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.
The atoms in a molecule are held together because they share or exchange electrons. Molecules are made from atoms of one or more elements. Some molecules are made of only one type of atom. For instance, two oxygen atoms bond together to form O 2, the part of the air that we need to breath to survive.