lecture01_notes - 17.1 Atomic Theory of Matter Matter is...

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17.1. Atomic Theory of Matter Matter is composed of atoms, or atoms that have joined to form molecules. Ordinary states of matter — seen in every day life (i.e., not inside neutron stars): 1. Solid – rigid, closely packed atoms or molecules 2. Liquid – fluid, less closely packed atoms / molecules 3. Gas – fluid, widely spaced atoms / molecules 4. Plasma – ionized gas (at least partly so); fluid, thermodynamically similar to gas; widely spaced atoms What’s an example of plasma seen in everyday experience? Poll: Which is the most prevalent observable state of matter in the universe? ————————————————– The masses of atoms primarily arise from protons & neutrons in nuclei. (Electron masses are smaller by about 1/1836.) Atomic masses are measured in atomic mass units (amu), or simply u: 1 u = 1 . 66 × 10 27 kg [I could also write this as 1.66e-27] 1 u is very close to the mass of either the proton or the neutron. The atomic mass of an atom A , expressed in u, is approximately the number of protons Z + the number of neutrons N n : A = Z + N n Carbon-12 ( 12 C) has Z =6 (this is what makes it carbon!) and N n =6, so A =12. In fact, for Carbon-12, its A is defined to be exactly 12. Carbon-14 ( 14 C) has Z =6 and N n =8, so A =14. (The atomic masses of elements listed on periodic tables are not exact integers. What are some reasons why?) ————————————————–
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– 2 – What are some other atoms or molecules with which we’re familiar, and their masses in u? ————————————————– Einstein (1905): Atoms are about 10 10 m ( 1 ˚ A= 1 angstrom) across. Ex. 17-1 from the book: Cu has an atomic mass of 63 u. What is its interatomic spacing? Answer: 2 . 3 × 10 10 m — this is about 2 ˚ A. ————————————————– Example: Similar to the book’s, what’s the intermolecular spacing of water at 4 C? Strategy: We want the length between molecules. Let’s go from kg / volume to get molecules / volume, then take the cube root to get molecules / length, then take the inverse to get length / molecule. At this temperature, the mass density of water is ρ 1g/cm 3 , or 10 3 kg/m 3 . Water has a molecular weight of 18 u — 16 u from O, and two 1 u’s for H. 10 3 kg m 3 1 u 1 . 66 × 10 27 kg molecule 18u = 3 . 3 × 10 +28 molecules m 3 Check units! Taking the cube root gives 3.2 × 10 9 molecules / m. Taking the inverse gives 3.1 × 10 10 m / molecule, or 3.1 ˚ A/ molecule.
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