1. CH-30:The Nature of the Atom In their operation, lasers depend on the structure of the atom, as this chapter discusses. Medical diagnostic and surgical procedures use lasers in a wide variety of situations, often with dramatic advantages over conventional techniques. The patient in this photograph is undergoing laser surgery to remove her tonsils and was ready to go out for ice cream ten minutes following the operation. (© AP/Wide World Photos)

2. Nuclear Atom An atom contains a small, positively charged nucleus (radius ≈ 10 –15 m), which is surrounded at relatively large distances (radius ≈ 10 –10 m) by a number of electrons. In the natural state, an atom is electrically neutral because the nucleus contains a number of protons (each with a charge of +e) that equals the number of electrons (each with a charge of –e). This model of the atom is universally accepted now and is referred to as the nuclear atom.

3. Early Model of The Atom Plum-Pudding Model The nuclear atom is a relatively recent idea. In the early part of the twentieth century a widely accepted model, due to the English physicist Joseph J. Thomson (1856–1940), pictured the atom very differently. In Thomson’s view there was no nucleus at the center of an atom. Instead, the positive charge was assumed to be spread throughout the atom, forming a kind of “paste” or “pudding,” in which the negative electrons were suspended like “plums.”

4. Rutherford Scattering Experiment In Rutherford scattering experiment α particles are scattered by a thin gold foil. The entire apparatus is located within in a vacuum chamber (not shown). The “plum-pudding” model was discredited in 1911 when the New Zealand physicist Ernest Rutherford (1871–1937) published experimental results that the model could not explain. Rutherford and his co-workers directed a beam of alpha particles at a thin metal foil made of gold. If the “plum-pudding” model were correct, the α particles would be expected to pass nearly straight through the foil. After all, there is nothing in this model to deflect the relatively massive α particles, since the electrons have a comparatively small mass and the positive charge is spread out in a diluted “pudding.” Not all the α particles passed straight through the foil. Instead, some were deflected at large angles, even backward. Rutherford concluded that the positive charge was concentrated in a small region called the nucleus.

5. 30.2. Line Spectra The line spectra for neon and mercury, along with the continuous spectrum of the sun. The dark lines in the sun’s spectrum are called Fraunhofer lines, three of which are marked by arrows. (Courtesy Bausch & Lomb)

6. Line spectrum of atomic hydrogen In these equations, the constant term R has the value of R = 1.097 × 10 7 m –1 and is called the Rydberg constant. Only the Balmer series lies in the visible region of the electromagnetic spectrum.

7. 30.3. The Bohr Model of the Hydrogen Atom In the Bohr model, a photon is emitted when the electron drops from a larger, higher-energy orbit (energy = E i ) to a smaller, lower-energy orbit (energy = E f ).

8. Niels Bohr By combining ideas from classical and modern physics, Niels Bohr was able to obtain values for the quantized energy levels of the electron in a hydrogen atom. He is shown here with his five sons. (Courtesy A.I.P. Niels Bohr Library, Margrethe Bohr Collection)

9. Bohr Model

10. Energy level diagram for the hydrogen atom

11. Lyman and Balmer series The Lyman and Balmer series of lines in the hydrogen atom spectrum correspond to transitions that the electron makes between higher and lower energy levels, as indicated here.