Physics Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 4

1. The atomic mass of an element is roughly equal to the sum of the number of protons and neutrons in the atomic nucleus. In this case, an atom of the isotope in question has 11 protons and 12 neutrons in its nucleus, so the atomic mass is approximately 11 + 12, or 23. The correct choice is C.

2. We're told that 3 kg of matter collide with 3 kg of antimatter. When equal amounts of matter and antimatter come together, we get pure energy as a result, with no matter or antimatter remaining. The samples completely annihilate each other. The answer is A.

3. Neutrons can exist indefinitely in the nucleus of an atom along with protons and other neutrons. However, if we isolate a single neutron, it will not retain its identity for very long. The average "life" of a lone neutron is only about a quarter of an hour. The correct choice is B.

4. When two or more atoms join to form a molecule, the atoms retain their individual identities insofar as nothing changes in the nuclei. However, the atoms share electrons, forming a so-called covalent bond that allows compounds to exist. The correct choice is B.

5. Figure 4-3 (on page 124) illustrates a water molecule, comprising two atoms of hydrogen in a covalent bond with one atom of oxygen. The inner shell of the oxygen atom appears as the smaller of two dashed circles surrounding the oxygen nucleus. Two electrons reside in this shell. The correct choice is A.

6. Different isotopes of a particular chemical element always have different numbers of neutrons in their nuclei, and they always have the same number of protons. For example, an atom of carbon-12 (C-12) has a nucleus containing six protons and six neutrons, while an atom of carbon-14 (C-14) has a nucleus with six protons and eight neutrons. Any atomic nucleus, no matter what isotope, can be surrounded by various numbers of electrons without losing its identity. The correct choice is A.

7. Scientists have created nuclei of elements never found in nature with the help of a powerful machine called a particle accelerator (informally known as an "atom smasher"). The correct choice is B.

8. By definition, an element's atomic number equals the number of protons that one of its nuclei contains. The answer is C.

9. An electron's mass is only a tiny fraction of a proton's mass. The answer is D.

10. Because a helium atom has an atomic number of 2, we know that its nucleus contains two protons. If only one electron "orbits" a nucleus that has two protons, we have an atom with two unit positive charges and one unit negative charge, forming a positive ion or cation. The correct choice is D.