2nd entry(chem) : What are molecules?

A molecule is a definite group of atoms that are chemically bonded together. They are tightly connected by attractive forces. A molecular formula is a chemical formula that gives the exact number of different types of atoms in a molecule. Some simple molecular substances are carbon dioxide, CO2; ammonia, NH3; and water, H2O.
The atoms that are in a molecule are not just stuffed together without any order. The atoms are chemically bonded to one another in order to form a definite arrangement. A structural formula is a chemical formula which shows how the atoms are bonded to one another to form a molecule. A good example is the structural formula for water, H-O-H. Those two horizontal lines connecting the H with the O (hydrogen and oxygen) represent the chemical bonds joining the atoms.

What are chemical bonds?
A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. The explanation of the attractive forces is a complex area that is described by the laws of quantum electrodynamics.

What is the laws of quantum electrodynamics?

It is a quantum field theory of the electromagnetic force.
Taking the example of the force between two electrons, the classical theory of electromagnetism would describe it as arising from the electric field produced by each electron at the position of the other.



References:
http://www.cartage.org.lb/en/themes/sciences/chemistry/Generalchemistry/Matter/Atoms/AtomsIons/AtonsIons.htm
http://hyperphysics.phy-astr.gsu.edu/Hbase/forces/qed.html
http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html
http://en.wikipedia.org/wiki/Chemical_bond