Why atoms bond together?

 Why atoms bond together?

All form of movement or interaction in the universe is for one goal only, reaching the stability.

when atoms associate with each other (the resultant product is called molecule) it forms a more stable product(molecule) from the former separated atoms each on its own. 

When we said stable its actually meant lower in energy i.e the atoms will release energy (usually in form of heat) to form a molecule which by default will contain lesser energy.

In contrast breaking bond needs to apply energy.

In general bond formation release energy ,bond breakage absorb energy.

Some groups in the periodic table contain elements that there atoms are stable, so they do not react at all, called NOBLE ELEMENTS those are group 8A in the periodic table and those elements contain eight electrons (octet) at there outer shell (valence shell) except Helium it contain only two electrons, it seems as if all other elements in the periodic table are trying to mimic those elements in groups eight by trying to reach eight or two (in case of Hydrogen) electrons at the valence shell, they are do so by either losing electrons (like the group 1A (alkali metals) making them positively charged or gaining electrons like group 7A(Halogens) making them negatively charged. 

In order to reach the octet its easy for those groups at the edges of periodic table to gain one to two electrons or losing them forming positive and negative ions (the positive and negative ions are formed simultaneously as one or two electrons are transferred from one (donor) atom ( the formation of positive ion) to the other atom (accept-or, forming the negative ion) and consequently those negative and positive charger attract each other and stick together threw electrostatic interaction, and this is the formation of ionic bond).

At the center of periodic table the are atoms needing to transfer more than one or two electrons, what to do?

In this case actually they tend to share electrons rather than transferring them, and this type is called COVALENT BOND it is more stronger and far more stable than the ionic bond.

the carbon atom have four electrons in it's outer shell it needs another four to reach the octet state so what it will do is seek to share those four electrons with four atoms or even just forming four bonds (in this case double and triple bonds are involved) by share other four electrons it will reach the octet.

so the Oxygen, Sulfur and Nitrogen. 

Hydrogen tend to form covalent bond with carbon ,nitrogen and so on, also it form covalent bond with HALOGENS the most electronegative atoms in the whole periodic table, by forming covalent bond with those how have enormous electron suction power and the huge deference in the electronegativity between hydrogen and Halogens the electrons use to spend a lot of time around the halogen atom rendering it as partially negative and the hydrogen partially position (and this is polarity) it called POLAR COVALENT bond.

Due to the huge difference in electronegativity between the hydrogen and halogen it almost as if the electron has completely transferred to halogen atom and is almost negative halogen and positive hydrogen. When this appears in media it increases the POSITIVE HYDROGEN ION (proton) in the media it called acidic media , and that's why when hydrogen form covalent bond with halogens it is always the product called acid.