The molecule: structure, size and shape

Abstract The relationships between atoms and molecules are mediated by the theory of chemical bonding. Atoms were forged in the extreme conditions that occurred at the cores of stars in the early stages of the development of the universe, but they did not survive as isolated systems in the much milder conditions of mature planets, where more stable structures are formed by highly stabilising interactions between electrons of different atoms, in terms of lowering of the potential energy and of spin pairing. This chapter discusses molecular structure, size and shape, along with classification concepts in many particle systems, symmetry and order, enthalpy and entropy, mass and length dimensions of a molecule, atomic radii, molecular volume and surface, and molecular size in terms of electron density.