HW From textbook “CHEMISTRY a Contemporary Approach”

• Read  p510 to top of p514

• Do questions 6 p535

Although most nuclei are stable, some are unstable and spontaneously decay, emitting radiation. The stability of isotopes is based on the ratio of neutrons and protons in its nucleus.

In the band of stability, atoms are stable. The ratio n/p  is 1 for low nuclear masses and it increases steadily up to 1.5 for high nuclear masses.

Example

C-12 has 6 n and 6 p therefore, n/p =1

Pb-206 has 124 n and 82 p therefore, n/p = 1.5

In region A, a nucleus with too many neutrons is unstable because not enough of them are paired with protons.

Example

H-3 has 2 n and 1 p therefore, n/p = 2 which is too high for a low molecular mass.

In region B, a nucleus with too many protons has too much repulsive electrical interactions to be stable.

Example

S-31 has 15 n and 16 p therefore, n/p = 0.94 which is too little for a relatively high molecular mass.

Elements with atomic number > 83 have no stable isotopes therefore they are all radioactive.

Spontaneous decay can involve the release of alpha particles, beta particles, positrons and/or gamma radiation from the nucleus of an unstable isotope. These emissions differ in mass, charge, and ionizing power, and penetrating power. See table O

*Penetration Power

Low = Stopped by a single sheet of paper.

Moderate = Penetrate only a few inches of solid materials.

High = Penetrate almost a foot of lead. They are similar to high-energy X-rays and very dangerous to living things.

**Ionization Power

Nuclear emanations can remove electrons from particles when striking them.

Each radioactive isotope has a specific mode of decay. See table N  

Example

Gold-198 decays by giving off beta particles.

TEST YOUR UNDERSTANDING

6/07

8/07

6/06

1/05

29 Which statement best describes gamma radiation?

                (1) It has a mass of 1 and a charge of 1. (2) It has a mass of 0 and a charge of –1.

                (3) It has a mass of 0 and a charge of 0. (4) It has a mass of 4 and a charge of +2.

42 For most atoms with an atomic number less than 20, nuclear stability occurs when the ratio of neutrons to protons is 1:1. Which of the following atoms would be most likely to have an unstable nucleus? (1) 4-He (2) 12-C (3) 16-N (4) 24-Mg

6/05

Base your answers to questions 61 through 63 on the information below, which relates the numbers of neutrons and protons for specific nuclides of C, N, Ne, and S.

61 Using the point plotted on the graph for neon, what is the neutron-to-proton ratio of this nuclide?

62 Explain, in terms of atomic particles, why S-32 is a stable nuclide.

63 Using the point plotted on the graph for nitrogen, what is the neutron-to-proton ratio of this nuclide?

1/04

29 The stability of an isotope is based on its  (1) number of neutrons, only (2) number of protons, only (3) ratio of neutrons to protons (4) ratio of electrons to protons

6/04

31 Which of these particles has the greatest mass? (1) alpha (2) beta (3) neutron (4) positron

33 Which of these types of radiation has the greatest penetrating power? (1) alpha (2) beta (3) gamma (4) positron 

1/03

Which radioisotope is a beta emitter? (1) Sr-90  (2) Fr-220   (3)K-37 (4) U-218

8/03

4 An electron has a charge of
                (1) -1 and the same mass as a proton (2) +1 and the same mass as a proton

                (3) -1 and a smaller mass than a proton (4) +1 and a smaller mass than a proton

31 Which of these types of nuclear radiation has the greatest penetrating power? (1) alpha (3) neutron (2) beta (4) gamma

32 Alpha particles and beta particles differ in (1) mass, only (2) charge, only (3) both mass and charge (4) neither mass nor charge

6/02

Which product of nuclear decay has mass but no charge?  (1) alpha particles   (2) neutrons   (3) gamma rays  (4) beta positrons

8/02

Which type of emission has the highest penetrating power?   (1) alpha      (2) beta    (3) positron       (4) gamma

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