an atom are called valence electrons.

fe 55.85 g/mol and h2o 18.02 g/mol.