The penultimate shell is the second-to-last shell in an atom, just before the valence shell. It is denoted by "n-1," where "n" represents the total number of shells in the atom. The electrons in the penultimate shell play a crucial role in determining the chemical properties of an atom, as they are involved in chemical bonding. The antipenultimate shell, also known as the pre-penultimate shell, is the shell just before the penultimate shell. It is denoted by "n-2" and is located closer to the nucleus than the penultimate shell. The concept of penultimate and antipenultimate shells is important in understanding the electronic configuration and chemical properties of elements.