Lesson_1 Introduction to materials (3)

Lesson_1 Introduction to materials (3) - 28 CHAPTER 2...

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neutrons in the atom, is also the mass in grams of the Avogadro number N A of atoms. The quantity N A ¼ 6.02 ± 10 23 atoms/mol is the number of atoms or molecules in a mole. Therefore, the atomic mass has units of g/mol. An alternate unit for atomic mass is the atomic mass unit , or amu, which is 1/12 the mass of carbon 12 (i.e., carbon atom with 12 protons). As an example, one mole of iron contains 6.02 ± 10 23 atoms and has a mass of 55.847 g, or 55.847 amu. Calculations including a material’s atomic mass and Avogadro’s number are helpful to understanding more about the structure of a mate- rial. Example 2-1 illustrates applications to magnetic materials. EXAMPLE 2-1 Nano-Sized Iron-Platinum Particles For Information Storage Scientists are considering using nano-particles of such magnetic materials as iron-platinum (Fe-Pt) as a medium for ultrahigh density data storage. Arrays of such particles potentially can lead to storage of trillions of bits of data per square inch—a capacity that will be 10 to 100 times higher than any other devices such as computer hard disks. Consider iron (Fe) particles that are 3 nm in diameter, what will be the number of atoms in one such particle? SOLUTION Let us assume the magnetic particles are spherical in shape. The radius of a particle is 1.5 nm. Volume of each iron magnetic nano-particle ¼ (4/3) p (1.5 ± 10 ² 7 cm) 3 ¼ 1.4137 ± 10 ² 20 cm 3 Density of iron ¼ 7.8 g/cm 3 . Atomic mass of iron is 56 g/mol. Mass of each iron nano-particle ¼ 7.8 g/cm 3 ± 1.4137 ± 10 ² 20 cm 3 ¼ 1.102 ± 10 ² 19 g. One mole or 56 g of Fe contains 6.023 ± 10 23 atoms, therefore, the num- ber of atoms in one Fe nano-particle will be 1186. This is a very small number of atoms. Compare this with the number of atoms in an iron particle that is 10 micrometers in diameter. Such larger iron particles often are used in breakfast cereals, vitamin tablets, and other applications.
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