Einstein ones said you should keep things as simple as possible! So I will try to keep it simple. Most atoms want to be happy, just like you. In chemistry a happy atom is an atom with a full atomic shell. That's it. I would like to make a comparison with chickens. Lets suppose you are a chicken and you have a number of eggs. read more
If the electronegativity of the two elements forming the bond is very different (elements from the 1st ,2nd groups with elements of 7th group) then ionic bond are formed. If the electronegativities are more similar, then overlapping is stronger, and covalent bonds are formed. read more
When two dissimilar nonmetals form bonds (e.g., hydrogen and oxygen), they will form a covalent bond, but the electrons will spend more time closer to one type of atom than the other, producing a polar covalent bond. read more