If you look up the definition of temperature you will probably find something like “the degree of heat of an object” and think to yourself, “Well, that’s not very illuminating, is it?” However, it is actually quite difficult to give a simple definition of temperature, (typically abbreviated as T). Up to now the major types of change we have considered are phase changes (solid to liquid, liquid to gas, etc.) Now we will look at the elements of a phase change in greater detail starting with temperature. Understanding these connections allows us to predict how and when chemical changes will occur, which is the heart of chemistry. Now we are ready to draw all these ideas together and make connections between the macroscopic and molecular levels. If you know the temperature at which phase changes occur in a material (solid to liquid, liquid to gas, etc.), you can make predictions about how much energy is required to overcome the interactions between the particles that make up the material.Some materials are continuous (diamond, metals, ionic compounds), whereas others are composed of discrete molecular units (water, methane, lipids, proteins). The macroscopic properties of materials depend upon the types of bonds present and their spatial organization, which influences molecular shape, the distribution of charges within the molecule, and intermolecular interactions.When atoms bond to form new materials (compounds), the properties of those compounds are emergent-that is, they are quite different from the properties of the isolated component atoms.Different types of atoms have different “internal” arrangements of electrons. The ways that atoms interact depend upon the arrangements of the electrons within them.Typically this energy is derived from collisions with surrounding molecules, although absorption of a photon can also overcome interactions. Whether weak or strong, all types of interactions require energy to overcome.The total energy of the interacting atoms (the system) can decrease if it is transferred to the surroundings, usually by collisions with other molecules or atoms but the emission of a photon is also possible. The potential energy of the system decreases but the total energy of the system remains constant. When atoms interact they form more stable systems, where the attractive and repulsive interactions are equal.
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