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.
Elements, Atoms, and Molecules. For a more advanced list of resources on atoms, elements and compounds. Elements are substances that cannot be separated into simpler substances. Salt is made up of the elements sodium and chloride. Water is made up of the elements hydrogen and oxygen. The smallest particles of matter are called atoms. Remember the carrot from the other chapter. If you continue to chop a carrot into smaller and smaller pieces, eventually you would reach a point where you could not cut up the carrot anymore, but still have carrot.