The Big Bang theory stipulates that the physical universe began at a single point in time when matter was created and spread outwards. The theory has been substantiated over time with the discoveries last century of the Universe's continued expansion and residual echo from the moment of the "Bang."  The original matter created at the moment of the Bang, and for some time afterwards, is not the same matter that we're familiar with on a daily basis -- as a matter of fact, the construction of large accelerators (such as CERN) have attempted to create some of those early existing particles.  In time, those original particles mostly settled into the more stable forms that we know, namely, electrons, neutrons, and protons.  These forms make up all the Elements, which are categorized in the Periodic Table.  However, the latter half of the Table (from the element Iron onward) is comprised of elements that were not created at the Bang, but much later -- actually when the first stars reached the end of their lives and exploded.  Chemistry can be considered as the study of the transfer of electrons amongst atoms -- how and when they react. Atoms are comprised of stable particles, which formed from the original matter, which was created at the moment of the Big Bang.