The nature of the bond between two carbon atoms or between carbon and another atom is a fundamental concept in organic chemistry. Atoms are discreet entities that differ from one another by the number of protons, neutrons, and electrons that make up each atom. Protons and neutrons are found in the nucleus, of course, and electrons are found outside of the nucleus in discreet energy levels. Valence is the number of bonds an atom can form with other atoms when molecules are formed and remain neutral. In molecules that contain carbon the orbitals associated with carbon are different from the orbitals associated with atomic carbon. The molecular orbitals of two atoms used to form a covalent bond are different from the atomic orbitals of the individual atoms. Bond breaking is an endothermic process because it requires energy.