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.
When two or more atoms combine, they form molecules. Molecules can be comprised of atoms that are different or alike. A water molecule is formed by the combination of two hydrogen atoms and a single oxygen atom. That molecular bond is better known as H2O. Atoms are made up of three sub-atomic protons called protons, neutrons and electrons.