We all want to have a relaxed life, and so do atoms. Atoms, when alone, are high in energy and are unstable. To be in a more stable state, atoms form bonds.
How does it form bonds?
There are three kinds of bonds and they all are formed differently.The three are: Ionic, Covalent and Coordinate covalent. Let's look at how they are formed.
A) Ionic Bond
Ionic bond is formed between metals and non-metals. Metals have 1 to 3 extra electrons. These electrons reside in the outermost shell. It has to loose these extra electrons in order to be stable. Because once a metallic atom loses these "valence" or outermost electrons, its next shell will become the outer most, which is already full. Hence, the stable condition of full outer shell is achieved.
On the other hand, a non-metal lacks 1 to 3 electrons. It needs these electrons in order to fill its last shell and be stable.
When bonding, the metallic atom gives its excess electrons to the non-metallic atom forming 2-ions: Positive ion and Negative ion. They now contain electric charge. The opposite charges attract each other and a very strong bond is formed. This attraction or bond is called ionic bond.
B) Covalent Bond
A covalent bond occurs when there is a mutual need of electrons. It generally occurs amongst non-metals. A good way to understand this bond is through example. Hydrogen and Chlorine both need one electron to fill their last shells. To do this, Hydrogen and Chlorine both share one-one electron, and form a electron pair. This pair fulfills their need of electrons. Each atom shares equal electrons. If they both share 2-2 electrons, a double bond is formed, likewise 3-3 electrons form a triple bond. It doesn't go higher than three.
Although there is no electrostatic force per say, some atoms tend to attract the shared pair of electrons more than other. This is due to electronegativity difference. This unequal force creates polar and non-polar substances. Polar, have unequal pull of shared pair of electrons, whereas non-polar have an equal pull on electrons. The mechanism of covalent bond is very well understood, but it is beyond the scope of this article.
C) Coordinate covalent bond
This bond is also called Bipolar bond or Dative bond. It occurs when an atom in a molecule has 2 excess electrons, and it donates them to another atom in need of 2 electrons. It is very similar to covalent bond, except both of the electrons are donated by the same atom. In fact, once it is formed, it is indistinguishable from a covalent formed. The pair of electrons donated is called "Lone pair". When writing a reaction in which a coordinate covalent bond is formed, an arrow is drawn from the donor atom, pointing to the acceptor atom.