19. In the Bohr theory of the hydrogen atom, an electron travels in a circular orbit around the nucleus. How did Bohr answer the objection that such an atom should radiate away all its energy and collapse?

20. What restrictions did Bohr place on permissible orbits jof the electron around the nucleus? How did it affect the energy that the hydrogen atom can have?

21. Why is the energy of the hydrogen atom in its ground state a negative number?

22. What is wrong with a Bohr orbit corresponding to n = 3.5?

23. In what way is Bohr's hydrogen atom "quantized"? In what way is a vibrating violin string "quantized"? How do the fingers of a violinist's left hand help him to get around this quantization?

24. What happens to the electron when the hydrogen atom absorbs radiation? What is the spectrum called that repre sents the selective removal of certain frequencies from the initial beam of radiation?