A Markov chain is a discrete random process with the property that the next state depends only on the current state (wikipedia). A Markov chain is a discrete random process with the property that the next state depends only on the current state (wikipedia). So $P(X_n | X_1, X_2, \ldots X_n-1) = P(X_n | X_{n-1})$.
Some people find molecules and compounds confusing. Here's how to remember the difference. If you join two different chemical elements together, you can often make a completely new substance. Glue two atoms of hydrogen to an atom of oxygen and you'll make a single molecule of water.
All Matter is made of Molecules. Molecules are made of Atoms. Atoms are made of Protons, Neutrons, and Electrons. Protons, Newtons and Electrons are made of Quarks. And some physicists believe that Quarks are made of Strings.