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.
This happens via a chemical reaction. A molecule is the smallest amount of a chemical substance that can exist. Essentially, a compound is a type of molecule. A molecule can be made up of two or more atoms of the same element or two or more atoms of different elements. However, compounds are molecules that are made up of atoms of different molecule. Atoms are the basic units that all matter is made of. Atoms are tiny, ranging from 0.1 to 0.5 nanometers in width.