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The modern theory of matter dates from the work of John Dalton at the beginning of the 19th cent. The atom is considered the basic unit of any element, and atoms may combine chemically to form molecules, the molecule being the smallest unit of any substance that possesses the properties of that substance. An element in modern theory is any substance all of whose atoms are the same (i.e., have the same atomic number), while a compound is composed of different types of atoms together in molecules.
Physical and Chemical Changes
The difference between a mixture and a compound helps to illustrate the difference between a physical change and a chemical change. Different atoms may also be present together in a mixture, but in a mixture they are not bound together chemically as they are in a compound. In a physical change, such as a change of state (e.g., from solid to liquid), the substance as a whole changes, but its underlying structure remains the same; water is still composed of molecules containing two hydrogen atoms and one oxygen atom whether it is in the form of ice, liquid water, or steam. In a chemical change, however, the substance participates in a chemical reaction,toms. As a result, it becomes a different substance with a different set of properties.
Many of the physical properties and much of the behavior of matter can be understood without detailed assumptions about the structure of atoms and molecules. For example, the kinetic-molecular theory of gases provides a good explanation of the nature of temperature and the basis of the various gas laws and also gives insight into the different states of matter. Substances in different states vary in the strength of the forces between their molecules, with intermolecular forces being strongest in solids and weakest in gases. The force holding like molecules together is called cohesion, while that between unlike molecules is called adhesion (see adhesion and cohesion). Among the phenomena resulting from intermolecular forces are surface tension and capillarity. An even larger number of aspects of matter can be understood when the nature and structure of the atom are taken into account. The quantum theory has provided the key to understanding the atom, and most basic problems relating to the atom have been solved.
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