Chemical Composition Atoms, Molecules, and Ions

The names and formulas of atoms, molecules, and ions were discussed in Lesson 2, Chemical Composition Atoms, Molecules, and Ions. The next step is to define how many atoms, molecules, and/or ions are in a sample. Because these particles are extremely small, a very large number would be required to describe a sample size. Eggs are sold by the dozen (12), soda by the case (24), and paper by the ream (500). Amedeo Avogadro s work on gases helped define a new unit of measurement for chemistry and physics the mole. A mole of a particular substance is equal to the number of atoms in exactly 12 g of the carbon-12 isotope. Experiments established that number to be 6.022142 10 particles. 

We learned in Lesson 2, Chemical Composition Atoms, Molecules, and Ions, that gases are fluid, compressible substances. All gases behave according to the following characteristics in the kinetic molecular theory (KMT) ... 

The meaning of a chemical formula was discussed in Chapter 5, and we learned how to interpret formulas in terms of the numbers of atoms of each element per formula unit. In this chapter, we will learn how to calculate the number of grams of each element in any given quantity of a compound from its formula and to do other calculations involving formulas. Formula masses are presented in Section 7.1, and percent composition is considered in Section 7.2. Section 7.3 discusses the mole—the basic chemical quantity of any substance. Moles can be used to count atoms, molecules, or ions and to calculate the mass of any known number of formula units of a substance. Section 7.4 shows how to use relative mass data to determine empirical formulas, and the method is extended to molecular formulas in Section 7.5. 

It was assumed in the preceding chapters that optical transitions between bonded states of molecules take place with no change in their chemical composition. Photodissociation and photoionization form a general class of photofragmentation processes in collisions between a molecule and a photon which lead to simple chemical reactions of disintegration into atomic or molecular fragments, or into ions and electrons. 

Our world is fuU of a variety of substances and materials that are used in human practice for very different purposes. Note that terms material and substance are not identical. A substance (chemical compound) is an ensem ble of a great number of atoms, molecules, ions, and/or radicals that define properties of the substance as an object for investigations, whereas materi als are the phase separated forms of substances (chemical individuals) or their ensembles (for example, specialty mixtures and composites) that feature a set of properties necessary for practical apphcations of the material. To create the material that possesses the required properties for particular practical applications, a specialty substance must be prepared and stabilized. Of importance is that the required substance modifications are often in a metastable state that differs from the thermodynamically equilibrium state and is, nevertheless, appropriate for the material storage and operation. 

Drawing the structure of a chemical compound is probably one of the first basic requirements of any chemist. It requires knowledge of the chemical composition of the structure to be drawn, an understanding of the type of bonding, and frequently a mental visualization of the arrangement of atoms (or ions). Once this has been assimilated it is not uncommon to draw a representation of the structure on paper. What is often lacking is the realization that the molecule should be represented in three dimensions. To some extent it is possible to represent a three-dimensional chemical structure on a piece of paper. Fig. 42.1 shows the structure of methane, CH4, where standard symbols e.g. the hatched hne, are used to imply a direction of the bond, and one that is different to, for example, the solid hne. This simple notation is commonly used to give a molecule the perception of three-dimensionality. 

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