Why atoms combine to from molecules

Why atoms combine ?

This is the first questions came to my mind when I started taking interest in Chemistry . 

First of all let's understand some basic concept so that it would be more easier for you to understand this .

What is atom simply ?

Atom is the smallest unit of matter which cannot be broken down further Chemically (but can be broken down into subatomic particles proton, Neutron and electron ).or in more simplified way atom is smallest particle of matter or basic unit of matter .

What is Molecule ?

Molecule is the smallest repeating unit of a compound (compounds are made up of 2 or more different atoms in fixed proportion).Molecule can be made up of same atoms or different atoms but the only condition is that more than one atoms must be Chemical combined .

Why atom combines to form Molecules ?

A simple and short answer is that atoms combine to form molecules to attain more stability.

How does this happen ?

Each atom tries to combine with other atoms , so that number of electrons in their valance shell become 8 (ns2 np6)  , so that they can attain noble gas configuration and become as stable as noble gases / Inert gases

Atom can donate , accept or share electrons with each other to complete their octet(8 electrons in valance shell/orbit) .

When metal combines with Non-Metal , metal loses electron and non metal gain electron to attain their noble gas configuration and form a strong ionic bond .

When two atoms with very less electronegative difference combines they generally share their electrons to attain noble gas configuration . 

Energy of a bonded atoms in a Molecule is less as compared to unbounded atoms . Lesser  the energy more will be the stability  of a particles.

Therefore ,  atoms combine to form molecules  to gain maximum stability by attaining noble gas configuration and decreasing energy .