HR40_ver1c - Chapter 40 All About Atoms In this chapter we...

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1 Chapter 40 In this chapter we continue with a primary goal of physics―discovering and understanding the properties of atoms. 100 years ago researchers struggled to find experiments that would prove the existence of atoms. Today, thanks to scientific and technological progress, we can manipulate atoms in amazing ways: we can image individual atoms using scanning tunneling microscopy; we can drag them on surfaces to make quantum corrals, and even hold an individual atom indefinitely in a trap in order to study its properties when isolated. All About Atoms 40-
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2 Basic Properties Atoms are stable. Essentially all atoms have remained unchanged for billions of years. Some Properties of Atoms 40- Atoms combine with each other. Atoms stick together to form molecules and stack up to form rigid solids. Even though atoms are mostly empty space, their interactions allow you to stand on a floor without falling through! These basic properties can be explained by quantum mechanics.
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3 Some Properties of Atoms 40- Fig. 40-2 Subtler Properties Atoms Are Put Together Systematically. There are repetitive (periodic) patterns in the properties of different atoms that allow them to be organized into a periodic table. Ionization energy vs. atomic number (number of protons in nucleus) Six periods with 2, 8, 8, 18, 18, and 32 atoms in each period, respectively. These numbers are predicted by quantum mechanics
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4 Some Properties of Atoms 40- Subtler Properties, cont’d Atoms Emit and Absorb Light: high low hf E E = - Fig. 40-3 “orbit” of each electron (more correct to think in terms of angular momentum of electronic state) can produce a magnetic moment. Atoms Have Angular Momentum and Magnetism:
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5 Some Properties of Atoms 40- Subtler Properties, cont’d Einstein-de Haas Experiment: Fig. 40-4 Angular momentum and magnetic moment of atoms are coupled. Aligning magnetic moments of iron atoms using an external magnetic field causes the iron cylinder to rotate in a direction opposite to the now aligned angular momenta of the iron atoms (conservation of angular momentum)
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6 Electron Spin 40- Trapped or free , electrons have intrinsic spin angular momentum S (spin). This is a basic characteristic like the electron’s mass or charge. This leads to two additional quantum numbers that are required to fully specify the electronic state: s (magnitude of the spin, which is always ½ for electrons) and m s (the component of spin along the z-axis). Electron States for an Atom Quantum Number Symbol Allowed Value Related to Principal n 1, 2, 3, … Distance from nucleus Orbital l 0, 1, 2, …, ( n- 1) Orbital Angular Momentum Orbital magnetic m l - l, -( l-1 ), …+( l -1), + l Orb. Ang. Mom. (z-component) Spin s ½ Spin Angular Momentum Spin magnetic m s ± ½ Spin Ang. Mom. (z-component) Table 40-1 States with same n form a shell.
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HR40_ver1c - Chapter 40 All About Atoms In this chapter we...

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